TLDR; While taking some time to flesh out the interactions between microbes in depositional environments further inland, I've developed some interesting ecological interactions. These might end up being applicable to both lake bed environments rich in organics, as well as the marine habitats I had discussed in the previous entry.
Now that I've finished fleshing out the broader ecological interactions in marine depositional habitats, I've had a moment to think. What sorts of behavioral and structural adaptations would these organisms have? Would they be distinct from microbes on Earth inhabiting similar environments, or would they experience extreme convergent evolution?
In order to answer this question, it's important to note that prokaryotes face severe energetic constraints due to their inability to effectively scale up respiration. Since there is an upper limit to how much energy these organisms could obtain, there are only so many structural modifications that they can invest in before maxing out their energetic carrying capacity.
This limited evolutionary flexibility often results in prokaryotes developing the same morphotypes over and over again, selecting for those that fall within their budget and provide clear ecological benefits (bacilli, cocci, spirilla, vibrio, etc.) Because of this, it's likely that our prokaryote analogues will be "pigeon-holed" into developing the same basic cell shapes when faced with similar environmental pressures. However, this isn't to say that there isn't room for contingency.
From time and time, we find cases in which structures and behaviors have arisen within a single prokaryotic lineage, only to have never reemerged again (and with no discernable reason). A good example of this are cannulae; hollow, proteinaceous appendages found in the genus Pyrodictium. These structures are incredibly durable, flexible, and heat-resistant.
Additionally, they permanently connect cells to their progeny, providing a network through which chemical signals and public goods could be exchanged, all without giving unrelated populations a competitive edge (This remains a speculative behavior for now, but there is a precedent for this idea, considering that cannulae are anchored directly to the periplasmic space.)
Given the role cannulae play in fortifying biofilms and (might) have in shuttling nutrients, it would seem that many organisms would benefit from developing structures with similar properties. However, as of the time this has been posted, no such instance of convergent evolution has ever been observed.
The same also goes for hami; tiny, grappling-hook like appendages that are commonly found amongst some groups of archaea. Despite being incredibly useful in allowing biofilms to resist strong currents, they have never been recapitulated in bacteria (At least, as far as we're aware of.)
As a result, we can infer that our alien microbes will exhibit many of the same cell morphologies and physiological adaptations as their terrestrial counterparts, but will also have unique structural and behavioral traits scattered throughout. With that said, I've been working on developing a few adaptations that would explore this theme of limited contingency.
However, before we jump in, I should offer a bit of a disclaimer. Trying to predict new behaviors and morphological adaptations for fictional microbes is a deceptively challenging process. Due to the strong pressures placed upon prokaryotes to make use of untapped resources and ecological associations within biofilms, there is a pretty high likely hood that whatever you come up already exists. It's also important to note that the absence of evidence is not the same proof of absence. In other words, just because you don't observe something you predicted, that doesn't mean it's not there.
The techniques used to study "microbial dark matter" and to model microbial ecology in the laboratory are still in their infancy. As a result, we're constantly learning new things all the time, even about organisms in well-studied environments.
Okay, with that out of the way, let's explore some speculative adaptations and ecological associations. In keeping with the theme of marine depositional environments, I'll be focusing on anoxic sediments in waters that lack euxinia.
As covered in the previous post, many of the microbes in these habitats have to navigate the difficult task of pairing the oxidation of sulfide to the reduction of nitrate. This means scaling several millimeters or centimeters of sediment in order to acquire both.
The most common way in which this is preformed on Earth and Leeuwenhoek is through physically migrating from one end of the redox gradient to the other. In doing so, cells collect a bit of sulfide and then oxidize it for energy upon reaching a source of nitrate. In many ways, this isn't too dissimilar from how whales will hunt for krill in deeper waters, before occasionally resurfacing to respire. However, much like whales filling up their lungs with air, many filamentous microorganisms are able to take a "gulp" of nitrate and store it within vacuoles for later use. This enables them to continue carrying out respiration in sulfidic sediments and to stockpile nitrate in the event the competition renders it difficult to come by.
As a result, organisms like Thioploca and Beggiatoa essentially operate as "little nitrate shuttles", gathering nitrate from the surface and bringing it down with them into sediments where electron acceptors are difficult to find. This offers smaller prokaryotes that are incapable of stockpiling nitrate the incentive to steal it through endobiosis (the act of living within another organism).
By boring into the host and adhering to the surface of the gas vacuole, the endobionts would be able to gain access to this highly sought after electron acceptor. Additionally, endobionts might start off as parasitic and later develop mutualistic endosymbiosis with their hosts. By supplying some sort of public good (such as the synthesis of amino acids), they could increase their hosts chances of survival and thereby increase their own reproductive fitness.
*NOTE : Of all of the speculative adaptations / associations listed here, this is admittedly the most unlikely to develop. However, non-motile sulfur oxidizers belonging to the genus Thiomargarita have been found to contain endobionts and it has been suggested that they might be there to harvest nitrate. However, little has been determined about the precise nature of this relationship, so it's still unclear as to whether or not they are after electron acceptors.
Alternatively, giant sulfide oxidizers could also serve as modes of transport for epibionts (organisms attached to the surface of a larger host, known as a "basibiont"), much like a whale transporting marine lice and barnacles. In so doing, they could carry them back and fourth, from the bottom of the redox gradient to the top. This in turn would supply them with the high energy sulfide and nitrate required to speed up their metabolism. In exchange, the epibionts (much like the endobionts mentioned earlier) may offer their host some sort of public good in order to increase their combined reproductive fitness.
*NOTE : Some gliding filamentous prokaryotes seem to support populations of epibionts. With that said, there is at least some precedent for gliding sulfide oxidizers partnering up with smaller organisms and offering them transportation.
Lastly, filamentous sulfide oxidizers may produce chemical signals that attract diverse, free-swimming microbes which supply them with waste electrons. In so doing, the swarming cells can reap the beneficial effects of nitrate reduction without having to spend energy on traveling long distances. Meanwhile, the host gets to supplement its diet with electrons obtained from a wide variety of substrates.
*NOTE : As mentioned in the previous entry, this process is widely observed occurring between cable bacteria and a host of free-swimming prokaryotes. Additionally, interspecies electron transfer is ubiquitous across both prokaryotic domains and has evolved several times, providing further support for this idea.
*NOTE : This sketch won't make its way into the book. The quality of the artwork will be far better in the finished version.
Citations :
https://schaechter.asmblog.org/schaechter/2008/02/archaeal-macaro.html
https://schaechter.asmblog.org/schaechter/2015/09/pictures-considered-29-archaeal-ninjas.html
https://experts.umn.edu/en/datasets/giant-sulfur-bacteria-host-intracellular-endobionts
Video of a filamentous bacterium with epibionts : https://www.youtube.com/watch?v=pBZk7Fgx-aw&list=PLGbE8ME5yo4AKJDlQwGOOBxQpWrTJSWd2&index=28
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