BIOL 212 Marine Biology and Ecology - Lecture 4: Marine Algae

Micro-algae Associations: Mutualism, Symbiosis, and Parasitism

• Diversity of Hosts for Micro-algae: Algae live in a variety of symbiotic relationships (mutualism, symbiosis, and parasitism) within diverse animal groups:

  • Dinoflagellates: Commonly found in corals, jellyfish, gastropods, and bivalves.

  • Chlorophyta (Tetraselmis): Known to live within flatworms.

  • Cyanobacteria: Colonial forms live within sea squirts; some are also parasitic.

• Mutualism in Colonial Sea Squirts:

  • Prochloron & Sea Squirts: The genus Prochloron (cyanobacteria) lives within the tunic of colonial sea squirts such as Didemnum molle.

  • Pigmentation: The sea squirts appear green due to the presence of the algae.

  • Cellular Characteristics: Prochloron cells are large and unusual because they contain both chlorophyll a and chlorophyll d within their chloroplasts.

• Mutualism in Golden Jellyfish (Palau):

  • Environment: Jellyfish Lake in Palau is a landlocked lagoon with no access to the sea, characterized by steep sides and shelter from the wind. It has high nutrient levels due to land drainage.

  • Trophic Shift: While jellyfish are typically predators using stinging cells on tentacles, those in this lake rely on algae called zooxanthellae found in the bell and tentacles.

  • Behavioral Adaptation: To support the algae, the jellyfish sink at night and migrate to surface waters during the day to maximize exposure to sunlight.

  • Reference: Dawson and Hamner (2003), Mar. Biol.

• Symbiosis in Hard Corals:

  • Coral Characteristics: Corals are predatory suspension feeders with hard, calcified internal skeletons.

  • Exchange of Materials: Corals obtain glucose from zooxanthellae (algal cells), while the algae receive protection and $CO_2$ from the coral host.

  • Thermal Tolerance: Different types of Symbiodinium (symbionts) can increase the thermal tolerance of the coral host, such as in Acropora millepora.

• Parasitism in Corals:

  • Certain cyanobacteria act as parasites, attacking organisms when they are under stress (e.g., during coral bleaching events).

  • These attacks can lead to diseases and eventual mortality of the coral.

• Associations with Flatworms:

  • Small acoel flatworms live among sand grains in estuarine sediments.

  • Convoluta roscoffensis is a larger species found in local estuaries.

  • These flatworms host Tetraselmis (Chlorophyta) as symbionts.

Macroalgae Overview and Classification

• General Characteristics:

  • Mostly attached to rock surfaces.

  • Morphology: Lack true leaves, stems, and roots. The body is the thallus; the blade is the primary photosynthetic surface. They are anchored by a holdfast and may have a stipe and pneumatocysts (air bladders).

• Classification (Three Main Phyla):

  • Chlorophyta (Green Algae): Approximately $125$ species, with only $10\%$ being marine. They have simple thallus structures and contain pigments and food reserves similar to land plants.

  • Phaeophyta (Brown Algae): Approximately $150$ species. They contain yellow and brown pigments and include the largest macroalgae, such as kelps.

  • Rhodophyta (Red Algae): Approximately $250$ species, mostly marine. Their green chlorophyll is masked by red pigments (phycobilins). They represent the highest species richness but have less diversity in form.

• Reproduction and Life Cycles:

  • Macroalgae exhibit both sexual (gametophyte) and asexual (sporophyte) phases.

  • These phases may be isomorphic (look the same) or heteromorphic (differ in size).

  • In most red algae, male and female thalli are separate.

Chlorophyta (Green Algae) and Ecological Impacts

• Habitats: Found in environments with a wide range of salinity.

• Nuisance Species and Blooms:

  • Macroalgal blooms of green seaweed are common globally, including in China and New Zealand.

  • Cause: Blooms occur in areas with high organic nutrient inputs.

  • Impact of Sea Lettuce: Forms deep mounds; as the tissue decays, the sediment below becomes anoxic (lacks oxygen). This leads to a decrease in species abundance.

  • Diversity: While large blooms are harmful, small amounts of algae can actually promote species diversity.

Phaeophyta (Brown Algae) Diversity and Structure

• Morphological Range: Varies from fine filaments to complex, massive structures.

• Examples of Genera:

  • Halopteris.

  • Macrocystis: Growth rates are very fast; can reach lengths of up to $30\,m$. It is a major habitat-forming and highly productive species.

  • Dictyota.

  • Hormosira.

  • Microzonia.

  • Laminaria sacculina: Structures include the thallus and sporangia which produce spores.

• Invasive Species: Undaria pinnatifida is a significant brown alga with a global distribution as an invasive species.

Rhodophyta (Red Algae) Characteristics

• Pigmentation: Uses phycobilins to mask green chlorophyll.

• Habitats: Primarily found in shallow rocky shore environments.

• Structure: Ranges from simple thalli to complex branching forms. The largest species, such as Gigartina, can have blade-like thalli up to $2\,m$ in size.

Epizoic Macroalgae: Macroalgae Attached to Animals

• Unique Associations:

  • Antarctic Feeding Triangle: Involves macroalgae (Phyllophora antarctica, Iridaea cordata), sea urchins (Sterechinus neumayeri), and sea anemones (Isotealia antarctica).

  • Defense Mechanism: The sea urchin does not eat the algae because the algae are protected by secondary metabolites (chemical defenses). This protects the algae from predation while it is carried by the urchin.

  • Decorator Crabs: Notomithrax sp. select specific red algae (e.g., Corallina sp.) to attach to their shells for camouflage.

  • Fish: Scorpion fish and stone fish often have macroalgae and microalgae attached for camouflage.

• Associations with Sea Slugs:

  • Sea hares use algae for food and camouflage.

  • Chemical Defense: Some sea slugs sequester chemicals from the algae they eat to deter predators.

  • Elysia chlorotica: Known as "solar-powered sea slugs" because they incorporate algal components to perform photosynthesis.

Parasitic Macroalgae

• Parasitism occurs even among macroalgae, predominantly within red algae which can be partially or fully parasitic.

• Examples:

  • Red algae living on Ascophyllum nodosum.

  • Ectocarpus growing as an epiphyte/parasite on larger algae.

  • Green alga specializing as a parasite on Corallina.

Importance and Uses of Macroalgae

• Ecological Roles:

  • Habitat formation and primary production.

  • Food source for myriad marine species and aquaculture.

  • Camouflage for macroinvertebrates.

  • Carbon sequestration tool (prolific growth in New Zealand helps environment).

• Commercial and Economic Uses:

  • Human Food: Used as a subsidiary food for relish, taste, texture, and as thickeners. The main market is Asia.

  • High-Value Species: Nori (Porphyra), Kombu (Laminaria), and Wakame (Undaria).

  • Hydrocolloids: Production of Agar and Carrageenan.

  • Other Uses: Fertilizers, drug development, and biofuels.

  • Fouling: Macroalgae can be a nuisance by fouling boats and underwater structures.

• Cultural Importance (New Zealand Māori):

  • Karengo: Porphyra sp. used in traditional recipes.

  • Rimurapa (Bull Kelp): Includes Durvillaea antarctica, D. poha, and D. willana.

• Future Applications:

  • Source of energy/compost via digestion.

  • Waste-water treatment.

  • Edible/Biodegradable Packaging: Research into carrageenan-based films and soft plastic alternatives. The company "Kelpn" in New Zealand is developing compostable food packaging from locally prolific seaweed that does not compete with food production.

Summary of Marine Algae

1. Algae are biologically diverse and belong to distinct taxa with unique features.

2. They play critical roles across various marine habitats.

3. They exhibit a wide range of lifestyles (mutualistic, parasitic, free-living).

4. They are fundamental to the ecology of marine environments.

5. They hold significant cultural and commercial value.

6. They form complex and interesting associations with other marine organisms.