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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