Biol120 - Lecture 11: Autotrophic Diversity

What Are “Algae”?

  • Functional – not evolutionary – category for diverse aquatic, “plant-like” autotrophs.
    • Range of body plans:
    • Single-celled phytoplankton (free-floating).
    • Large attached seaweeds/macroalgae.
    • Habitats: oceans, lakes, rivers, ponds, damp soils, snow.
  • NOT a monophyletic group; photosynthetic capability evolved repeatedly or was transferred by endosymbiosis.

Autotrophy, Photon Energy & Photosynthesis

  • Photosynthesis equation (oxygenic):
    • 6CO2 + 12H2O \xrightarrow{\text{light}} C6H{12}O6 + 6O2 + 6H_2O
  • Autotrophs use light (phototrophy) or chemical energy (chemotrophy). Mixotrophs combine strategies.
  • Up to 50 % of global primary productivity comes from marine phytoplankton (mainly algae).

Eukaryote Supergroups – Which Lineages Are Photosynthetic?

  • 5 supergroups (Excavata, SAR, Archaeplastida, Amoebozoa, Opisthokonta) + minor lineages.
  • Photosynthetic members highlighted:
    • Excavata → Euglenozoans (many mixotrophic).
    • “SAR” clade → Stramenopiles (diatoms, golden algae, brown algae), Alveolates (dinoflagellates), Rhizarians (chlorarachniophytes via secondary plastids).
    • Archaeplastida → Red algae, Green algae, Land plants (shared primary plastid).
  • Term “algae” ≙ photosynthetic protists across these clades.

Capturing Light Energy & Pigments

  • Photosynthetically Active Radiation (PAR): ~400–700 \text{ nm}.
  • Each pigment absorbs a characteristic band; algae pack multiple pigments to widen capture range.

Chlorophylls

  • Universal primary electron donor = Chlorophyll a (in all oxygenic photosynthesizers).
  • Chlorophyll b → land plants & green algae (chlorophytes + charophytes).
  • Chlorophyll c → red algae, brown algae, diatoms, dinoflagellates.

Accessory / Auxiliary Pigments

  • Carotenoids (orange-brown) – esp. in brown algae & diatoms; limited photosynthesis + photoprotection.
  • Phycocyanin (light-blue) – cyanobacteria; absorbs orange/red.
  • Phycoerythrin (red) – red algae; absorbs green/blue.

Light Penetration & Depth Zonation

  • Blue wavelengths penetrate deepest; red attenuate fastest.
  • Typical depth niches (approx.):
    • Chlorophytes (\to) shallower (red light still present).
    • Rhodophytes (\to) mid-depths (green/blue capture via phycoerythrin).
    • Dinoflagellates & diatoms – broad ranges depending on pigments.
  • Graph highlights open-ocean vs. coastal attenuation to (~200 \text{ m}).

Endosymbiosis & Plastid Evolution

  • Plastids = double-membrane organelles with shared genome (chloroplast, chromoplast, leucoplast, etc.).
  • Primary endosymbiosis: heterotrophic eukaryote engulfed cyanobacterium → ancestral red & green algae (and glaucophytes).
    • Some descendant lineages lost 1 membrane (now two total).
  • Secondary endosymbiosis: other eukaryotes engulfed a red/green alga → more complex plastids (3–4 membranes) in:
    • Stramenopiles
    • Alveolates
    • Euglenids (green‐derived)
    • Chlorarachniophytes (green‐derived; nucleomorph relic of algal nucleus).

Major Photosynthetic Protist Groups

Excavata → Euglenozoans

  • Morphology: 1–2 flagella; pellicle for flexibility.
  • Nutrition spectrum: predatory heterotrophs, parasitic forms (e.g.
    Trypanosoma → sleeping sickness), mixotrophs (euglenids).
  • Mixotrophic strategy: photosynthesize in light, absorb/ingest organics in dark.
  • Habitat: ponds, lakes, wetlands.

SAR Clade

Alveolates → Dinoflagellates

  • Morphology:
    • Cellulose “armor” of 2 plates.
    • Two perpendicular flagella in grooves → characteristic whirling motion (“dinos”).
    • Nucleus with permanently crystalline chromosomes (large DNA content).
  • Pigments: carotenoids (often red/pink).
  • Nutritional modes: many mixotrophs; some purely heterotrophic.
  • Ecological roles:
    • Constitute ~90 % of marine plankton in certain regions.
    • Together with diatoms supply up to 50 % of global primary production.
    • Base of marine food webs (→ consumed by zooplankton).
  • Harmful Algal Blooms (HABs):
    • “Red tides” = explosive dinoflagellate growth; some species produce neurotoxins → paralytic shellfish poisoning (PSP).
  • Bioluminescence:
    • Flash emission when mechanically stimulated (anti-predator deterrent?).

Stramenopiles → Diatoms (Bacillariophyta)

  • Pigments: chlorophylls (a,c) + carotenoids (fucoxanthin gives golden color).
  • Cell wall = silica frustule (two valves, petri-dish overlap) → withstands high pressure; excellent fossils.
  • Forms: solitary or colonial; inhabit freshwater, marine, damp soils.
  • Diversity: >100{,}000 species (many undescribed) → arguably most species-rich protists.
  • Environmental importance:
    • Provide O$_2$ for ~every 4th breath (≈45 % of oceanic primary production).
    • Biological carbon pump: sinking frustules export organic C to sediments.
    • Paleoclimatology: diatom assemblages reconstruct past lake/ocean conditions.
    • Commercial: diatomaceous earth (filtration, abrasives, insecticide).

Stramenopiles → Golden Algae (Chrysophyta)

  • Pigments: chlorophylls (a,c) + carotenoids (golden hue).
  • Nutrition: phototrophs OR mixotrophs; become predatory on bacteria/diatoms under low light or high dissolved organics.
  • Morphology: typically biflagellate; some filamentous/colonial.
  • Adaptations: lipid storage (buoyancy); silica cysts enable decades-long dormancy—used for paleoenvironment reconstructions.

Stramenopiles → Brown Algae (Phaeophyta)

  • Pigments: chlorophyll (a), (\beta)-carotene, high fucoxanthin.
  • 99 % marine; body plans from filaments to giant kelps (>70 \text{ m}).
  • Cell wall: cellulose + alginic acid (a mucopolysaccharide).
  • Tissue differentiation (kelps):
    • Holdfast (attachment), stipe (support), blade (photosynthesis).
    • Internal conducting tissue parallels evolutionary convergence with land-plant vasculature.
  • Ecological role: kelp forests (e.g. Pacific NW) create 3-D habitat; rapid growth (up to 60 \text{ cm day}^{-1}).
  • Human uses: alginates (food thickener, dental molds, textile printing, fertilizers; brown-alga extract sprays).

Archaeplastida

Red Algae (Rhodophyta)

  • Ancient multicellular lineage; some of the oldest non-bacterial photo-organisms.
  • Pigments: phycoerythrin (red), phycocyanin (blue), + chlorophyll (a).
  • Stress tolerance: intertidal Porphyra withstands temperature swings, UV, salt, desiccation.
  • Coralline red algae deposit CaCO$_3$ → reef builders; contribute to coral framework & bone grafts.
  • Commercial uses:
    • Polysaccharides agar & carrageenan (thickeners: yogurt, chocolate milk, vegan candies).
    • Food crop “nori” (Porphyra) – highest-value marine crop worldwide.
    • Agar plates for microbiology; coralline skeletons for bone replacement.

Green Algae (Chlorophyta + Charophytes)

  • Pigments identical ratios to higher plants (chlorophylls (a,b), carotene, xanthophyll).
  • Cell features: central vacuole, plastids, two-layered cellulose/pectin wall; starch storage.
  • Morphological diversity: unicells, motile/immotile; filaments; sheets; giant coenocytes.
  • Habitats: predominantly freshwater (also marine & terrestrial crusts); form scums, biofilms, plankton.
  • Ecological functions:
    • Key primary producers & O$_2$ source; nutrient uptake → influence water quality.
    • Foundation of many freshwater food webs.
  • Evolutionary importance: charophyte relatives gave rise to land plants (see Lectures 12–16).

Applications, Implications & Human Relevance

  • Carbon sequestration: diatom frustule sinking; kelp farming proposals for climate mitigation.
  • Food & feed: nori, kelp snacks, algal oil (ω-3 fatty acids), animal & aquaculture feed.
  • Biotechnology & energy:
    • Algal biofuels (lipid-rich microalgae) – “AI + Algae = Future Energy Revolution or Hype?”.
    • Pigments & antioxidants (e.g. astaxanthin from microalgae) for nutraceuticals.
  • Environmental monitoring: diatom and algal assemblages as water-quality bioindicators.
  • Ethical/practical considerations: HABs affect fisheries & public health; climate change alters phytoplankton community structure.

Quick Quiz (Answers in parentheses)

  1. Algae are an evolutionary group derived from a single photosynthetic common ancestor. → False (polyphyletic; term is functional).
  2. Light-capturing pigments shared between some “algae” and land plants → Chlorophyll a and b (Correct option: “Chlorophyll a and b”).

Connections to Prior & Future Lectures

  • Builds on Campbell Biology Ch. 10 (light reactions & pigments) and Ch. 28 (endosymbiosis, eukaryote evolution).
  • Sets stage for Lectures 12-16 on land-plant diversity (derived from green algal ancestors).

The Future Is Algae

  • Vision of algae in sustainable energy, AI-optimized bioreactors, carbon capture, and novel biomaterials – active research frontier.