Marine Biology - Marine Algae Lecture Notes

Introduction to Marine Algae and Their Habitats

• Conceptual Overview: Algae are a surprisingly broad group of organisms occurring in a range of marine habitats. This study guide explores their diversity, size, structure, evolutionary relationships, and ecological roles, including their significance in global biogeochemical cycles.

• General Definition: Algae is an informal name, not a formal taxonomic group. It encompasses organisms from approximately $11$ major evolutionary lineages, each equivalent to the lineage leading to animals, plants, or fungi.

  • Common Features: Most algae are oxygenic, photoautotrophic organisms containing chlorophyll a, excluding land plants (Embryophyta).

  • Exceptions: Some algae are parasitic and have lost photosynthetic capabilities.

  • Cellular Structure: Includes both eukaryotic cells (e.g., diatoms) and prokaryotic structures (Cyanobacteria).

• Biodiversity Statistics:

  • Known species: approximately $40,000$ across marine, freshwater, and land-based (soil) habitats.

  • Potential discovered species: Tens of thousands are likely yet to be described.

  • Phylogeny: Based on $16S\,rDNA$ gene sequencing, algae fall within the domains Eukarya and Bacteria.

Size, Scale, and Primary Productivity

• Size Range: Spans $9$ orders of magnitude.

  • Smallest: Prokaryotic unicells at approximately $0.6\,\mu m$ in diameter (e.g., Prochlorococcus).

  • Largest: Complex multicellular seaweeds exceeding $60\,m$ in length (e.g., the brown alga Macrocystis).

• Micro-algae vs. Macro-algae:

  • Micro-algae: Loosely defined as those requiring a microscope for identification.

  • Macro-algae: Identifiable with the unaided eye; most are "seaweeds."

• Global Carbon Cycle Roles:

  • Algae are massive primary producers, absorbing $CO_2$ via photosynthesis.

  • Net Productivity Statistics (Geider et al. 2001):

    • Phytoplankton: $46.4\,PgC$

    • Salt marshes, estuaries, macrophytes: $1.2\,PgC$

    • Coral reefs: $0.7\,PgC$

    • Total Marine: $48.3\,PgC$

    • Total Terrestrial: $56.4\,PgC$

  • Unit Conversion: $1\,PgC$ (Petagrams carbon) = $10^{15}\,g$; equivalent to $1\,GtC$ (Gigatons carbon) = $10^9\,t$.

Phytoplankton Characteristics and Global Impact

• Environment: Growing in the illuminated surface waters of coastal seas and open oceans, covering roughly $70\%$ of the Earth.

• Biological Carbon Pump: The proportion of dead cells sinking into deep waters influences nutrient distribution and atmospheric $CO_2$ concentrations.

• Limiting Resources:

  • Light intensity and daylight duration (seasonal changes in temperate and polar waters).

  • Mineral nutrients: Nitrogen (as $NO_3^-$ and $NH_4^+$), Phosphorus (as $PO_4^{3-}$), and micronutrients like iron ($Fe$).

• Species Estimates: Approximately $5,000$ marine phytoplankton species (Tett & Barton 1995), though molecular studies suggest tens of thousands.

• Major Groups:

  • Diatoms: Phylum Ochrophyta (Heterokontophyta), Class Bacillariophyceae.

  • Dinoflagellates: Phylum Pyrrhophyta.

  • Coccolithophores: Phylum Haptophyta (Prymnesiophyta).

  • Cyanobacteria: Prokaryotic members.

Key Phytoplankton: Cyanobacteria and Nitrogen Fixation

• Prochlorococcus and Synechococcus:

  • Found via epifluorescence microscopy (1979 for Synechococcus) and flow cytometry (1987 for Prochlorococcus by Sally Chisholm).

  • Prochlorococcus is the smallest known cyanobacterium ($0.6\,\mu m$), with an estimated $3 \times 10^{27}$ cells in the ocean. They provide $20\%$ of marine primary production ($10\%$ of the global total).

  • Synechococcus: Highlighted by phycobilin pigments which glow orange-yellow under UV light.

• Nitrogen-Fixation: Reducing dissolved nitrogen ($N_2$) to ammonia ($NH_3$).

  • Trichodesmium: A large cyanobacterium fixing about $62\,million\,tons$ of nitrogen annually ($60-70\%$ of total marine biological nitrogen fixation).

  • Crocosphaera: A unicellular nitrogen-fixing genus.

  • Mutualisms: Richelia is an intracellular symbiont within diatoms like Rhizosolenia and Hemiaulus. In nutrient-poor waters, up to $98\%$ of Rhizosolenia cells contain Richelia filaments to provide nitrogen.

Dinoflagellates, Diatoms, and Coccolithophores

• Dinoflagellates:

  • $50\%$ are photosynthetic; $50\%$ are phagotrophic chemoheterotrophs. Many are mixotrophic.

  • Features: Two flagella in grooves; contain chlorophylls a and c and the carotenoid peridinin.

  • Toxins: Roughly $60$ of $4,000$ species produce toxins (e.g., saxitoxins causing Paralytic Shellfish Poisoning or PSP). Gymnodinium catenatum is a known producer.

• Diatoms:

  • Responsible for $40\%$ of marine primary productivity ($20\%$ global, equivalent to rainforests).

  • Thrive in HNLC (High Nutrients, Low Chlorophyll) regions where growth is often limited specifically by iron ($Fe$).

  • Structure: Build siliceous cell walls (frustules).

• Coccolithophores:

  • Surrounded by calcite scales (calcium carbonate). Emiliania huxleyi is the most abundant.

  • Climate Impact: Scales release $CO_2$ during formation but sequester carbon when sinking. They produce dimethyl sulphide (DMS), which leads to cloud formation and sunlight reflection.

  • History: Formed the White Cliffs of Dover durante the Cretaceous period ($100\,million\,years\,ago$).

Micro-Algae in Diverse Benthic Habitats

• Epiphytic Algae: Attached to marine flowering plants like Seagrasses (Zostera capricorni in NZ). Epiphytes can provide up to $60\%$ of the primary productivity in seagrass beds.

• Floating Mats: Sargassum macro-algae support Dichothrix, a nitrogen-fixing cyanobacterium with specialized heterocytes.

• Mangrove Roots: Pneumatophores host nitrogen-fixers like Scytonema, which features "double false-branching."

• Intertidal Sediments:

  • Epipelon: Communities on the surface.

  • Endopelon: Communities within the sediment.

  • Epipsammon: Flora attached to sand grains (e.g., Martyana martyi and Cymbellonitzschia diluviana).

  • Lyngbya aestuarii: A filamentous cyanobacterium that fixes nitrogen in the dark to avoid oxygen inhibition of the nitrogenase enzyme.

• Rocks and Substrates:

  • Epilithic: On rock surfaces (e.g., the dark crusts of Gloeocapsa in the supralittoral).

  • Endolithic: Inside rocks. Chasmoendolithic (in cracks) and Euendolithic (active borers like Hyella or the green alga Ostreobium found in coral limestone).

Special Ecological Structures and Symbioses

• Stromatolites: Layered microbial consortia, often including Schizothrix. Modern examples exist in hypersaline lagoons like Shark Bay, Western Australia. Fossil evidence dates back $3,500\,million\,years$.

• Sea-Ice Algae: Dominated by diatoms that color the ice brown. In Antarctica, Euphausia superba (krill) depend on these for food.

• Epizoic Algae: Attached to animal surfaces. Diatoms like Benettella grow on whale skin. Red algae Phyllophora antarctica provide physical protection for sea urchins from predatory anemones.

• Endozoic Symbioses:

  • Coral and Zooxanthellae: Dinoflagellates (genus Symbiodinium) live intracellularly. Coral receives $95\%$ of their photosynthetic product. Coral bleaching occurs when these symbionts are lost due to temperature stress.

  • Flatworms: Convoluta roscoffensis and Symsagittifera roscoffensis host the green alga Tetraselmis. The worms lose their mouths and rely entirely on the algae.

  • Ascidians (Sea Squirts): Tropical testate ascidians host Prochloron, a cyanobacterium containing chlorophyll b.

  • Acaryochloris: A cyanobacterium containing chlorophyll d, enabling it to capture far-red/infra-red light beneath sea squirt colonies.

Kleptoplasty and Predatory Relationships

• Sacoglossan Molluscs (Elysia): These sea slugs graze on algae but sequester functional chloroplasts.

  • Elysia viridis feeds on Codium.

  • Elysia chlorotica feeds on Vaucheria. Genes from the alga have laterally transferred to the slug genome to support chloroplast function.

• Parasitic Algae:

  • Black Band Disease: Caused by the cyanobacterium Roseofilum reptotaenium, which migrates across coral at $3\,to\,10\,mm/day$, killing tissue via anoxia and toxins.

  • Red Algal Parasites: Evolutionarily independent lineages. Adelphoparasites (kin-parasites like Hypneocolax) and Alloparasites (distantly related hosts).

Macro-Algal Diversity and Distributions

• Phyla Composition:

  • Ochrophyta (Phaeophyceae): Brown algae (~$1,800$ species).

  • Rhodophyta: Red algae (~$6,000$ species).

  • Chlorophyta: Green algae (~$1,500$ species).

• New Zealand Context: Over $800$ seaweeds, many endemic. Includes ~$525$ reds, ~$150$ browns, and ~$125$ greens.

• Key Brown Algae:

  • Hormosira: Southern Hemisphere exclusive.

  • Fucus: Northern Hemisphere exclusive.

  • Macrocystis: Giant kelp with gas-filled bladders and a holdfast. Diplotic sporophyte is the visible stage.

• Siphonous Algae: Lack distinct cells; one large multinucleate cell.

  • Caulerpa taxifolia: An invasive "killer alga" that has dominated Mediterranean habitats since an accidental release in 1984.

• Biofouling: Growth on human structures costs $US\$6\,billion$ annually. Major contributors include the green alga Enteromorpha and the brown alga Undaria pinnatifida (widespread invasive).

Questions & Discussion

• Laboratory Focus: Studies examine freshly collected micro-algae from contrasting habitats and local seaweeds (benthic macro-algae).

• Evolutionary Application: Parasitic red algae are used as model systems to understand genomic changes in eukaryotic parasites, potentially aiding research on malaria (Plasmodium) and potato blight (Phytophthora).