lecture 4

• Algae


  • Photosynthetic organisms that are not land plants
• Algae are not a monophyletic group, they are found across diverse groups of the tree of life
• All algae live in aqueous environments
• How did photosynthesis spread across such diverse groups of algae?
• Monophyletic group


  A subset of the tree of life that contains a common ancestor and all its descendants
• Solely unicellular fungi are not a monophyletic group
• Ecological group


  A set of taxa that share common ways of life, often in a community context
• Ecological groups may or may not overlap with phylogenetic relatedness
• Primary endosymbiosis


  1. Cyanobacterium engulfed by heterotrophic eukaryote

  2. Plastid membranes formed
• Secondary endosymbiosis


  1. Heterotrophic eukaryote engulfs red or green alga

  2. Plastid membranes formed
• Algae groups we will focus on are: unicellular (cyanobacteria, diatoms, dinoflagellates) and multicellular (brown, red, green algae)
• Algae, especially unicellular ones, are huge contributors to global net primary productivity
• Diatoms


  • Unique glass-like cell walls made of silica
  • Contribute 25% of global net primary productivity
• Dinoflagellates


  • Have two flagella, one spiral
  • Some are heterotrophic, others are mixotrophs
  • Cause red tides and toxin production
  • Symbionts of corals (zooxanthellae)
• Brown algae


  • Marine algae including kelps
  • Keystone species of intertidal and deepwater
  • Can grow up to 200 ft long
  • Cell walls produce alginate used in food, cosmetics, and laboratories
• Red algae


  • Often red due to pigments tuned to absorb blue light
  • Diverse in color and form
  • Economically important (e.g. nori, dulse)
• Green algae


  • Closest relatives to the plant kingdom
  • Very diverse forms and life cycles from unicellular to multicellular
  • Found in freshwater, seawater, and high elevation snow fields
• Generalized sexual life cycles of algae


  1. Haplontic: Haploid unicellular/multicellular organism

  2. Diplontic: Diploid multicellular organism

  3. Haplodiplontic: Alternation of haploid and diploid multicellular stages
• The alternation of generations life cycle with two multicellular stages of alternating ploidy has evolved multiple times independently in red, brown, and green algae, as well as plants
• Life really plants a flag on land only ~500 mya => still pretty new all things considered
• Different groups arrive at land at different times
• Soil


  One key need for life to thrive on land
• Lichens


  Where soil initially comes from
• Lichens


  • Combo of at least one heterotrophic fungus (the mycobiont) with one phototrophic alga or cyanobacterium (the photobiont)
  • Both species take on a form completely distinct from how they would grow individually
  • Neither the mycobionts or the photobionts that lichenize are monophyletic
  • Capacity to lichenize has evolved many many times (20% of fungi lichenize)
  • Non-exclusive associations: fungi and algae or cyanobacteria usually can pair with multiple partners
• Photobiont


  Provides sugars (and fixed nitrogen if cyanobacterium)
• Mycobionts


  • Provides moisture, shelter, UV protection, and minerals obtained from dust or leached from substrate
  • Secrete acids like usnic acid
• Lichens


  • Pioneer species! Make soil by both physical and chemical weathering
• Hypothesis of lichens being a symbiosis was initially dismissed but then became dominant paradigm
• Even then, the story of lichens is not yet complete. 2016: Discovery that lichens often have basidiomycete yeasts as a third partner
• Lichen reproduction


  1. Together and asexually: fragmentation, soredia - bundle of fungi and algae

  2. Alone and sexually: fungi produce their own fruiting body
• Key terms


  • monophyletic group
  • ecological or functional group
  • primary / secondary endosymbiosis
  • haplodiplontic life cycle / alternation of generations
  • sporophyte / gametophyte
  • diatoms
  • dinoflagellates
  • brown / red / green algae
  • phycocolloid
  • lichen
  • photobiont / mycobiont
  • soredia