Algae

"Algae" is an informal, polyphyletic term for eukaryotic microbes (and, loosely, cyanobacteria) that carry out oxygenic photosynthesis — splitting water, releasing O₂, and fixing CO₂ into organic carbon via the Calvin cycle — but that lack the true roots, stems, leaves, and vascular tissue of land plants. Because the label is defined by function rather than phylogeny, algae are scattered across the eukaryotic tree: green algae and land plants form one clade (Archaeplastida), red algae belong to that same clade, but diatoms, dinoflagellates, and brown algae belong to entirely separate eukaryotic super-groups (Stramenopiles and Alveolates) that acquired photosynthesis independently by engulfing already-photosynthetic eukaryotes. This is the central twist that distinguishes algal biology from plant biology: in plants there was one plastid-founding event, but in the algal world plastids have been passed sideways between unrelated lineages multiple times. Two organizing axes make sense of algal diversity. The first is pigment composition — which chlorophylls, phycobiliproteins, and carotenoids a lineage uses, which determines the color of the organism and the wavelengths of light it can exploit. The second is plastid membrane topology — how many membranes surround the chloroplast, which is a direct readout of how many rounds of endosymbiosis produced it. A chloroplast with two membranes reflects a primary endosymbiosis (host membrane + the symbiont's own outer membrane, with the symbiont's plasma membrane retained as the innermost). A chloroplast with three or four membranes reflects a secondary (or tertiary) endosymbiosis, in which a photosynthetic eukaryote was itself engulfed by another eukaryote, and remnants of the engulfed cell's own membranes were retained. These two axes are not independent: pigment identity often travels with plastid origin. Green algae (Chlorophyta) use chlorophylls a and b, like land plants, because both derive from the same primary endosymbiotic lineage. Red algae (Rhodophyta) use chlorophyll a plus phycobiliproteins (phycoerythrin, phycocyanin) organized in phycobilisomes, an inheritance from their cyanobacterial ancestor that never lost these accessory pigments. Diatoms, brown algae, and dinoflagellates — descended from a secondary endosymbiosis with a red alga — use chlorophylls a and c plus fucoxanthin, a carotenoid that gives them their characteristic golden-brown color, without phycobilisomes. This layered inheritance is why an algal biologist can often guess a lineage's evolutionary history just by looking at its pigmentation and counting plastid membranes under an electron microscope.

Step 1 — Light capture in the thylakoid membrane. Inside the chloroplast, thylakoid membranes house two photosystems, Photosystem II (PSII) and Photosystem I (PSI), arranged in an electron transport chain analogous to (and evolutionarily continuous with) the one in cyanobacteria. Antenna pigments — chlorophylls and accessory pigments specific to each algal lineage — absorb photons and funnel excitation energy to reaction-center chlorophylls.

Step 2 — Water splitting and electron flow. At PSII, the oxygen-evolving complex (a Mn₄CaO₅ cluster) strips electrons from water: 2 H₂O → O₂ + 4 H⁺ + 4 e⁻ Electrons flow from PSII through plastoquinone, the cytochrome b₆f complex, and plastocyanin (or cytochrome c₆ in some algae) to PSI, where re-energized electrons reduce ferredoxin and ultimately NADP⁺ to NADPH. Proton accumulation in the thylakoid lumen drives ATP synthase, generating ATP by chemiosmosis — the same chemiosmotic logic taught for mitochondrial respiration, run here in reverse polarity relative to the stroma.

Step 3 — Carbon fixation in the stroma (Calvin cycle). ATP and NADPH power RuBisCO-catalyzed fixation of CO₂ onto ribulose-1,5-bisphosphate, producing two 3-phosphoglycerate molecules, which are reduced and cycled to regenerate RuBP and export triose phosphates for biomass and storage-carbohydrate synthesis. Many algae additionally run a carbon-concentrating mechanism — active bicarbonate transport and pyrenoid-localized carbonic anhydrase — to overcome RuBisCO's poor affinity for CO₂ in water, where diffusion is roughly 10,000-fold slower than in air.

Step 4 — Pigment tuning to the underwater light field. Water absorbs red and infrared light rapidly, so at depth, light becomes progressively blue-green. Chlorophyll a alone absorbs poorly in this window (its main peaks are ~430 nm and ~662 nm). Accessory pigments extend usable wavelengths: phycoerythrin in red algae absorbs strongly around 490–570 nm (green light), letting red algae live at greater depths than green algae; fucoxanthin in diatoms and brown algae absorbs blue-green light (~510–525 nm) efficiently, suiting them to turbid coastal and planktonic environments. This is why intertidal zonation on a rocky shore often runs, from shallow to deep, green algae → brown algae → red algae — a rough but real correlation between pigment absorption range and depth tolerance, first rationalized as the "complementary chromatic adaptation" hypothesis.

Step 5 — Endosymbiotic plastid assembly and protein targeting. A secondary plastid (as in diatoms) is bounded by four membranes: the innermost pair are homologous to a primary chloroplast's envelope, the next is a vestige of the engulfed alga's plasma membrane, and the outermost is derived from the host's phagosomal/endomembrane system, often continuous with the host's endoplasmic reticulum (giving the "chloroplast ER"). Because most of the original endosymbiont's genes were lost or transferred to the host nucleus, plastid-targeted proteins are synthesized on host ribosomes and must cross up to four membranes, guided by bipartite N-terminal signal sequences (a signal peptide for ER entry, followed by a transit peptide for the remaining membranes) — a trafficking puzzle that itself is strong indirect evidence for the multi-membrane, multi-engulfment history.

Endosymbiotic theory's core proofs. The endosymbiotic origin of chloroplasts (and mitochondria) was proposed in embryonic form by Konstantin Mereschkowski (1905) and revived with rigorous molecular argument by Lynn Margulis in her 1967 paper "On the Origin of Mitosing Cells." Margulis was rejected by roughly fifteen journals before the paper was accepted, largely because the idea — that eukaryotic organelles were once free-living bacteria — seemed radical against the era's assumption that organelles arose by gradual internal differentiation. The decisive evidence accumulated over the following two decades: 1. Plastid DNA. Chloroplasts were shown to contain their own circular, bacterial-style DNA, distinct from the nuclear genome, first demonstrated biochemically in the 1960s (Ris and Plaut showed DNA fibrils in Chlamydomonas chloroplasts by electron microscopy in 1962). 2. Plastid ribosomes. Chloroplast ribosomes are the 70S bacterial type, sensitive to antibiotics like streptomycin and chloramphenicol that block bacterial but not eukaryotic cytoplasmic (80S) ribosomes — a functional fingerprint of bacterial ancestry. 3. Membrane topology. Electron microscopy consistently revealed a double membrane around primary plastids, exactly what is predicted if a Gram-negative-style bacterium (with its own outer and inner membrane) was engulfed by phagocytosis, retaining the host's phagosomal membrane as a third boundary that was later lost in the primary lineage, leaving two. 4. Molecular phylogeny. When 16S rRNA gene sequences from chloroplasts were compared to rRNA from free-living cyanobacteria and to nuclear-encoded eukaryotic genes, chloroplast rRNA consistently grouped with cyanobacteria, not with the host nuclear lineage — a result reproduced across hundreds of algal and plant species since the 1970s–80s using Carl Woese's rRNA-based phylogenetic methods (the same methods that established the three-domain tree). 5. Ongoing secondary endosymbiosis caught in the act. Perhaps the most vivid direct evidence comes from nucleomorphs — tiny, vestigial second nuclei found in cryptophyte and chlorarachniophyte algae, discovered via electron microscopy in the 1970s–80s and sequenced in the 1990s. A nucleomorph is the shrunken, relict nucleus of the engulfed alga itself, still transcribing a handful of genes needed for plastid maintenance, sandwiched between the extra plastid membranes. Its existence is essentially a fossil endosymbiont caught mid-reduction, directly confirming that secondary plastids came from whole engulfed eukaryotic cells rather than from lateral gene transfer alone.

Complementary chromatic adaptation. The idea that algal pigment composition is tuned to the light available at depth traces to careful spectrophotometric and depth-distribution surveys in the mid-20th century, later reinforced by action-spectrum experiments: measuring the rate of oxygen evolution (via a Clark electrode or manometric method, following the logic pioneered by Engelmann's 1882 bacterial-motility photosynthesis assay) as a function of wavelength shows that red algae fix carbon efficiently under green light exactly where phycoerythrin absorbs strongly, while green algae do not. Thomas Engelmann's original 1882 experiment — projecting a light spectrum across a filament of the alga Cladophora and tracking oxygen-seeking motile bacteria that clustered specifically near the chlorophyll absorption bands — was the first direct visual demonstration that photosynthetic oxygen output tracks pigment absorption, a logic still used (with modern oxygen electrodes) to map action spectra in different algal groups today.

Primary production quantification. The claim that microscopic algae rival land plants in global photosynthesis rests on satellite ocean-color measurements (chlorophyll concentration inferred from reflected light, calibrated against shipboard ¹⁴C-uptake incubations) combined with models of light and nutrient availability. Longhurst et al. (1995) and later refinements estimated marine net primary production at roughly 45–50 gigatonnes of carbon per year, comparable to terrestrial NPP (~56–60 Gt C/yr), despite phytoplankton constituting less than 1% of Earth's photosynthetic biomass — a striking mismatch between standing biomass and productivity explained by phytoplankton's extremely fast turnover (days, versus years to decades for forests).

Example 1 — Quantifying phytoplankton turnover versus a forest. Suppose a patch of ocean has a standing phytoplankton biomass of 1 g carbon/m² and a net primary productivity of 200 g C/m²/year. A nearby temperate forest has a standing biomass of 20,000 g C/m² (20 kg/m², a modest but realistic value for aboveground + some root biomass) and a similar net primary productivity of 1,000 g C/m²/year. Turnover time = biomass ÷ productivity. Phytoplankton: 1 g C/m² ÷ 200 g C/m²/yr = 0.005 yr ≈ 1.8 days Forest: 20,000 g C/m² ÷ 1,000 g C/m²/yr = 20 years Even though the forest fixes only about 5 times more carbon per square meter per year, its biomass turns over roughly 4,000 times more slowly. This is why oceanic phytoplankton can rival terrestrial vegetation in annual carbon fixation while holding a tiny fraction of the standing carbon stock — reproductive rate and cell turnover, not standing crop, drive their global significance. It also explains why a bloom of algae can appear and vanish within a single week, tracking nutrient pulses far faster than any forest could respond to a rain event.

Example 2 — A life-cycle genetics problem: isomorphic alternation of generations in a green alga. Consider a hypothetical green alga, similar in life cycle to Ulva, that alternates between a haploid gametophyte and a diploid sporophyte, both morphologically identical (isomorphic alternation of generations) — distinct from the land-plant pattern where sporophyte and gametophyte look very different. Suppose a gene for flagellar length has two alleles: L (long flagella, dominant) and l (short flagella, recessive), and this gene is expressed in the haploid gametophyte stage (where gametes are produced and the phenotype is directly observable, since there's no diploid masking in haploid cells). A diploid sporophyte is heterozygous Ll (having arisen from fusion of an L gamete and an l gamete). This sporophyte undergoes meiosis to produce haploid spores.

• Meiosis of an Ll sporophyte, absent linkage complications (single gene, no crossover issue since it's the only locus considered), yields spores in a 1:1 ratio: half L, half l.
• Each spore germinates directly into a haploid gametophyte expressing its own allele: L spores make long-flagellum gametophytes, l spores make short-flagellum gametophytes.
• Observed phenotypic ratio among the gametophyte generation: 1 long-flagellum : 1 short-flagellum — not the 3:1 a student might reflexively expect from "dominant/recessive" pattern-matching to a diploid F2 cross.

The key teaching point: dominance/recessiveness is a relationship between alleles in a diploid cell; in the haploid gametophyte generation, each spore's single allele is simply and fully expressed, so meiotic segregation ratios (1:1) — not Mendelian F2 ratios (3:1) — govern the phenotype distribution. This distinction becomes essential once gametophytes of each mating type fuse: if a long-flagellum gametophyte (L gamete) fuses with a short-flagellum gametophyte (l gamete), the resulting diploid zygote/sporophyte is again Ll, heterozygous, and — because L is dominant — would show the long-flagellum phenotype if the sporophyte itself expressed the gene, illustrating how the same allele pair can produce different apparent ratios depending on which generation (haploid vs. diploid) is scored.

Lynn Margulis (1938–2011) trained initially in genetics at the University of Chicago and Wisconsin, but her defining contribution came from synthesizing decades of scattered microbiological and cytological observations — bacterial-type ribosomes in chloroplasts, circular organellar DNA, membrane topology — into a single, falsifiable evolutionary framework: that mitochondria and chloroplasts originated as free-living bacteria engulfed by ancestral eukaryotic cells and retained as permanent, heritable endosymbionts. Her 1967 paper was rejected repeatedly before publication in the Journal of Theoretical Biology, partly because serial endosymbiotic theory required biologists to accept that some of the most fundamental "parts" of a eukaryotic cell were, in a real evolutionary sense, formerly separate organisms — a claim that ran against a prevailing gradualist, autogenous view of organelle origin. Margulis persisted through the 1970s and 1980s as molecular evidence — rRNA sequence comparisons enabled by Carl Woese's methods, direct plastid genome sequencing — repeatedly and independently confirmed her predictions, turning what had been a fringe hypothesis into standard textbook fact by the 1980s.

Climate and carbon cycling. Marine phytoplankton, dominated by diatoms, coccolithophores, and cyanobacteria, drive the biological carbon pump: carbon fixed at the surface sinks as dead cells, fecal pellets, and marine snow, sequestering carbon in deep water and sediment on timescales of centuries to millions of years. Coccolithophore calcite shells and diatom silica frustules also form the chalk and diatomaceous earth deposits used today in everything from toothpaste abrasives to industrial filtration, and their fossil records (recovered via ocean drilling cores) provide high-resolution paleoclimate archives.

Biotechnology and biofuel. Microalgae such as Chlorella and Nannochloropsis are cultivated in photobioreactors for lipid-rich biomass that can be converted to biodiesel, and engineered strains are explored as chassis for producing pigments (astaxanthin, phycocyanin as a natural food dye), omega-3 fatty acids, and recombinant proteins, exploiting the same core photosynthetic machinery described above.

Public health and harmful algal blooms (HABs). Certain dinoflagellates (e.g., Karenia brevis, Alexandrium species) and some diatoms produce potent neurotoxins (brevetoxin, saxitoxin, domoic acid) that accumulate in filter-feeding shellfish, causing paralytic or amnesic shellfish poisoning in humans; monitoring programs rely directly on the pigment- and morphology-based identification methods rooted in classical algal taxonomy, feeding forward into Microbial Ecology and the Microbiome as blooms reshape entire coastal microbial communities.

Aquaculture and agriculture. Algae are cultivated as feed for shellfish and fish larvae, and as biofertilizers and soil conditioners (particularly nitrogen-fixing cyanobacterial-algal associations in rice paddies). Symbiotic algae (zooxanthellae, themselves dinoflagellates) sustain coral reef ecosystems, and their thermally induced expulsion is the direct mechanistic cause of coral bleaching, linking single-cell photosynthetic physiology to whole-ecosystem collapse under ocean warming.

Ask the tutor: How does the number of membranes around a plastid let biologists reconstruct an algal lineage's endosymbiotic history?