Chapter 7 Multicellular Primary Producers Karleskint Turner Small

Chapter 7 Multicellular Primary Producers Karleskint Turner Small

Key Concepts • Multicellular marine macroalgae, or seaweeds, are mostly benthic organisms that are divided into three major groups according to their photosynthetic pigments. • The distribution of seaweeds depends not only on the quantity and quality of light but also on a complex of other ecological factors.

Key Concepts • Marine algae supply food and shelter for many marine organisms. • Flowering plants that have invaded the sea exhibit adaptations for survival in saltwater habitats. • Seagrasses are important primary producers and sources of detritus, and they provide habitat for many animal species.

Key Concepts • Salt marsh plants and mangroves stabilize bottom sediments, filter runoff from the land, provide detritus, and provide habitat for animals.

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

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 1800 s, 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.

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

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

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

HAPLOID DIPLOID Germinating zygote Zygote (diploid) Sporophyte (diploid) Gametes fusing Sporangium Spores Gametangium (haploid) +Spore +Gametophyte Germinating (haploid) +spore –Spore +Gametophyte +Gametes (haploid) –Gametes (haploid) Germinating –spore –Gametophyte (haploid) 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

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

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-tograze 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

HAPLOID DIPLOID Germinating carpospore Sporangia Tetraspores (haploid) Diploid carpospores Tetrasporophyte (diploid) Egg (haploid) Growth Filament Female gametophyte (haploid) Young carposporophyte Zygote nucleus (diploid) Germinating tetraspores Zygote nucleus (diploid) Male gametophyte (haploid) Sperm and egg Sperm fuse (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

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

Brown Algae • Distribution of brown algae – more diverse and abundant along the coastlines of high latitudes – most are temperate – sargassum weeds are tropical

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

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

HAPLOID DIPLOID Zygote (diploid) Sperm and egg fuse Gas bladders Young sporophyte (diploid) Receptacle Sperm (haploid) Eggs (haploid) Receptacles Egg Gametangium containing eggs (haploid) Sporophyte (diploid) Cross-section of a receptacle Sperm Gametangium containing sperm (haploid) Magnified view of a conceptacle Stepped Art Fig. 7 -11, p. 172

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

Invasion of the Sea by Plants • Flowering plants evolved on land 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 – 12 genera in 5 families of 3 clades (groups with a common ancestor) • 1 clade = eelgrasses and surf grasses • 2 nd clade = paddle grasses (Halophila), turtle grasses, and Enhalus • 3 rd clade = paddle grass (Ruppia), manatee grasses, and shoal grasses

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

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

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

Seagrasses • Reproduction in seagrasses – some use fragmentation, drifting and rerooting 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

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

Salt Marsh Plants • Much less adapted to marine life than seagrasses; must be exposed to air by ebbing tide • Classification and distribution of salt marsh plants – salt marshes are well developed along the low slopes of river deltas and shores of lagoons and bays in temperate regions – salt marsh plants include: • cordgrasses (true grasses) • needlerushes • various shrubs and herbs, e. g. , saltwort, glassworts

Salt Marsh Plants • Structure of salt marsh plants – smooth cordgrass, initiates salt marsh formation, grows in tufts of vertical stems connected by rhizomes, dominates lower marsh • culm: vertical stem • tillers: additional stems produced by a culm at its base, gives a tufted appearance – aerenchyme allows diffusion of oxygen from blades to rhizomes and roots – flowers are pollinated by the wind – seeds drop to sediment or are dispersed by water currents

Salt Marsh Plants • Adaptations of salt marsh plants to a saline environment – facultative halophytes—tolerate salty as well as fresh water – leaves covered by a thick cuticle to retard water loss – well-developed vascular tissues for efficient water transport – Spartina alterniflora have salt glands, secrete salt to outside – shrubs and herbs have succulent parts

Salt Marsh Plants • Ecological roles of salt marsh plants – contribute heavily to detrital food chains – stabilize coastal sediments and prevent shoreline erosion – serve as refuge, feeding ground and nursery for other marine organisms – rhizomes of cordgrass help recycle phosphorus through transport from bottom sediments to leaves – remove excess nutrients from runoff – are consumed by (at least in part) by crabs and terrestrial animals (e. g. insects)

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

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

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