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Forms of Algae
  • Unicells: single cells, motile or nonmotile

  • Colonies: Assemblage of individual cells with variable or constant number of cells that remain constant throughout the colony life

  • Coenobium: Colony with constant number of cells, which cannot survive alone; specific „tasks“ among groups of cells is common
  • Filaments: daughter cells remain attached after cell division and form a cell chain; adjacent cells share cell wall (distinguish them from linear colonies!); maybe unbranched (uniseriate) or branched (mutiseriate)

  • Coenocytic or siphonaceaous forms: one large, multinucleate cell without cross walls
  • Parenchymatous and pseudoparenchymatous algae: mostly macro-scopic algae with tissue of undifferentiated cells and growth originating from a meristem with cell division in three dimensions; pseudoparenchymatous superficially ressemble parenchyma but are composed of appressed filaments

Phylogeny of Algae
  • Algae did not evolve from one single, common ancestor but evolved at different times from different ancestors. 
  • Algae are, therefore, not monophyletic
  • Some algae are closer related to heterotrophic flagellates or ciliates than to other algae
  • Prokaryotic phototrophs: Cyanobacteria („blue-green algae“)
  • Eukaryotic algae: possess a doublemembrane enclosed nucleus and ususal other membrane-enclosed organelles (mitochondria, chloroplasts)
  • Origin of organelles: symbiotic enclosure of free living bacteria or cyanobacteria, horizontal transfer of genes from endosymbionts to host nucleus

Nutrition of Algae
  • Photosynthesis is the primary mode of nutrition (C uptake as CO2)
  • Photosynthetic algae were mostly considered photoautotrophic
  • Numerous phototrophic algae can, however, take up dissolved organic matter or engulf bacteria and other algal cells as particulate prey; they are referred to as mixotrophic
  • Uptake of particulate prey = phagotrophy; Uptake of dissolved substances = osmotrophy
  • The degree of mixotrophie in chlorophyll-containing algae varies with environmental conditions (light, prey abundance)
  • Some chlorophyll-containing species can even lose their pigments when living heterotrophic
  • Most algal phyla/divisions contain mixo- or heterotrophic species
  • Some algae cannot synthesize essential vitamins (biotin, thiamine, cobalamin = B12) and have to import them: auxotrophic species