Chlorophyta

Chlorophyta (from the Ancient Greek chloros, "green," and phyton, "plant") is the phylum of core green algae comprising approximately 8,000 described extant species of marine, freshwater, and terrestrial photosynthetic eukaryotes. Together with its sister lineage Streptophyta—which unites the charophycean freshwater green algae (Charophyta: Chara, Klebsormidium, Spirogyra) and all terrestrial land plants (Embryophyta)—Chlorophyta forms the subkingdom Viridiplantae ("Green Plants") within the primary-plastid supergroup Archaeplastida.

Diverging from Streptophyta during the Late Mesoproterozoic–Early Neoproterozoic (~1,000 to 900 million years ago), Chlorophyta encompasses an extraordinary range of cellular architectures: from the world's smallest free-living eukaryote—the 0.8-micrometer marine picoprasinophyte Ostreococcus tauri—and biflagellate swimming unicells (Chlamydomonas reinhardtii) to spherical multicellular colonies (Volvox carteri), sheet-like marine sea lettuces (Ulva), terrestrial lichen photobionts (Trebouxia), and macroscopic, meter-long single-celled multinucleate siphonous seaweeds (Caulerpa taxifolia and the "mermaid's wineglass" Acetabularia).

All members of Viridiplantae (both Chlorophyta and Streptophyta) share three fundamental plastid innovations that distinguish them from red algae (Rhodophyta):

1. Chlorophylls a and b with Stacked Thylakoid Grana: Having lost cyanobacterial phycobilisomes, green-plant chloroplasts (bounded by two membranes) harvest red and blue light using chlorophyll a and accessory chlorophyll b bound inside membrane-intrinsic light-harvesting complex (LHC) proteins alongside β-carotene, lutein, violaxanthin, and neoxanthin, with thylakoids stacked into appressed thylakoid disks.
2. Intraplastidial True Starch Storage (Amylose + Amylopectin): Unlike red algae and glaucophytes (which synthesize starch in the cytosol), green algae and land plants relocated ADP-glucose pyrophosphorylase and starch synthases inside the chloroplast stroma, depositing semicrystalline granules of true starch (α-1,4-linked amylose and α-1,6-branched amylopectin) directly around the pyrenoid or between thylakoids.
3. Cytokinetic Divergence: The Phycoplast (Chlorophyta) vs. The Phragmoplast (Streptophyta): How do cytologists distinguish the core Chlorophyta clade from the Streptophyta clade that gave rise to land plants? By the orientation of their mitotic spindle and cytokinetic microtubules at cell division (Pickett-Heaps & Marchant, 1972):
   • In advanced Streptophyta (Charophyceae, Coleochaetophyceae, Zygnematophyceae, and all land plants), the mitotic spindle persists at telophase to form a phragmoplast—an array of microtubules oriented perpendicular (parallel to the spindle axis) to the plane of cell division, guiding Golgi vesicles to build a centrifugal cell plate from the inside out (accompanied by plasmodesmata and peroxisomal glycolate oxidase photorespiration).
   • In the crown UTC clade of Chlorophyta (Chlorophyceae and Trebouxiophyceae), the mitotic spindle collapses immediately after nuclear division (often inside an intact nuclear envelope, closed mitosis) and is replaced by a phycoplast—a flat system of microtubules oriented parallel to the plane of cell division between the two daughter nuclei, guiding cleavage-furrow ingression or cell-plate vesicle fusion!

1. Liquid–Liquid Phase Separation of the RuBisCO Pyrenoid (Chlamydomonas reinhardtii): Because aquatic CO₂ diffuses 10,000 times more slowly in water than in air and RuBisCO has a slow catalytic turnover and poor specificity for CO₂ over O₂, green algae evolved a biophysical CO₂-Concentrating Mechanism (CCM) centered on a dense microcompartment inside the chloroplast called the pyrenoid. For over a century, cell biologists thought the pyrenoid was a rigid crystalline protein solid. In 2016–2017, Martin C. Jonikas, Elizabeth S. Freeman Rosenzweig, Benjamin D. Engel, Luke Mackinder, and colleagues discovered that the Chlamydomonas pyrenoid is actually a liquid–liquid phase-separated (LLPS) biomolecular condensate! An intrinsically disordered repeat protein called EPYC1 (Essential Pyrenoid Component 1) bears five multivalent binding motifs that cross-link RuBisCO holoenzymes into a liquid droplet traversed by carbonic-anhydrase-containing thylakoid tubules (which pump concentrated CO₂ straight into the droplet), dissolving and re-condensing like an oil droplet during every cell division!
2. Channelrhodopsins (ChR1 and ChR2) and the Birth of Optogenetics: How does a single-celled Chlamydomonas reinhardtii swim toward dim light (positive phototaxis) and away from blinding sunlight (photoshock)? Inside an orange carotenoid-rich patch at the equator of the chloroplast (the eyespot / stigma), the overlying plasma membrane contains two seven-transmembrane retinal-binding proteins discovered and characterized electrophysiologically in 2002–2003 by Georg Nagel, Peter Hegemann, and Ernst Bamberg: Channelrhodopsin-1 (ChR1) and Channelrhodopsin-2 (ChR2)—the first known light-gated cation channels in nature! When blue light (~470 nm) isomerizes all-trans-retinal inside ChR2, the channel pore opens within <50 microseconds to conduct Na⁺, H⁺, and Ca²⁺ ions across the membrane, depolarizing the algal cell and steering its two flagella. In 2005, Karl Deisseroth, Edward S. Boyden, Feng Zhang, Nagel, and Bamberg expressed Chlamydomonas ChR2 in mammalian neurons, allowing millisecond-precision control of action potentials with fiber-optic blue light and launching the field of optogenetics!
3. The Volvocine Evolutionary Transition from Unicell (Chlamydomonas) to Germ–Soma Multicellularity (Volvox): Within a single green-algal order (Chlamydomonadales / Volvocales), living species preserve a complete, 200-million-year step-by-step evolutionary series from a single cell to differentiated multicellularity with mortal somatic cells and immortal germ cells:
   • Unicellular Chlamydomonas reinhardtii (1 cell) → Gonium pectorale (flat plate of 8 or 16 identical cells) → Pandorina morum (16–32-celled morula ball) → Eudorina elegans (32–64 cells in an extracellular matrix sphere) → Pleodorina californica (64–128 cells with partial anterior somatic specialization) → Volvox carteri (a hollow sphere of 2,000 to 4,000 small, terminally differentiated biflagellate somatic cells on the outside that swim and inevitably die via programmed senescence, enclosing ~16 large, non-flagellated immortal reproductive germ cells [gonidia] on the inside!).
   • Comparative genomics of Chlamydomonas, Gonium, and Volvox (Prochnik et al., 2010; Hanschen et al., 2016) revealed that evolving multicellular Volvox required almost no new protein-coding domains—only expansion of pherophorin extracellular matrix glycoproteins, cyclin D1 cell-cycle genes, and the master transcription factor regA, which suppresses chloroplast biogenesis in small cells (<8 μm) after asymmetric cleavage (controlled by glsA) to lock them into a mortal somatic fate!

How can the invasive "killer alga" Caulerpa taxifolia grow leaves, stems, and roots up to a meter long inside a single cell without dividing?

When you look at a frond of the marine ulvophycean green alga Caulerpa taxifolia or Caulerpa prolifera, you see a horizontal stolon ("runner"), downward-growing rhizoids ("roots"), and upright flattened blades ("leaves") measuring 30 cm to >1 meter in length. Yet if you examine the entire plant under a microscope, there are zero internal cell walls—the entire organism is a single giant multinucleate cell (siphonous coenocyte) containing millions of nuclei and chloroplasts streaming through a continuous cytoplasm braced against turgor pressure by internal cross-beams of cell-wall polysaccharides (trabeculae)! In 2014, spatial transcriptomics by Daniel H. Chitwood, Neelima R. Sinha, and colleagues demonstrated that Caulerpa establishes distinct apical-to-basal mRNA transcript gradients (expressing root-like, stem-like, and leaf-like homologous transcription factors in different regions of the same shared cytoplasm!), proving that complex plant organ morphology can arise entirely through intracellular sub-compartmentalization independent of multicellularity.

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5. Chitwood, D. H., Ranjan, A., Kumar, R., Ichihashi, Y., Zumstein, K., Headland, L. R., ... & Sinha, N. R. (2014). Resolving distinct molecular signatures for organ-like subregions of the giant single-celled Caulerpa taxifolia. PLOS Genetics, 11(1), e1004900. doi:10.1371/journal.pgen.1004900
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