i'm writing up my final exam for my plant ecology class and i just accidentally found out isoetes is one of the few aquatic plants that do cam photosynthesis and i'm losing my damn mind
you know it!!!!!! i love that about them tbh. i think i’ve talked about this before on here but i’m gonna talk about it again because this ask reminded me of it and oh man..... its so wild
so, some basic background to this, there are multiple different kinds of photosynthesis, with the main ones being the normal kind, C4, and CAM. these exist because to take in CO2 for photosynthesis, plants need to open pores in their leaves called stomata...which also means that as long as they’re open, they can lose water through the same pores. this isn’t a big deal if you’re a plant in a shady/not too humid environment, but in hot/water scarce environments this can be a big deal. C4 photosynthesis is what corn actually does, and it gets around this problem by having two ‘compartments’ for photosynthesis instead of one; this is a whole other thing that doesn’t have anything to do with this story. CAM photosynthesis on the other hand is primarily in desert plants like cacti and succulents, and gets around it by just opening the pores at night and taking in CO2 when the temperatures are lower, then closing the pores during the day to actually process the CO2 while temperatures are higher.
aquatic members of the notorious ancient seedless plant family Isoetes are also plants that do CAM photosynthesis. i will again emphasize the fact that like 90% of Isoetes species are aquatic, which is like.... literally the direct opposite of why CAM is a thing to begin with, or so that was the impression up until the 1980s. the thinking was that CAM was only used by desert plants, and it evolved specifically to subvert the whole water scarcity problem in the environment, and to be fair this is still true for most of the plants it happens in, but like...not always and not always for the same reasons, lmao.
the whole ‘Isoetes doing CAM’ thing was discovered accidentally by a plant researcher named Jon Keeley who was demonstrating a kind of metabolic pathway to a plant physiology class. they were doing an experiment where he had the students collect a bunch of plants from a local vernal pool ecosystem, and then they’d be able to measure their respiration and demonstrate function of the pathway to compare it to local aquatic animal species, which worked fine until they tested the local Isoetes species (Isoetes howellii) and it was just vibing doing CAM while submerged in water, which is like...you know...bizarre, on first inspection.
fast forward several years. Keeley has gotten funding to test this and has gotten enough data to prove that this plant was, indeed, VERY CLEARLY doing CAM photosynthesis underwater. he submits the manuscript to Science, one of the most prominent academic journals in the world, and it gets rejected because half the editors don’t believe it. like, they took one look at it, decided he was a nutjob who didn’t know the first thing about photosynthesis, and just got rid of it because there was OBVIOUSLY no way an aquatic plant could do CAM, just because there was no reason for it to happen.
but. it turns out that there is a reason for it to happen, and like all things relating to Isoetes it’s like, a prehistoric hindbrain sort of thinking. as it turns out it’s not for survival so much as they use it for convenience, because aquatic plants get their CO2 from the surrounding water....but the CO2 dissolves in the water at night and burns off into the air during the day as temperatures go up. so like, it’s not necessary, but there’s nothing saying they HAVE to have their stomata open during the day when there’s no CO2 in the water to collect. therefore, aquatic Isoetes collect CO2 at night and actually process it into sugars during the day, which is CAM photosynthesis, just like....in a completely different context, for different reasons. it makes a lot of sense.
so. dude submits it to Nature, the other most prominent academic journal in the world, and it also gets rejected from publication before even being reviewed. it ends up being published in The American Journal of Botany, which is a major journal in the field, but one of the writers at Nature believes him and thinks it’s significant enough to give it a mention in the news section of an edition, and thus the academic community was introduced to the idea of aquatic plants doing CAM.
so....several decades down the line we know that CAM photosynthesis is actually present in all aquatic Isoetes species, while no known terrestrial Isoetes species do it, which is gloriously ironic; the current idea is that it evolved way back when the ancestors of modern aquatic Isoetes were all still vibing in shallow prehistoric swamp pools, after the terrestrial species made it to land, and it just stayed in the aquatics as they diversified. it also has turned out to be present in several other aquatic plant families for the same reasons, so like, in short......once again Isoetes proves to Just Be Like That, You Know?
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