lichens and photobionts

 what if there was a variety of mushrooms that also plants? thats lichens: a tube of fungus, filled with chloroplasts in the form of either green algae or cyanobacteria (or both). critically, this changes the typical fungus strategy from decomposer to primary producer - they can simply sit on a rock and live off sunlight, just like a plant!

they are essentially the 'ultra-plant'. lichens compose the majority of biomass in a variety of habitats (like Sequoia groves) for this reason. they are very successful, can resist drying out, so can colonize almost any habitat or surface. 

what came first? cyanobacteria! (which came from deep sea thermophilic bacteria)

cyanobacteria, the 'first plants', invented chlorophyll. fungus probably evolved next, separately, as a marine organism. fungus would become terrestrial and steal the cyanobacterial cells, creating lichens. and, proto-algae (eukaryotic protists probably) would steal them too via endosymbiosis, creating chloroplasts and therefore algae. algae evolved and gave off true plants.

fungi see these photobionts (cyanobacteria and/or algae) as simply gelatinous balls dripping sugar syruip when exposed to light. so, fungi wrap their arms around them and absorb up all that sweet sugar. 

the fungus seems to remain in control, though, so lichens generally just do fungus things. they crawl outwards and fruit with caps (usually cup-shaped apothecia) and spread out spores. the photobionts give up 80% of their sugar production to the fungus, and allow the fungus to penetrate them with special hyphae, so they do seem like slaves. crucial, however, since that same fungus isolated in culture without its partner would simply grow as an undifferentiated mass of hyphae. 

can fungus copy that proto-algae, and themselves engulf a photobiont via endosymbiosis? yes! but its only happened once: Geosiphon pyriformis, the only member of its genus, a fungus whose cells have engulfed Nostoc cyanobacteria cells, creating a 'chloroplast' again. its not a lichen, because the symbiosis is intracellular, not extracellular. fungal-bacterial endosymbiosis has occurred a few other times as well, but apparently never involving chlorophyll/photosynthesis. so, if Geosiphon eventually internalizes the Nostoc genome into its own nucleus, it would become a new type of organism. 

mycorrhizae are fungi that are symbiotes with plants. there is a plant analogue to lichens: plants will wrap themselves (their roots, specifically) around fungal hyphae, surrounding them and feeding off each other - the ectomycorrhizae. even more common are the endomycorrhizae - fungi that dont just wrap gently around, but instead penetrate and invaginate into the plant cells - an intracellular endosymbiosis akin to Geosiphon, except in plants they are much more common: ~85% of all plant families have had endomyco's found, compared to simply 1 species of the fungal-algal endosymbiosis.