marine biology indexado para clase microorganismo

CastroHuber: Marine Biology, Fourth Edition

II. Life in the Marine Environment

5. The Microbial World The McGrawHill Companies, 2003

91

T his chapter, the first in our surveyof life in the ocean, is devoted tomarine microorganisms, whichare easily the most abundant forms of ma-rine life. Microorganisms live everywherein the ocean, from the deepest trenches tothe highest tide pools. The microbial worldincludes an incredibly diverse assortmentof organisms, and new microorganisms arediscovered regularly. Except for beingsmall, the various groups of microorgan-isms discussed in this chapter have little incommon. All three biological domains, themost basic divisions of life (see The Tree

of Life, p. 87), include microorganisms.The diversity of the microbial world can bebewildering even to biologists. Indeed,much of the scientific debate about how togroup organisms into kingdoms within do-mains centers on microorganisms. Onething is for sure: The familiar concepts ofplants and animals have been modifiedin classification schemes that consist ofmore than two kingdoms.

Though marine microorganisms in-clude some of the smallest and struc-turally simplest organisms on earth, theyhave played critical roles in the evolu-

tion of life on our planet. Withoutthem, the world we live in would be verydifferent. Microorganisms are the mostimportant primary producers in manymarine environments, and directly or in-directly feed most marine animals. Somemicroorganisms make essential nutrients

Marine diatom (Arachnodiscus).

The Microbial World

c h a p t e r

5

DomainBacteria DomainArchaea

DomainEukaryota

KingdomPlantae Kingdom

Animalia

KingdomProtista

KingdomFungi

Prokaryotes

Eukaryotes

Primary Producers Organisms thatmanufacture organic matter from CO2,usually by photosynthesis.

Chapter 4, p. 73




available to other primary producers, ei-ther for the first time or by recyclingthem. Others swim about in the waterto ingest food very much like animalsdo, and play critical roles in marine foodchains.

PROKARYOTES

Prokaryotes are the smallest and struc-turally simplest organisms, and the oldest forms of life on earth. Nonethe-less, they carry out nearly all the chemi-cal processes performed by supposedlymore complex organismsmost ofwhich evolved in prokaryotes in the firstplaceas well as some that are unique toprokaryotes.

Prokaryotes cells are enclosed by aprotective cell wall. The plasma mem-brane, or cell membrane, lies immedi-ately inside the cell wall (see Fig. 4.7). Inaddition to lacking membrane-bound or-ganelles, prokaryotes differ from eukary-otes in the circular shape of the DNAmolecules that encode their genetic infor-mation, in the size of their ribosomes,and in a number of other ways.

Despite their similarities, the two pro-karyotic domains, Bacteria and Archaea,also have important differences, includingthe chemistry of their cell walls (Table 5.1)and plasma membranes, and the cellularmachinery that manufactures proteins. Infact, genetic analyses indicate that bacteriaand archaea are as different from each otheras they are from humans!

Bacteria

Bacteria (domain Bacteria) appear to havebranched out very early on the tree of lifeand are genetically distant from both ar-chaea and eukaryotes. Because they areprokaryotes, bacteria (singular, bacterium),have remained structurally simple, but theyhave evolved a great range of metabolicabilities and chemical features. They areabundant in all parts of the ocean.

Bacterial cells have many shapes, including spheres, coils, rods, or rings (Fig. 5.1) depending on the species. Thecells are very small (see Appendix A forrelative size), much smaller than those ofsingle-celled eukaryotes. About 250,000average bacterial cells would fit on the pe-riod at the end of this sentence. There are

92 Part Two Life in the Marine Environment www.mhhe.com/marinebiology

Photosynthetic Pigments Major Storage Products Major Cell-Wall Components

Bacteria (except Bacteriochlorophyll a, b, c, d, e Variety of types Peptidoglycan containing Cyanobacteria) muramic acid

Cyanobacteria Chlorophyll a Cyanophycean starch Chains of amino sugars and amino acids

Phycobilins (phycocyanin, Cyanophycin (protein)phycoerythrin, and others)

Carotenoids

Archaea Bacteriorhodopsin Variety of types Variety of types, no muramic acid

Diatoms Chlorophyll a, c Chrysolaminarin Silica

Carotenoids Oil Pectin

Dinoflagellates Chlorophyll a, c Starch Cellulose

Carotenoids Oil

Green algae Chlorophyll a, b Starch Cellulose

Carotenoids Carbonates in calcareous algae

Brown algae Chlorophyll a, c Laminarin Cellulose

Carotenoids (fucoxanthin, Oil Alginatesand others)

Red algae Chlorophyll a Starch Agar

Phycobilins (phycocyanin, Carrageenanphycoerythrin)

Carotenoids

Flowering plants Chlorophyll a, b Starch

Carotenoids

Chemical Compounds Used in the Characterization of Marine Photosynthetic OrganismsTable 5.1Table 5.1

Cellulose

Carbonates in coralline algae

Cellulose




exceptions, however, such as a sedimentbacterium discovered off Namibia, south-west Africa, with cells in filaments as wideas 0.75 mm (0.03 in), large enough to beseen with the naked eye (Fig. 5.2)! An-other giant bacterium (0.57 mm or 0.02 inlong) is found inside the intestines of acoral reef fish.

The unique chemistry of bacterialcell walls (Table 5.1) makes them rigid

and strong. A stiff or gelatinous coveringoften surrounds the cell wall as addi-tional protection or a means to attach tosurfaces.

Bacteria can grow to extremely highnumbers in favorable environments suchas sediments or decomposing organicmatter. In large numbers, marine bacteriaare sometimes visible as iridescent or pinkpatches on the surface of stagnant poolsin mudflats and salt marshes. Sometimesgigantic strands of cells can be seen aswhitish hairs on rotting seaweed.

Heterotrophic Bacteria

Most bacteria, like animals, are het-erotrophs and obtain their energy fromorganic matter. Most of these bacteriaare decomposers, which break downwaste products and dead organic matterand release nutrients into the environ-ment. These bacteria, the decay bacteria,are vital to life on earth because they en-sure the recycling of essential nutrients.They are found everywhere in the marineenvironmenton almost all surfaces aswell as in the water column. They are es-pecially abundant in bottom sediments,

which are usually rich in organic matter.All organic matter is sooner or later de-composed, though in very deep, coldwater the process is slower than elsewhere(see Bacteria in the Deep Sea, p. 374).

Decay and other types of bacteriaplay a crucial role because they constitutea major part of the organic matter thatfeeds countless bottom-dwelling animals.Even many organic particles in the watercolumn are composed mostly of bacteria.Marine bacteria are also important in re-cycling dissolved organic matter inoceanic food webs (see The MicrobialLoop, p. 343). Other marine bacteria arebeneficial because they are involved indegrading oil and other toxic pollutantsthat find their way into the environment.The same process of decomposition isunfortunately involved in the spoilage ofvaluable fish and shellfish catches.

Autotrophic Bacteria

Autotrophic bacteria make their own or-ganic compounds and thus are primary pro-ducers. Some of them are photosynthetic(or photoautotrophs; see Fig. 4.7). Theycontain chlorophyll or other photosyn-thetic pigments (Table 5.1) and, like sea-weeds and plants, tap light energy tomanufacture organic compounds fromCO2. Photosynthetic bacteria are nowknown to account for much of the primaryproduction in many open-ocean areas. Thebiochemistry of bacterial photosynthesis,however, is different from that of algae and

Chapter 5 The Microbial World 93

Prokaryotes Organisms with cells thatdo not have a nucleus or otherorganelles.

Chapter 4, p. 74; Figure 4.7

Eukaryotes Organisms with cells thatcontain a nucleus and other organellesthat are enclosed by membranes.


Heterotrophs Organisms that cannotmake their own food and must eat theorganic matter produced by autotrophs.Autotrophs Organisms that can useenergy (usually solar energy) to makeorganic matter.

Chapter 4, p. 72

Cell wall

FIGURE 5.1 A high-magnification photograph taken with an electron microscope showingCyclobacterium marinus, a ring-forming marine bacterium. The bar represents 0.1 micron (m).

FIGURE 5.2 Three cells ofThiomargarita namibiensis, the largest bacteriumknown so far. Cells can be 0.75 mm (0.03 in)wide. The round, white structures are sulfurgranules.





Some bacteria live in close, orsymbiotic, associations with othermarine organisms. Some symbi-otic bacteria are parasites thatmay ultimately cause a disease.Others, on the other hand, bene-fit their hosts. These symbioticbacteria began their association byincreasing the chances of survivalof the host and evolve to becomeessentialhosts cannot survivewithout them. In many cases thesymbiotic bacteria are even shel-tered in special tissues or organsthat evolve in the host.

All eukaryotic organisms, in-cluding humans, shelter bacteriawithout which life would be im-possible. The chloroplasts andmitochondria of eukaryotic cellsevolved from symbiotic bacteria(see From Snack to Servant:How Complex Cells Arose,p. 77). Indeed essential, these bacteria have become an integral partof all complex cells.

There are many cases of symbiotic bacteria among marine or-ganisms. Symbiotic bacteria, for example, are involved in digestingthe wood that is ingested by shipworms (Teredo), which happen tobe bivalve molluscs, not worms. Like all wood-eating animals, ship-worms lack cellulase, the enzymes needed to digest cellulose, themain component of wood. Wood is a surprisingly common habitatin many marine environments, and so everything from driftwood toboat bottoms is exploited by shipworms thanks to their symbioticbacteria.

Symbiotic bacteria are also responsible for the light, or bio-luminescence, that is produced by some fishes, squids, octopuses,and other animals of the deep (see Bioluminescence, p. 365).The bacteria are usually sheltered in special light-producing or-gans, or photophores. These deep-sea animals, which live indarkness, use light to communicate with other members of theirspecies, lure prey, blend with the light that filters from the surface,and for other functions. Flashlight fishes (Anomalops), which livein shallow tropical waters, lodge their symbiotic bacteria in anorgan beneath each eye. A shutter mechanism controls the emis-sion of light so that the fish are provided with the ability of blink-ing at night! Groups of fish blink in synchrony, a behavior that isinvolved in attracting prey.

Chemosynthetic bacteria symbiotic with mussels, clams, and tubeworms that live around deep-sea hydrothermal vents have a very par-ticular role: manufacturing organic matter from CO2 and the abun-dant hydrogen sulfide (H2S) from the vents. The symbiotic bacteria

live in a special organ, the feeding body, of the giant hydrothermal-vent tube worm Riftia (see Fig. 16.31).

Symbiotic bacteria are also involved in some unexpected ways.Pufferfishes are known to store a toxin that is deadly to any predators(or humans) who eat the fish. The fish, or fugu, is a delicacy in Japan.Licensed chefs must prepare the fish, otherwise the toxin(tetrodotoxin) will kill anyone feasting on fish that has been improp-erly prepared. Tetrodotoxin is a deadly neurotoxin (that is, it affectsthe nervous system). In fact, it is one of the most powerful poisonsknown, and there is no antidote! The deadly toxin is stored mostly inthe liver and gonads of the fish, so the internal organs must be ex-pertly removed. Mistakes do happen and numerous deaths (includingthe suicides of disgraced cooks) take place every year in Japan. Cooksmay still be guilty, but not the puffers: The toxin is now known to beproduced not by the fish but by symbiotic bacteria. No one knowsyet how the fish is immune to the bacteria toxins.

Tetrodotoxin and very similar toxins have also been found in avariety of marine organisms such as flatworms, snails, crabs, seastars, and several species of fishes. It has also been found in theblue-ring octopus, a notoriously toxic animal. We are not yet sure ifin all of these animals the toxin is produced, as in fugu, by symbioticbacteria or if it is accumulated from their food. Tetrodotoxin of anunknown source has also been found in dead sea urchins and is sus-pected as their cause of death.

The production of powerful toxins that are used by other or-ganisms is just one example of the amazing abilities of bacteriainvisible to the eye but powerful giants when it comes to their rolein the environment.

SYMBIOTIC BACTERIATHE ESSENTIAL GUESTS

A Japanese pufferfish, or fugu (Takifugu niphobles), and an inflated pufferfish(Diodon histrix).




plants, and varies considerably among dif-ferent groups of bacteria. Some bacteria,for example, have a kind of chlorophyllunique to prokaryotes and produce sulfur(S) instead of oxygen (O2).

Other bacterial autotrophs, calledchemosynthetic or chemoautotrophic,derive energy not from light but fromchemical compounds such as hydrogen(H2), ammonia (HN3), hydrogen sulfide(H2S), and other sulfur or iron com-pounds. Many other ways of obtainingenergy to manufacture organic matter arefound among chemosynthetic bacteria.

Bacteria are structurally simple microorgan-isms that are especially significant as decom-posers, breaking down organic compounds intonutrients that can be used by other organisms.They are also an important food source andhelp degrade pollutants. Some species are auto-trophic, and account for much of the oceanicprimary production.

Cyanobacteria

Cyanobacteria, once known as blue-green algae, are a group of photosyntheticbacteria. In addition to having chloro-phyll, most contain a bluish pigmentcalled phycocyanin (Table 5.1). Mostmarine cyanobacteria also have a reddishpigment, phycoerythrin. The visiblecolor of the organism depends on the rel-ative amounts of the two pigments.When phycocyanin predominates, thebacteria appear blue-green, when phyco-erythrin predominates they appear red.Photosynthesis takes place on foldedmembranes within the cell (as in Fig. 4.7)rather than in chloroplasts as in eukary-otic photosynthetic organisms such asseaweeds and plants. Unlike other photo-synthetic bacteria, cyanobacteria do sharewith eukaryotic photosynthetic organismsa type of chlorophyll, chlorophyll a(Table 5.1), and liberate gaseous oxygenin photosynthesis. Most marinecyanobacteria are microscopic, thoughsome form long filaments, strands, orthick mats visible to the naked eye.

Cyanobacteria were perhaps amongthe first photosynthetic organisms onearth. They are thought to have had an im-

portant role in the accumulation of oxygenin our atmosphere. Fossil stromatolites,massive calcareous mounds formed bycyanobacteria, are known to date backsome 3 billion years. Stromatolites are stillbeing formed in tropical seas (Fig. 5.3).

Many species of cyanobacteria can tol-erate wide ranges of salinity and tempera-ture and are therefore found practicallyeverywhere in the marine environment, in-cluding some unexpected places like thehair of polar bears! Some cyanobacteria,called endolithic, burrow into calcareousrocks and coral skeletons. Others formthick, dark crusts along the wave-splashedzone of rocky coasts. Some cyanobacteriaexploit oxygen-poor sediments, which mayinclude polluted sites. Planktonic speciesmay rapidly multiply and change the colorof the water. Even some of the so-calledred tides (see Red Tides and HarmfulAlgal Blooms, p. 326) are caused bycyanobacteria that contain a red pigment.A few species are responsible for skinrashes on swimmers and divers.

Many cyanobacteria are known tocarry out nitrogen fixation, converting

gaseous nitrogen (N2) into other nitrogencompounds that can be used by other pri-mary producers (see Cycles of EssentialNutrients, p. 227). Some cyanobacterialive on the surface of seaweeds and sea-grasses. Photosynthetic organisms thatlive on other algae or plants are calledepiphytes. Some cyanobacteria live insidealgae and are called endophytes. Othercyanobacteria are known to lose theirability to photosynthesize, thus becomingheterotrophs.

Cyanobacteria are photosynthetic bacteriawidely distributed in the marine environment.


FIGURE 5.3 Stromatolites, calcareous mounds deposited by cyanobacteria are frequentlyfound as fossils. These, however, are living stromatolites growing in shallow water in the ExumaCays, Bahama Islands.

Calcareous Made of calcium carbonate(CaCO3).

Chapter 2, p. 33

Plankton Primary producers(phytoplankton) and consumers(zooplankton) that drift with the currents.





Archaea

Archaea (domain Archaea), often calledarchaebacteria, are the simplest, mostprimitive forms of life. Some look verysimilar to the oldest fossils ever found,cells estimated to be at least 3.8 billionyears old. Like bacteria, their cells aresmall and may be spherical, spiral, orrod-shaped (Fig. 5.4). In fact, until re-cently archaea (singular, archaeum) werethought to be bacteria. It is now thought,however, that despite being prokaryotic,archaea are more closely related to eu-karyotes than to bacteria.

Archaea may be heterotrophs or autotrophs and like bacteria exhibit agreat variety of metabolic processes. Onegroup, the methanogens is the only groupof organisms that produces methane(CH4). They are important decomposersand extremely abundant in sediments, es-pecially in organic-rich environments likesalt marshes and mudflats. Some are ni-trogen fixers. They are also important inbreaking down organic matter in sewagetreatment plants and landfills.

Other groups of archaea were discov-ered only recently, first in extreme envi-ronments on land such as hot sulfursprings, saline lakes, and highly acidic oralkaline environments. Marine archaeawere subsequently found in extreme ma-rine environments, such as in very deepwater, where they survive at pressures of300 to 800 atmospheres. Other archaeaare associated with hydrothermal ventsor coastal saltpans.

Because these groups of archaea werefirst known only from such extreme envi-ronments, they were thought to depend onthem. Indeed, the term extremophiles(lovers of extremes) became almost syn-onymous with archaea. Some archaea, forexample, cannot grow in temperatures lessthan 70 to 80C (158 to 176F), and onespecies can live at 113C (235F), the high-est of any living organism known. Othersdepend on extremely acid, alkaline, or saltyconditions. New techniques based on de-tecting nucleic acid sequences characteristicof archaea, however, have shown that theyare exceedingly common in the water col-umn and other marine environments (seeTiny Cells, Big Surprises, p. 99). Thus,the hypothesis that archaea are restrictedto extreme environments was proven false.

Archaea are prokaryotic microorganisms oncethought to be bacteria, but are more closely re-lated to eukaryotes. Most were thought to berestricted to extreme environments but are nowknown to be very common in the water columnand elsewhere.

UNICELLULAR ALGAE

Algae (singular, alga) are a very diversegroup of simple, mostly aquatic (that is,marine and freshwater), mostly photo-synthetic organisms. They are eukaryotic,and as such their cells contain organellesenclosed by membranes. Photosynthesistakes place in chloroplastsgreen,brown, or red organelles with layers of in-ternal membranes that contain photosyn-thetic pigment (see Fig. 4.8b). The colorof algae is a reflection of the pigmentsand their concentration. In contrast tothe land plants with which we are famil-iar, algae lack flowers and have relativelysimple reproductive structures. Theirnonreproductive cells are also mostly sim-ple and unspecialized. Algae lack trueleaves, stems, and roots.

Biologists used to refer to algae asplants. Many of the unicellular algae,however, show animal-like characteris-tics. Some swim by moving their flagella.Distinguishing these free-swimmingalgae from some of the structurally sim-

pler animals becomes rather difficult, es-pecially when there are species that carryout photosynthesis like plants, and verysimilar ones move and eat food particleslike animals. Some are claimed by bothbotanists (plant biologists) and zoologists(animal biologists)! These unicellular organisms are grouped in a separate king-dom, Protista. Seaweeds, though multi-cellular, are algae that are generallyplaced in the kingdom Protista mostlybecause they lack the specialized tissuesof plants. Seaweeds, together with othermulticellular primary producers, the ma-rine plants, are discussed in Chapter 6.

Algae are protists. They are mostly aquatic pri-mary producers that lack the specialized tissuesof plants. They range in size and complexityfrom single cells to large multicellular seaweeds.

Diatoms

Diatoms (phylum Bacillariophyta) areunicellular, though many species aggregateinto chains or star-like groups. Diatomcells are enclosed by cell walls made largelyof silica (SiO2), a glass-like material(Table 5.1). This glassy shell, or frustule,consists of two tightly fitting halves oftenresembling a flat, round, or elongate box(Fig. 5.5). The frustule typically has intri-cate perforations and ornaments such asspines or ribs, making diatoms strikinglybeautiful when seen under a microscope(see figure on page 91). The frustule al-lows light to pass through, so that the con-spicuous golden-brown chloroplasts cancapture light energy for photosynthesis.The minute perforations allow dissolvedgases and nutrients to enter and exit. Thesinking of open-ocean diatoms below thewell-lit surface layer is often slowed by oildroplets in their cells and spines on thefrustules. (see Staying Afloat, p. 333).

The characteristic color of diatoms isdue to yellow and brown carotenoid pig-ments present in addition to two types ofchlorophyll, a and c (Table 5.1). Diatomsare efficient photosynthetic factories, pro-ducing much-needed food (the food beingthe diatoms themselves), as well as oxygenfor other forms of life. They are very im-portant open-water primary producers in


FIGURE 5.4 Pyrobacterium aerophilumis a rod-shaped marine archaeum that growsbest at temperatures of 100C. Each cell isabout 0.5 microns (m) in diameter.




half (Fig. 5.6). Diatoms may also repro-duce by sexual reproduction. Some cellsdevelop eggs, others develop flagellatedsperm (Fig. 5.6). Fertilization then re-sults in the development of resistantstages known as auxospores.

Favorable environmental conditions,such as adequate nutrients and tempera-ture, trigger periods of rapid reproductioncalled blooms. This is a general phenome-non that also occurs in other algae. Duringblooms, most diatoms get progressivelysmaller, partly because they use up the sil-ica dissolved in the water and partly be-cause the smaller frustule becomes thelarger one in many of the new cells. Fullsize may be regained by the developmentof auxospores. The auxospores eventuallygive rise to larger cells that display thefrustule characteristic of the species.

The glassy frustules of dead diatomseventually settle to the bottom of the seafloor. Here they may form thick deposits ofsiliceous material that cover large portionsof the ocean floor. Such sediments areknown as diatomaceous ooze. Huge fossil

deposits of these sediments can now befound inland in various parts of the world.The siliceous material, or diatomaceousearth, is mined and used in products suchas filters for swimming pools, for clarifyingbeer, as temperature and sound insulators,and as mild abrasives that may find theirway into toothpaste.

Dinoflagellates

The dinoflagellates (phylum Dinofla-gellata or Pyrrhophyta) make up anotherlarge group of planktonic, unicellular or-ganisms. Their most outstanding charac-teristic is the possession of two flagella,one wrapped around a groove along themiddle of the cell (Fig. 5.7) and one trail-ing free. These flagella direct movementin practically any direction. Most dinofla-gellates have a cell wall that is armored


Ribosomes Structures inside the cell,consisting of proteins and RNA, whereproteins are manufactured.


Hydrothermal Vents Undersea hotsprings associated with mid-ocean ridgesand other geologically active environments.

Chapter 2, p. 38

Nucleic Acids DNA and RNA,complex molecules that store andtransmit genetic information.

Chapter 4, p. 70

Hypothesis A statement about theworld that might be true.

Chapter 1, p. 13

Types of reproduction:Asexual (or vegetative) The productionof new individuals by simple division (or other means) without involvinggametes, so that the offspring aregenetically identical to the parents.

Chapter 4, p. 82

Sexual The production of newindividuals by the formation of gametes(sperm and eggs), so that the offspring aregenetically different from the parents.

Chapter 4, p. 83

Frustule

Upper frustule(epitheca)Oil droplet

Nucleus

Lower frustule(hypotheca)Chloroplast

FIGURE 5.5 Diagrammatic representation of a diatom cell (also see Fig. 15.3).

temperate and polar regions (seeEpipelagic Food Webs, p. 341). In fact,billions of diatom cells in the ocean ac-count for a hefty share of the organic car-bon and oxygen produced on planet earth.

Around half the estimated 12,000living species of diatoms are marine.Most are planktonic, but many produce astalk-like structure for attachment torocks, nets, buoys, and other surfaces.The brownish scum sometimes seen onmudflats or glass aquaria often consists ofmillions of diatom cells. Some are able toslowly glide on surfaces. A few are color-less and, having no chlorophyll, live onthe surfaces of seaweeds as heterotrophs.

Diatoms are unicellular organisms that livemostly as part of the plankton. A silica shell istheir most distinctive feature. They are importantopen-water primary producers in cold waters.

Diatoms reproduce mostly by cell di-

vision, a type of asexual reproduction.The overlapping halves of the frustuleseparate, and each secretes a new, smaller




with plates made of cellulose, the char-acteristic component of the cell walls ofseaweeds and land plants (Table 5.1).The plates may have spines, pores, orother ornaments.

Though most dinoflagellates photo-synthesize, many can also ingest food par-ticles. A few have a light-sensitive pigmentspot that acts as a crude eye. It has beensuggested that during their evolution di-noflagellates gained the ability to functionas primary producers by capturing andusing chloroplasts from other algae. Al-most all known dinoflagellates, around1,200 living species, are marine. Dinofla-gellates are important planktonic primaryproducers, especially in warm water.

Dinoflagellates reproduce almost exclusively by simple cell division (see Fig. 4.18). They sometimes form bloomsthat color the water red, reddish-brown,yellow, or other unusual shades (see RedTides and Harmful Algal Blooms,p. 326). Some of these dinoflagellates re-lease toxic substances, and seafood col-lected during red tide periods may bepoisonous. Other dinoflagellates are noted


Asexual reproduction

Sexual reproduction

Smallerfrustules

Auxospore

Fertilization

Sperm cellEgg

FIGURE 5.6 The usual method of reproduction in diatoms is by cell division. Most, but not all, frustules get smaller with time. Resistant cellscalled auxospores are produced two ways: directly from the expansion of a smaller frustule or by sexual reproduction when an egg is fertilized by a spermcell, both of which are liberated from separate frustules.

FIGURE 5.7 Dinoflagellates such as Gonyaulax polyedra have a cell wall, or theca, thatconsists of cellulose plates. The theca is marked by grooves for the flagella; only one groove isvisible here (arrow). This species is bioluminescent and also responsible for red tides.




for the production of light, or biolumines-cence (see The Bay of Fire, on page 100).Though bioluminescence has also beenobserved in some bacteria and many typesof animals, dinoflagellates are generally re-sponsible for the diffuse bioluminescencesometimes seen on the sea surface. This ef-fect is of course seen only at night. It is es-pecially bright if the water is disturbed by aboat or when a wave crashes on the shore.

A variety of marine animals containin their tissues round, golden-brown pho-tosynthetic cells known as zooxanthellae.These cells are actually dinoflagellatesadapted to live in close association with ananimal host. Though hosts range fromsponges and sea anemones to giant clams,it is perhaps in reef-building corals where

zooxanthellae are most significant (seeFig. 14.7). They fix carbon dioxide byphotosynthesis, release organic matterused by the coral, and help in the forma-tion of the coral skeleton (see Coral Nu-trition, p. 301).

A few highly modified dinoflagel-lates are parasites of seaweeds and somemarine animals. Like zooxanthellae, thesehighly specialized forms reveal their truenature by having life cycles that includefree-swimming stages resembling typicaldinoflagellates.

One such dinoflagellate is Pfiesteria,sometimes called the phantom dinofla-gellate because they spend most of theirlives as harmless resting stages, or cysts,in the sediments. Coastal pollution in the


Cellulose Complex carbohydratecharacteristic of plants and other primaryproducers.

Chapter 4, p. 70

It is surprising how little biologists know about the lives of marinemicrobes. We know they are everywhere but we are not sure howmany types there are, how common they are, and what exactly theyare up to. The situation is rapidly changing as we apply new tech-niques and approaches studying the private lives of marine microor-ganisms that call the ocean home.

Information about marine microbes was traditionally gatheredlargely by growing them in artificial cultures inoculated with sea-water samples. This information (what nutrients the microbes needto grow, the type of compounds they produce, and such) was gath-ered mostly from organisms collected in water samples and thengrown under artificial conditions. Even worse, many organismswould not grow in the lab and their presence in the sample went un-detected. At least some of this information was used by marine ecol-ogists to deduce the role of microbes in the big ocean worldtherole of decay bacteria in releasing nutrients, for example, and of bac-teria that convert free nitrogen into nitrates or make dissolved or-ganic matter (DOM) available to larger forms of life. Still, bacteriaand other marine microbes remained vital but little-understoodforms of life. Some groups of microbes remained undiscovered for along time. Most archaea, for instance, have been known only sincethe 1970s. Certainly, other groups remain to be discovered.

The minute prokaryotic cells of bacteria and archaea and thesmallest eukaryotic cells of some algae seem to be everywhere butare very difficult to collect and characterize by the microscopic ex-amination of cells. Instead of culturing or filtering out these cells,fragments of nucleic acid (see Nucleic Acids, p. 70) are collectedfrom the water and sequenced. The presence of organisms can bedetected by identifying characteristic DNA or RNA sequences. Fur-thermore, the partial sequences can yield information about theunique characteristics of these microbes, even if whole cells or wholenucleic acid molecules are not collected.

The presence of RNA sequences characteristic of archaea inseawater and sediment samples has shown archaea to be abundant.In 2001 two groups of archaea were found to be extremely abundantin water samples collected from depths below the photic zone, thatis, where light does not penetrate (see Chapter 16). The samples,collected from the Pacific Ocean south of Hawaii, showed that ar-chaea were rare at the surface but sharply increased in abundancebelow depths of 250 m (720 ft). They were as common as bacteriabelow 1,000 m (3,280 ft). Judging from their numbers and the hugevolume of the ocean in question, these archaea are among theoceans most abundant forms of life.

Similar techniques based on the detection of characteristicnucleic acid sequences have been used to identify previously un-known eukaryotic microorganisms. These eukaryotes are largerthan prokaryotes but still minute (smaller than 2 to 3 m), mak-ing them difficult to collect and study. This sharply contrastsother unicellular but larger eukaryotes in the phytoplankton,which are relatively easy to collect using plankton nets (see ThePlankton: A New Understanding, p. 324). In 2001 two newgroups related to dinoflagellates were discovered at depths of 250to 3,000 m (720 to 9,840 ft) near Antarctica. Similar but uniquegroups of eukaryotic microorganisms (including yet another newgroup related to dinoflagellates) were reported at the same time byanother team working at depths of 75 m (246 ft) in the tropicalPacific. The fact that these microorganisms collected in relativelyshallow water (where light still penetrates) are photosyntheticadds a new dimension to the discovery. They represent a previ-ously unknown source of primary production in typically unpro-ductive tropical waters. Other groups of minute algae have beendiscovered in recent years, including three groups that are relatedto dinoflagellates. It will not be surprising if additional groups arediscovered in the future.

TINY CELLS, BIG SURPRISES

form of excessive nutrients can triggerblooms of Pfiesteria, which release power-ful poisonous substances that stun fishes.Pfiesteria and other Pfiesteria-like mi-croorganisms have been linked to deadlyopen sores on the fishes. These parasitesare also known to be harmful to crabs,oysters, and clams, and have been impli-cated with sores and symptoms such astemporary memory loss in humans.




Dinoflagellates are unicellular organisms thathave two unequal flagella. They are mostlymarine and are particularly common in thetropics. Some are noted for their emission oflight; others are closely associated with marineanimals, especially reef corals.

Other Unicellular Algae

Three additional groups of primary pro-ducers may be abundant in some areas.Silicoflagellates (phylum Chryosphyta)are characterized by a star-shaped inter-nal skeleton made of silica (Fig. 5.8) anda single flagellum. Coccolithophorids(phylum Haptophyta) are flagellated,spherical cells covered with button-like

structures called coccoliths that are madeof calcium carbonate (Fig. 5.9). Coccolithmay be found in sediments as fossils.Cryptomonads (phylum Cryptophyta)have two flagella and lack a skeleton.Members of these three groups are sosmall that hundreds could fit into a largediatom or dinoflagellate cell.

PROTOZOANS:THE ANIMAL-LIKEPROTISTS

Protozoans are structurally simple andvery diverse organisms that are tradition-ally considered to be animal-like. Mostbiologists agree that protozoans (meaning


FIGURE 5.8 Skeleton of Dictyochaspeculum, a silicoflagellate.

Imagine entering a quiet tropical bay at night. There are no lights onshore, and on a moonless night the spectacle is unforgettable. As thepropeller of your boat churns the black water, an intense blue-greenlight is left behind, like an eerie trail of cool fire. Outside the bay yousee a few sparks of light as you approach, but inside the bay thingsare very different. Long streaks of light shoot like fireworks beneaththe boat. They are created by fleeing fish. A swim in the bay is evenmore spectacular. Your dive into the water is accompanied by ablinding flood of light, and sparks scatter out as you wave your arms.

Such light effects result from an unusual concentration of bio-luminescent dinoflagellates, a natural and permanent phenomenonin a few select locations such as Baha Fosforescente, or Phosphores-cent Bay, in southwestern Puerto Rico.

Bioluminescent Bay might be a more apt name because the lightis produced by living organisms. The bays most important source ofbioluminescence is Pyrodinium bahamense, a unicellular, photosyn-thetic dinoflagellate about 40 microns (m) (0.004 cm or 0.0015 in)in diameter. As many as 720,000 individuals are found in a gallon of water, many more than outside the bay. The bay is small, about 90 acres, and fan-shaped, and it does not exceed 4.5 m (15 ft) indepth. The main reason that the dinoflagellates are concentrated hereis that the bay is connected to the open sea by a narrow, shallow chan-nel. As water containing the dinoflagellates flows into the bay, evapo-ration in the shallow water causes the surface water to sink because ofthe increase in salinity, and therefore density. Evaporation is enhancedby the dry and sunny days. The Pyrodinium stays near the surface andis therefore not carried out as the denser water flows out along thebottom of the shallow entrance. The tidal range here is only about afoot at the most, so water exchange with the outside is limited. Thebay thus acts as a trap that keeps the dinoflagellates from leaving.

Of all the planktonic organisms that enter the bay, why is Pyro-dinium favored? A key factor seems to be the thick mangrove treesbordering the bay. They grow along the muddy shores, togetherwith all kinds of organisms living attached to their roots. Mangroveleaves fall into the water, where intense bacterial decomposition in-creases the organic nutrient levels in the water. Some nutrients es-sential to the growth of Pyrodinium, perhaps vitamins, may bereleased by bacteria or other microorganisms.

Baha Fosforescente is protected to keep the critical natural bal-ance intact, but bioluminescence has noticeably decreased during re-cent years. The same phenomenon in a bay in the Bahamasdisappeared when its narrow entrance was dredged to allow largerboats, with larger loads of tourists, to pass through.

THE BAY OF FIRE

Baha Fosforescente in Puerto Rico.





FIGURE 5.9 Coccolithophorids, likeUmbilicosphaera sibogae from Australia, aretiny single-celled phytoplankton that can bevery important primary producers in the openocean. The plates that cover the cell are madeof calcium carbonate.

the first animals) are actually composedof several groups of urelated origins. Likeanimals, they ingest food (that is, they areheterotrophs) and are eukaryotic. Someprotozoans, however, contain chlorophylland photosynthesize. Although a fewform colonies, most are single-celled, un-like true animals, and only visible under amicroscope.

Having a single cell is about the onlything that all protozoans, which show anenormous diversity in structure, function,and lifestyle, have in common. There islittle agreement as to how to classify theestimated 50,000 species of protozoans,but for now biologists group them in thekingdom Protista with unicellular algaeand seaweeds.

Protozoans are the most animal-like of theprotists. They are eukaryotic and unicellular.They are heterotrophic and ingest food like trueanimals.

The minute size and apparent sim-

plicity of protozoans disguise a complexnature. Each cell might be described as asuper cell because it can perform many

of the same functions carried out by themultitude of cells in the structurally morecomplex organisms. They inhabit watereverywhere, living not only in marine andfreshwater environments, but even insideother organisms. Many kinds of marineprotozoans can be readily collected fromsediments rich in organic debris, the sur-face of seaweeds, the guts of animals, andplankton samples.

Foraminiferans

The foraminiferans, (phylum Forami-nifera), better known as forams, are ma-rine protozoans that usually have a shell,or test, made of calcium carbonate(CaCO3). The test is usually micro-scopic and may have several chambers(Fig. 5.10a) that increase in size as theforam grows. Pseudopodiaextensionsof the jelly-like contents of the cell, or cytoplasmare thin, long, and retract-able in forams. The pseudopodia protrudethrough pores in the shell and form a network used to trap diatoms and otherorganisms suspended in the water (seeFig. 15.5). Food is then moved into theinterior of the cell as if on a conveyor belt.

Most forams live on the bottom,either free or attached. Attached foramsmay develop into conspicuous growthsas much as 5 cm (2 in) in diameter (Fig. 5.10b). Each growth consists of asingle cell that forms a shell, though

some may be covered with sand grains orother materials instead. The shells ofbottom-living forams can be importantcontributors of calcareous material oncoral reefs and sandy beaches. Only a fewspecies are planktonic, but these can bevery abundant. Their shells are smallerand thinner than those that live on thebottom and may have delicate spines thataid in flotation. The shells of planktonicforams eventually sink to the bottom insuch high numbers that large stretches ofthe ocean floor are covered by forami-niferan ooze, a type of calcareous ooze.Many limestone and chalk beds aroundthe world, like the white cliffs of Doverin England, are products of foram sedi-ments that were uplifted from the oceanfloor.

Forams are protozoans characterized by a shellusually made of calcium carbonate. Most liveon the bottom. The shells of planktonic speciesare important components of marine sediments.

FIGURE 5.10 Foraminiferans. (a) Foraminiferans typically have calcareous shells thatconsist of a spiral arrangement of chambers. The thin, long strands are the pseudopodia used tocapture food. (b) Homotrema rubrum is a foraminiferan that forms bright red calcareous growthsseveral millimeters in diameter at the base of corals in the tropics. It is so common in Bermuda thattheir skeletons are responsible for the islands famous pink beaches.

(a) (b)

Biogenous Sediments Sediments madeof the skeletons and shells of marineorganisms; calcareous ooze is made ofcalcium carbonate and siliceous ooze ofsilica.

Chapter 2, p. 33




Most species of forams are knownonly as microfossils. The distribution ofthese microfossils in sediments is veryimportant to geologists. Shells of warm-water species are slightly larger and moreporous than those from colder water. Pastwater temperatures can be estimated fromthe distribution of certain marker species.Their distribution is also valuable in thesearch for oil because it is used to deter-mine the age of sediments.

Radiolarians

Radiolarians (phylum Polycystina) areplanktonic marine protozoans that se-crete elaborate and delicate shells madeof glass (silica) and other materials. Typ-ical shells are spherical with radiatingspines (Fig. 5.11), though the structurevaries. Thin, needle-like pseudopodiacapture food, as in forams.

Most radiolarians are microscopic,but some form bizarre, sausage-shapedcolonies that reach 3 m (9 ft) in length,making them true giants among proto-zoans. Radiolarians inhabit open waterthroughout the oceans. When abundant,the remains of their shells settle to the

bottom and form a siliceous ooze knownas radiolarian ooze. This ooze is moreextensive in deep water because radiolar-ian shells are more resistant to dissolvingunder pressure than those of forams.

The shells of radiolarians are made primarilyof silica (glass). These shells form siliceous sedi-ments that cover large areas of the ocean floor.

Ciliates

Ciliates (phylum Ciliophora) are proto-zoans that have many hair-like cilia thatare used in locomotion and feeding. Themost familiar ciliates are freshwater formssuch as Paramecium. Marine ciliates areusually found creeping over seaweeds andin bottom sediments. Some live on thegills of clams, in the intestines of seaurchins, on the skin of fish, and in otherunusual places. Other ciliates live attachedto surfaces, even forming branchedcolonies of tiny individuals. Tintinnids arecommon ciliates that build vase-like cases,or loricas, loosely fitting shells that drift inthe water (Fig. 5.12). The cases may betransparent or made of bits of particles.

FUNGI

Fungi (kingdom Fungi) are eukaryoticand mostly multicellular, although some,such as molds and yeasts, are unicellular.Many fungi appear superficially plant-like but are heterotrophs that lack chloro-plasts and chlorophyll and cannotperform photosynthesis.

There are at least 500 known speciesof marine fungi. They are mostly micro-scopic. Many, like bacteria, are decom-posers of dead organic matter. They arethe most important decomposers of man-grove plants and thus contribute to therecycling of nutrients in mangrove forests(see Mangroves, p. 115). Some are par-asites of seagrasses or borers in molluscshells. Others are parasites that cause dis-eases of economically important seaweeds,sponges, shellfish, and fish. Some marinefungi are being investigated as a source ofantibiotics for use in medicine. Still otherslive in close association with algae to formunique entities, the lichens. In lichens thelong, filament-like growths of the fungiprovide support, while the algae providefood manufactured by photosynthesis. Ma-rine lichens can be found as thick, dark-brown or black patches in the wave-splashedzone of rocky shores, particularly on NorthAtlantic shores. By comparison with themultitude of lichen species on land, thereare very few species of marine lichens.


Food particle

Skeleton

Pseudopod Bandof cilia

Test(lorica)

Nuclei

FIGURE 5.11 A typical radiolarian cell consists of a dense central portion surrounded by aless dense zone that is involved in the capture of food particles and in buoyancy.

FIGURE 5.12 Tintinnopsis is amarine tintinnid that forms a vase-like loricamade of sand grains. Specialized cilia on oneend are used in feeding.




103

i n t e r a c t i v e e x p l o r a t i o nCheck out the Online Learning Center at www.mhhe.com/marinebiology and click on thecover of Marine Biology for interactive versions of the following activities.

Jarrell, K. F., D. P. Bayley, J. D. Correia and N. A. Thomas,1999. Recent excitement about the Archaea. BioScience,vol. 49, no. 7, July, pp. 530541. The archaea are useful tools in efforts to understand some of the basiccharacteristics of life.

Knight, J., 1999. Blazing a trail. New Scientist, vol. 164, no. 2213, 20 November, pp. 2832. Bioluminescentphytoplankton is used to study the swimming ability of dolphins.

Leslie, M., 2001. Tales of the sea. New Scientist, vol. 169, no. 2275, 27 January, pp. 3235. New techniques allow thesampling of previously unknown unicellular organisms in theocean.

Zimmer, C., 2000. Sea sickness. Audubon, vol. 102, no. 3,MayJune, pp. 3845. Bacteria, fungi, and othermicroorganisms cause serious diseases in marine life, fromseagrasses and corals to cetaceans.

In DepthCahoon, L. B., 1999. The role of benthic microalgae in neritic

ecosystems. Oceanography and Marine Biology: An AnnualReview, vol. 37, pp. 4786.

Fleming, L. E., J. Easom, D. Baden, A. Rowan and B. Levin,1999. Emerging harmful algal blooms and human health:Pfiesteria and related organisms. Environmental ToxicologicPathology, vol. 27, pp. 573581.

Hyde, K. D., E. B. G. Jones, E. Leano, S. B. Pointing, A. D. Poonyth and L. L. Vrijmued, 1998. Role of fungi inmarine ecosystems. Biodiversity and Conservation, vol. 7,pp. 11471161.

McFall-Ngai, M. J., 1999. Consequences of evolving withbacterial symbionts: Insights from the squid-Vibrioassociations. Annual Review of Ecology and Systematics,vol. 30, pp. 235256.

Rowan, R., 1998. Diversity and ecology of zooxanthellae on coralreefs. Journal of Phycology, vol. 34, pp. 407417.

See It in MotionVideo footage of the following can be found for this chapter onthe Online Learning Center.

Brown algae moving in surge (Honduras)

?^

Do-It-Yourself SummaryA fill-in-the-blank summary is available in the Online LearningCenter, which allows you to review and check your understandingof this chapters subject material.

Key TermsAll key terms from this chapter can be viewed by term, or bydefinition, when studied as flashcards in the Online LearningCenter.

Critical Thinking1. Scientists use the particular structure of nucleic acids and

other chemical differences to separate the archaea from thebacteria. Can you think of other characteristics that could beused to separate not only these two kingdoms but alsobetween them and members of the kingdom Protista?

2. An autotrophic protist, such as a diatom or a dinoflagellate,can evolve into a heterotrophic protist (and therefore aprotozoan) simply by losing its chloroplasts. Under whatconditions could such a situation take place?

For Further ReadingSome of the recommended readings listed below may be avail-able online. These are indicated by this symbol , and willcontain live links when you visit this page in the Online Learn-ing Center.

General InterestBurkholder, J. M., 1999. The lurking perils of Pfiesteria.

Scientific American, vol. 281, no. 2, August, pp. 4249. The life cycle and other aspects of the biology of thispotentially harmful parasite.

Furlow, B., 2001. The freelance poisoner. New Scientist,vol. 169, no. 2274, 20 January, pp. 3033. As in the case of the deadly toxins found in pufferfishes, similar toxicchemicals in other marine organisms may actually beproduced by bacteria.

Hoppert, M. and F. Mayer, 1999. Prokaryotes. AmericanScientist, vol. 87, no. 6, NovemberDecember, pp. 518525.Prokaryotic cells show a complex organization, even whenorganelles are absent.




Pfiesteria Diversity of archaea Marine protozoans and invertebrates Diversity of fungi Lichens

Quiz YourselfTake the online quiz for this chapter to test your knowledge.

Marine Biology on the NetTo further investigate the material discussed in this chapter, visitthe Online Learning Center and explore selected web links to re-lated topics.

Photosynthetic marine organisms Diversity of algae Economic and ecological importance of algae Diversity of eubacteria Cyanobacteria


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