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Karleskint
Small
Turner
Chapter 7Multicellular Primary Producers
Multicellular Algae
• Most primary production in marine ecosystems takes place by phytoplankton but seaweed and flowering plants contribute especially in coastal areas
• Seaweeds are multicellular algae that inhabit the oceans
• Major groups of marine macroalgae:– red algae (phylum Rhodophyta)– brown algae (phylum Phaeophyta)
– green algae (phylum Chlorophyta)
Multicellular Algae
• Scientists who study seaweeds and
phytoplankton are called phycologists or algologists
• Seaweeds contribute to the economy of coastal seas
• Produce 3 dimensional structural habitat for other marine organisms
• Consumed by an array of animals, e.g., sea urchins, snails, fish
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Distribution of Seaweeds
• Most species are benthic, growing on rock, sand, mud, corals and other hard substrata in the marine environment as part of the fouling community
• Benthic seaweeds define the inner continental shelf, where they provide food and shelter to the community– compensation depth: the depth at which the
daily or seasonal amount of light is sufficient for photosynthesis to supply algal metabolic needs without growth
• Distribution is governed primarily by light and temperature
Distribution of Seaweeds
• Effects of light on seaweed distribution
– chromatic adaptation, proposed in the 1800s, was accepted for 100 years
• chromatic adaptation: the concept that the distribution of algae was determined by the light wavelengths absorbed by their accessory photosynthetic pigments, and the depth to which these wavelengths penetrate water
– distribution now believed to be more dependent on herbivory, competition, pigment concentration, etc.
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Distribution of Seaweeds
• Effects of temperature on seaweed distribution– diversity of seaweeds is greatest in tropical
waters, less in colder latitudes– temperature not a limiting factor for algae in
tropical/subtropical seas
– many colder-water algae are perennials (living more than 2 years)
• only part of the alga survives colder seasons• new growth is initiated in spring• freezing and ice scouring can eliminate seaweeds in
high latitudes
– intertidal algae can be killed if temperatures become too hot or cold
Structure of Seaweeds
• Thallus: the seaweed body, usually
composed of photosynthetic cells
– when flattened, called a frond or blade
• Holdfast: the structure attaching the thallus to a surface
• Stipe: a stem-like region between the holdfast and blade of some seaweeds
• Lack vascular (conductive) tissue, roots, stems, leaves and flowers
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Biochemistry of Seaweeds
• Major distinctions among seaweed phyla is based on biochemistry
• Photosynthetic pigments– Color of thallus due to wavelengths of light not absorbed
by the seaweed’s pigments– All have chlorophyll a plus:
• chlorophyll b in green algae• chlorophyll c in brown algae
• chlorophyll d in red algae
– Chlorophylls absorb blue/red wavelengths of light, pass green light
– Accessory pigments absorb various colors• e.g. carotenes, xanthophylls, phycobilins pass energy to
chlorophylls for photosynthesis
Biochemistry of Seaweeds
• Composition of cell walls– Primarily cellulose
– May be impregnated with calcium carbonate in calcareous algae
– Many seaweeds secrete slimy mucilage (polymers of several sugars) as a protective covering
• holds moisture, and may prevent desiccation
• can be sloughed off to remove organisms
– Some have a protective cuticle—a multi-layered protein covering
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Biochemistry of Seaweeds
• Nature of food reserves
– Excess sugars are converted into polymers
– Stored in cells as starches
– Chemistry of starches differs among groups of macroalgae
– Unique sugars and alcohols may be used as antifreeze substances by intertidal seaweeds
during cold weather
Reproduction in Seaweeds
• Fragmentation: asexual reproduction in which the thallus breaks up into pieces, which grow into new algae– drift algae: huge accumulations of seaweeds
formed by fragmentation, e.g., some sargassum weeds
• Asexual reproduction through spore formation– haploid spores formed within an area of the
thallus (sporangium) through meiosis
– sporophyte (diploid): stage of the life cycle that produces spores, which is diploid
Reproduction in Seaweeds
• Sexual reproduction
– gametes fuse to form a diploid zygote
– Gametophyte (usually haploid): stage of the life
cycle that produces gametes
– gametangia: structures in the gametophytes
where gametes are typically produced
• Alteration of generations: the possession of 2 or more separate multicellular stages
(asexual sporophtye, sexual gametophyte) in
succession
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+Gametes(haploid)
Germinatingzygote
Sporophyte(diploid)
Zygote(diploid)
Gametesfusing
+Spore+Gametophyte
(haploid)Germinating+spore
+Gametophyte
Spores(haploid)
Sporangium
–Gametes(haploid)
Gametangium
HAPLOIDDIPLOID
–Gametophyte(haploid)
Germinating–spore
–Spore
–Gametophyte
Stepped Art
Fig. 7-3, p. 164
Green Algae (Phylum: Chlorophyta)
• Diverse group of microbes and multicellular organisms that contain some pigments found in vasculaar plants, chlorphyll a & b and certain carotenoids
• Structure of green algae– Most are unicellular or small multicellular
filaments, tubes or sheets
– Some tropical green algae have a coenocytic thallus consisting of a single giant cell or a few large cells containing more than 1 nucleus and surrounding a single vacuole
• the cell grows but doesn’t divide, the nucleus divides
– There is a large diversity of forms among green algae
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Green Algae
• Response of green algae to herbivory
– Tolerance: rapid growth and release of huge numbers of spores and zygotes
– Avoidance: small size allows them to occupy out-of-reach crevices
– Deterrence:
• calcium carbonate deposits require herbivores with strong jaws and fill stomachs with non-nutrient minerals
• many produce repulsive toxins
Green Algae
• Reproduction in green algae
– the common sea lettuce, Ulva, has a life cycle that is representative of green algae
– basic alternation of generations between the sporophyte and gametophyte stages
• large, leafy sporophytes and gametophytes are nearly identical
• spores and gametes are similar, but spores have 4 flagella while gametes have 2
• gametes of opposite mating types must fuse for fertilization to occur
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Red Algae (Phylum: Rhodophyta)
• Primarily marine and mostly benthic
• Highest diversity among seaweeds
• Red color comes from phycoerythrins
– Thalli can be many colors, yellow to black
• Structure of red algae
– Almost all are multicellular
– Thallus may be blade-like or composed of
branching filaments or heavily calcified
• algal turfs: low, dense groups of filamentous red (along with greens, browns) and branched thalli that carpet the seafloor over hard rock or loose sediment
Red Algae
• Annual red algae are seasonal food for sea urchins, fish, molluscs and crustaceans
• Response of red algae to herbivory– making their thalli less edible by incorporating
calcium carbonate
– changing growth patterns to produce hard-to-graze forms like algal turfs
– evolving complex life cycles which allow them to rapidly replace grazed biomass
– avoiding herbivores by growing in crevices
Red Algae
• Reproduction in red algae
– 2 unique features of their variety of life cycles:
• absence of flagella
• occurrence of 3 multicellular stages:
– 2 sporophytes in succession and one gametophyte
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Germinatingcarpospore
Diploidcarpospores
Sporangia
Tetrasporophyte(diploid)
Growth
Sperm and eggfuse
Youngcarposporophyte
Zygotenucleus(diploid)
Zygotenucleus(diploid)
Egg(haploid)
Filament
Tetraspores(haploid)
Germinatingtetraspores
Malegametophyte
(haploid)
Sperm(haploid)
HAPLOIDDIPLOID
Female
gametophyte
(haploid)
Stepped Art
Fig. 7-6, p. 168
Red Algae Life Cycle
• sperm from male gametophyte forms zygote on part of egg-containing female gametophyte, then divides while still attached to the gametophyte to form unique red algal stage called a carposporophyte
• carposporophyte produces non-motile diploid spores called carpospores
• carpospores settle, germinate, and grow into an adult alga called a tetrasporophyte
• tetrasporophyte releases non-motile haploid tetraspores which grow into gametophytes
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Red Algae
• Ecological relationships of red algae
– a few smaller species are:
• epiphytes—organisms that grow on algae or plants
• epizoics—organisms that grow on animals
– red coralline algae precipitate calcium carbonate from water and aid in consolidation of coral reefs
Red Algae
• Human uses of red algae
– phycocolloids (polysaccharides) from cell walls are valued for gelling or stiffening properties
• e.g. agar, carrageenan
– Irish moss is eaten in a pudding
– Porphyra are used in oriental cuisines
• e.g. sushi, soups, seasonings
– cultivated for animal feed or fertilizer in parts of Asia
Brown Algae (Phylum: Phaeophyta)
• Familiar examples:– rockweeds
– kelps
– sargassum weed
• 99.7% of species are marine, mostly benthic (sargassum – not benthic)
• Olive-brown color comes form the carotenoid pigment fucoxanthin, masks green pigment of chlorophylls a & c
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Brown Algae
• Distribution of brown algae
– more diverse and abundant along the coastlines of high latitudes
– most are temperate
– sargassum weeds are tropical
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Brown Algae
• Structure of brown algae
– most species have thalli that are well differentiated into holdfast, stipe and blade
– bladders—gas-filled structures found on larger blades of brown algae, and used to help buoy the blade and maximize light
– cell walls are made up of cellulose and alginates (phycocolloids) that lend strength and flexibility
– trumpet cells—specialized cells of kelps that conduct photosynthetic products (e.g. mannitol)
to deeper parts of the thallus
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Brown Algae
• Reproduction in brown algae– usual life cycle, i.e., alternation of generations
between a sporophyte (often perennial) and a gametophyte (usually an annual)
– rockweed (Fucus) eliminates gametophyte stage; meiosis occurs on inflated tips (recepticles) of the sporophyte in chambers called conceptacles, fertilization occurs in the water column
– rhizoids—root-like structures which attach the fertilized egg and grow into a holdfast
Magnifiedview of a
conceptacle
Cross-sectionof a receptacle
Sporophyte(diploid)
Receptacle
Young sporophyte(diploid)
Zygote(diploid)
Sperm andegg fuse
Gametangiumcontaining
sperm(haploid)
Sperm(haploid)
Sperm
Eggs(haploid)
Gametangiumcontaining eggs
(haploid)
Egg
Gas bladders
Receptacles
HAPLOIDDIPLOID
Stepped Art
Fig. 7-11, p. 172
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Brown Algae
• Brown algae as habitat
– kelp forests house many marine animals
– sargassum weeds of the Sragasso Sea form floating masses that provide a home for unique organisms
• Human uses of brown algae
– thickening agents are made from alginates
– once used as an iodine source
– used as food (especially in Asia)
– used as cattle feed in some coastal countries
Marine Flowering Plants
• Seagrasses, Marsh Plants, Mangroves
• General characteristics of marine flowering plants– vascular plants are distinguished by:
• phloem: vessels that carry water, minerals, and nutrients
• xylem: vessels that give structural support
– seed plants reproduce using seeds, structures containing dormant embryos and nutrients surrounded by a protective outer layer
Marine Flowering Plants
– 2 types of seed bearing plants:
• conifers (bear seeds in cones)
• flowering plants (bear seeds in fruits)
– all conifers are terrestrial
– marine flowering plants are called halophytes, meaning they are salt-tolerant
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Invasion of the Sea by Plants
• Flowering plants evolved on land and then adapted to estuarine and marine environments
• Flowering plants compete with seaweeds for light and with other benthic organisms for space
• Their bodies are composed of polymers like cellulose and lignin that are indigestible to most marine organisms
• Have few competitors and often form extensive single-species stands on which other members of the community depend
Seagrasses
• Seagrasses are hydrophytes (generally live beneath the water)
• Classification and distribution of seagrasses– 7 Species in Florida (see articles)
Seagrasses
• Structure of seagrasses
– vegetative growth—growth by extension and branching of horizontal stems (rhizomes) from
which vertical stems and leaves arise
– 3 basic parts: stems, roots and leaves
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Seagrasses (Structure)
– stems • have cylindrical internodes (sections) separated by nodes (rings)
• rhizomes—horizontal stems with long internodes with growth zones at the tips, usually lying in sand or mud
• vertical stems arise from rhizomes, usually have short internodes, and grow upward toward the sediment surface
• grow slowly ensuring leaf production keeps up with sediment accumulation
– roots• arise from nodes of stems and anchor plants
• usually bear root hairs—cellular extensions
• Absorb mineral nutrients
• allow interaction with bacteria in sediments
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Seagrasses (Structure)
– leaves
• arise from nodes of rhizomes or vertical stems
• scale leaves—short leaves that protect the delicate growing tips of rhizomes
• foliage leaves—long leaves from vertical shoots with 2 parts
– sheath that bears no chlorophyll
– upper blade that accomplishes all photosynthesis of the
plant using chloroplasts in its epidermis undergo periods of growth and senescence
– blade life cycles affect epiphytes on seagrasses
Seagrasses (Structure)
– aerenchyme—an important gas-filled tissue in seagrasses
• lacunae—spaces between cells in aerenchyme tissues throughout the plant
– provide a continuous system for gas transport
• aerenchyme provides buoyancy to the leaves so they can remain upright for sunlight exposure
• tannins—antimicrobials produced as a chemical defense against invasion of the aerenchyme by pathogenic fungi or labyrinthulids
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Seagrasses
• Reproduction in seagrasses– some use fragmentation, drifting and re-
rooting and do not flower
– inconspicuous flowers are usually either male or female and borne on separate plants
– hydrophilous pollination• sperm-bearing pollen is carried by water currents
to stigma (female pollen receptor)
– a few species produce seedlings on the mother plant (viviparity)
Seagrasses
• Ecological roles of seagrasses
– highly productive on local sale
– role of seagrasses as primary producers
• less available and less digestible than seaweeds
• contribute to food webs through fragmentation and loss of leaves
– sources of detritus
– role of seagrasses in depositing and stabilizing sediments
• blades act as baffles to reduce water velocity
• decay of plant parts contributes organic matter
• rhizomes and roots help stabilize the bottom
• reduce turbidity—cloudiness of the water
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Seagrasses (Ecological Roles)
– role of seagrasses as habitat
• create 3-dimensional space with greatly increased area on which
other organisms can settle, hide, graze or crawl
• rhizosphere—the system of roots and rhizomes also increases
complexity in surrounding sediment
• the young of many commercial species of fish and shellfish live in seagrass beds
– human uses of seagrass
• indirect – fisheries depend on coastal seagrass meadows
• direct – extracted material used for food, medicine and industrial
application
Mangroves
• Classification and distribution of mangroves
– mangroves include 54 diverse species of trees, shrubs, palms and ferns in 16 families
– ½ of these belong to 2 families:
• red mangrove (Rhizophora mangle)
• black mangrove (Avicennia germinans)
– others are white mangroves, buttonwood, and Pelliciera rhizophoreae
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Mangroves (Distribution)
– thrive along tropical shores with limited wave action, low slope, high rates of sedimentation,
and soils that are waterlogged, anoxic, and high in salts
– low latitudes of the Caribbean Sea, Atlantic Ocean, Indian Ocean, and western and eastern Pacific Ocean
– associated with saline lagoons and tropical/subtropical estuaries
– mangal: a mangrove swamp community
Mangroves
• Structure of mangroves
– trees with simple leaves, complex root systems
– plant parts help tree conserve water, supply
oxygen to roots and stabilize tree in shallow, soft sediment
– roots: many are aerial (above ground) and contain aerenchyme
• stilt roots of the red mangrove arise high on the trunk (prop roots) or from the underside of branches (drop roots)
• lenticels: scarlike openings on the stilt root surface connecting aerenchyme with the atmosphere
Mangroves (Structure)
• anchor roots: branchings from the stilt root beneath the mud
• nutritive roots: smaller below-ground branchings from anchor roots which absorb mineral nutrients from mud
• black mangroves have cable roots which arise below ground and spread from the base of the trunk
• anchor roots penetrate below the cable root
• pneumatophores: aerial roots which arise from the upper side of cable roots, growing out of sediments and into water or air
• lenticels and aerenchyme of pneumatophores act as ventilation system for black mangrove
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Mangroves (Structure)
– leaves
• mangrove leaves are simple, oval, leathery and thick, succulent like marsh plants, never submerged
• stomata: openings in the leaves for gas exchange and water loss
• salt is eliminated through salt glands (black mangroves) or by concentrating salt in old leaves that shed
Mangroves
• Reproduction in mangroves
– simple flowers pollinated by wind or bees
– mangroves from higher elevations have buoyant seeds that drift in the water
– mangroves of the middle elevation and seaward fringe have viviparity
• propagule: an embryonic plant that grows on the parent plant,
breaks through fruit wall and grows an elongated cigar-shaped stem (hypocotyl)
• propagule falls from parent tree and may drift in currents by the
buoyant hypocotyl for as long as 100 days
Mangroves
• Ecological roles of mangroves
– root systems stabilize sediments
• aerial roots aid deposition of particles in sediments
– epiphytes live on aerial roots
– canopy is a home for insects and birds
– mangals are a nursery and refuge
– mangrove leaves, fruit and propagules are consumed by animals
– contribute to detrital food chains
