Chapter 7
Key Concepts
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Multicellular marine macroalgae, or seaweeds, are mostly benthic organisms that
are divided into three major groups according to their photosynthetic pigments.
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The distribution of seaweeds depends not only on the quantity and quality of light
but also on a complex of other ecological factors.
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Marine algae supply food and shelter for many marine organisms.
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Flowering plants that have invaded the sea exhibit adaptations for survival in
saltwater habitats.
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Seagrasses are important primary producers and sources of detritus, and they
provide habitat for many animal species.
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Salt marsh plants and mangroves stabilize bottom sediments, filter runoff from the
land, provide detritus, and provide habitat for animals.
Multicellular Algae
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Seaweeds - multicellular algae in the oceans
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Major groups of marine macroalgae:
– red algae (phylum Rhodophyta)
– brown algae (phylum Phaeophyta)
– green algae (phylum Chlorophyta)
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Seaweeds economically important
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Provide habitat for other organisms
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Provide food for other organisms
Distribution of Seaweeds
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Most species are benthic
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Benthic seaweeds define the inner continental shelf
– compensation depth
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Distribution is governed primarily by light and temperature
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Effects of light on seaweed distribution
– chromatic adaptation?
– Distribution dependent on herbivory, competition, pigment concentration,
etc.
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Effects of temperature on seaweed distribution
– Diversity is greatest in tropical waters
– Temperature not a limiting factor for algae in tropical/subtropical seas
– Many colder-water algae are perennials
– Extreme temperatures can kill algae
Structure of Seaweeds
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Thallus
– when flattened, called a blade
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Holdfast
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Stipe
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Lack vascular tissue, roots, stems, leaves and flowers
Biochemistry of Seaweeds
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Major distinctions based on photosynthetic pigments
– All have chlorophyll a plus:
• chlorophyll b in green algae
• chlorophyll c in brown algae
• chlorophyll d in red algae
– Accessory pigments absorb various colors
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Composition of cell walls
– Primarily cellulose
– May have calcium carbonate
– Many secrete slimy mucilage
– Some have a protective cuticle
Reproduction in Seaweeds
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Fragmentation
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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
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Sexual reproduction
– gametes fuse to form a diploid zygote
– Gametophyte
– gametangia
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Alteration of generations
Green Algae (Phylum: Chlorophyta)
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Diverse group containing chlorophyll a & b and carotenoids
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Structure of green algae
– Most are unicellular
– Some have a coenocytic thallus
– Large diversity of forms
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Response of green algae to herbivory
– Tolerance
– Avoidance
– Deterrence
Red Algae (Phylum: Rhodophyta)
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Phycoerythrins
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Structure of red algae
– multicellular
– Diversity of forms
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food for sea urchins, fish, molluscs and crustaceans
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Response of red algae to herbivory
– calcium carbonate
– hard-to-graze forms
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– complex life cycles
– growing in crevices
Ecological relationships of red algae
• epiphytes
• Epizoics
Human uses of red algae
– Phycocolloids
– Irish moss is eaten in a pudding
– Porphyra are used in oriental cuisines
• e.g. sushi, soups, seasonings
– animal feed or fertilizer
Brown Algae (Phylum: Phaeophyta)
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Familiar examples:
– rockweeds
– kelps
– sargassum weed
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99.7% of species are marine
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fucoxanthin
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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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Structure of brown algae
– holdfast, stipe and blade
– Bladders
– cellulose
– alginates
– trumpet cells
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Brown algae as habitat
– kelp forests
– sargassum weeds
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Human uses of brown algae
– thickening agents
– iodine source
– used as food
– cattle feed
Marine Flowering Plants
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vascular plants are distinguished by:
• phloem
• xylem
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2 types of seed bearing plants:
• conifers
• flowering plants
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all conifers are terrestrial
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halophytes
Seagrasses
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Hydrophytes
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Classification and distribution of seagrasses
– 12 genera in 5 families of 3 clades (groups with a common ancestor):
• 1 clade = eelgrasses and surf grasses
• 2nd clade = paddle grasses (Halophila), turtle grasses, and Enhalus
• 3rd clade = paddle grass (Ruppia), manatee grasses, and shoal grasses
Seagrasses (Structure)
– aerenchyme
• lacunae
• buoyancy
• tannins
Seagrasses
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Reproduction in seagrasses
– fragmentation, drifting and re-rooting
– inconspicuous flowers
– hydrophilous pollination
– viviparity
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Ecological roles of seagrasses
– primary producers
– depositing and stabilizing sediments
– reduce turbidity
– habitat
– human uses of seagrass
Salt Marsh Plants
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Less adapted to marine life
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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
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Structure of salt marsh plants:
– smooth cordgrass
• culm
• tillers
– aerenchyme allows diffusion of oxygen
– flowers are wind pollinated
– seeds drop to sediment
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Adaptations of salt marsh plants:
– facultative halophytes
– thick cuticle
– well-developed vascular tissues
– salt glands
– succulence
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Ecological roles of salt marsh plants:
– contribute to detrital food chains
– Stabilize sediments
– habitat for other marine organisms
– phosphorus cycling
– remove excess nutrients from runoff
– Consumed by crabs and terrestrial animals
Mangroves
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Classification and distribution of mangroves:
– 54 diverse species of trees, shrubs, palms and ferns in 16 families
– 2 main families:
• red mangrove
• black mangrove
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
– saline lagoons and tropical/subtropical estuaries
– mangal
Mangroves
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Structure of mangroves
– 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)
– roots
• stilt roots
• prop roots
• drop roots
• lenticels
• anchor roots
• nutritive roots
• cable roots
• pneumatophores
– Leaves
• simple, oval, leathery and thick, succulent
• stomata
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salt glands
concentrating salt in shedding leaves
Mangroves
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Reproduction in mangroves
– simple flowers pollinated by wind or bees
– buoyant seeds
– viviparity
• propagule
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Ecological roles of mangroves:
– stabilize 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