TLDR; In the previous entry, we introduced some of the ecosystems present on Leeuwenhoek and added a few more to our list. These new ecosystems would begin as depositional environments. In which, organic matter from more productive habitats would accumulate in waters with minimal access to sunlight, giving rise to vast communities of decomposers and chemoautotrophs (those that use chemical energy to fix inorganic carbon and construct biomolecules). Here, we'll be examining how these ecosystems might function, starting with marine depositional environments :
Along coastlines, organics generated within photosynthetic mats would be carried upwards into the water column by passing tides. Once shuttled into deeper waters, much of this organic matter would begin to drift downward, sedimenting on the ocean floor.
Since Leeuwenhoek is a "microbial planet", these environments would lack bioturbation (sediment-mixing) from burrowing animals, allowing organics to pile up into massive debris fields. In some cases, this layer of detritus could grow to be several meters thick, providing a vast habitat from microorganisms.
*Note : Interestingly, as more organic material is deposited, increased pressures and temperatures deep within the debris field will eventually yield the production of petroleum and coal.
During early colonization, the organic matter would attract aerobic scavengers, which would end up depleting the available oxygen in the sediment, preventing communities of microorganisms further down from acquiring it. This may also use up the available oxygen in the overlying water as well, leading to widespread anoxia. However, due to tides, storms, and winds at the surface churning the water and providing aeration, this condition will likely be restricted to the lowermost depths and is unlikely to penetrate into the photic zone above.
In this dark, nutrient-rich habitat, organisms would have to get clever, utilizing other electron acceptors in place of oxygen. These could include nitrate, nitrite, elemental sulfur, sulfate, iron, manganese, carbon dioxide, and a host of trace metals (vanadium, molybdenum, cobalt, chromium, palladium, uranium, arsenic, mercury, etc.).
Most of these electron acceptors would be sparse and difficult to come by. As a result, organisms would be forced to fiercely compete for them or to rely more heavily upon less efficient forms of energy production, such as various modes of fermentation.
However, in competing for resources, microbes would naturally begin to outcompete those with less energetic systems of slower respiration, forcing them to flee into environments deeper within the sediment. Over time, the addition of progressively more efficient forms of respiration creates a stratified ecosystem in which layers nearest to the surface participate in more efficient forms of energy production and support faster rates of division relative to those at the bottom (This form of ecological stratification is commonly observed on Earth and will likely arise elsewhere as a natural consequence of thermodynamics.)
*Note : Once stratification occurs, organisms will often form beneficial relationships in which resources and ecological services are exchanged by members of neighboring layers. For instance, methanotrophs (methane-consuming archaea) living alongside methanogens will often use nanowires to exchange electrons with sulfate reducers in order to acquire greater energy yields.
Returning back to the seabed, the sulfide produced within the sulfidic layer would also cause the debris fields to turn putrid, releasing an odor similar to rotten eggs which would bubble up to the surface. Meanwhile, the hydrogen sulfide would readily react with ferric iron in the sediment to generate pyrite, mackinawite, and other iron-sulfur minerals, causing the substrate to turn pitch black in color.
(Above) Here we have an image showing the bottom of the black sea. Within this depositional environment, algae residing within oxygenated waters die during the winter and rain down into the anoxic zone, leaving behind glass shells. Collectively referred to as "Siliceous Ooze", this material forms a fine, cream-color crust the overlies the black sediment beneath. In contrast, life on Leeuwenhoek has never evolved the capacity to use silica in this way. As a result, the global ocean contains concentrations of dissolved silica that are orders of magnitude greater than what we observe within Earth's oceans. During winter, this silicic acid experiences decreased solubility and precipitates onto the seafloor, adhering to clay particles and organics. As a result, our depositional habitats on Leeuwenhoek would have a similar coating of minerals. Additionally, if you were to take a cross section of the sediment beneath, you would see many thin, alternating bands of white silica and black, organic rich sediment. These mark the passage of time in yearly intervals, much like the rings of a tree.
During the late stages of colonization, sulfide would begin to accumulate in the overlying water column, turning the water euxinic. This would begin to attract sulfide-oxidizing microorganisms which would settle along the surface of the sediment in order to capitalize on the energetically favorable blend of nitrate and sulfide.
Since sulfide is the most energetically favorable electron donor in this ecosystem and nitrate is the most viable acceptor, this form of respiration would provide them with unrivaled growth rates and allow them to monopolize more space and resources than there contemporaries. Eventually, they would go on to take over much of the sediment-water interface, creating dense microbial mats (of which, some may resemble the "Thiodendron" consortium in Entry #1).
Alternatively, within less productive environments (such as those that exist further away from continental margins where most deposition occurs), euxinia would be less widespread. Within these habitats, nitrate would remain constrained to the sediment-water interface and sulfide would only be found deep within the substrate.
In order to bridge this gap and attain the highest potential energy yields, many organisms will end up making the trek vertically through the sediment, either through swimming or gliding. Over time, these "nomads" would grow to dominate over the sedentary sulfide oxidizers deeper in the sediment and the nitrate reducers at the surface. In addition, roving herds of filamentous organisms could end up forming large, conspicuous mats along the seabed (much like those of Thioploca and Beggiatoa mentioned in Entry #2).
However, stochastic processes (such as viral infections and storms) could threaten to disturb these mats, providing an opportunity for new communities with unique compositions to replace previously established ones. This would cause the microbial mats to bear a "patchy" appearance, with different regions supporting different primary producers, each capable of sulfide oxidation and nitrate reduction.
Because of this, we might also see a wide diversity of lineages, each with their own unique strategies for linking the two processes together. As opposed to migrating, some organisms for instance may use insulated, electrically-conductive wires to transport electrons from sulfide to nitrate, much like the muticellular cable bacteria found in terrestrial oceans. Others, similar to Lysinibacillus varians GY32 (shown below), may form giant, hyper-polyploid cells (those containing many genomes) that span the redox gradient. Alternately, smaller organisms may swarm around these biological wires in order to use them for vertical electron transport, much like free-swimming prokaryotes associated with cable bacteria do today.
Lastly, some microbes may use a radically different approach. As a opposed to traveling up and down through the sediment, developing systems of relaying electricity, or relying upon other organisms to transport electrons for them, some colonies of microorganisms might develop structures referred to as "veils" that transport resources through "bioconvection".
By generating currents through the rotary action of their flagella, cells within the veil could create currents that mixing the water. This in turn would provide them with a continuous source of sulfide and nitrate. In addition, some organisms might take advantage of this ecological service, attaching to the veils so that they could benefit from the transport of resources.
Now that we've covered marine depositional ecology, let's leave behind the seafloor behind and check out what's going on the water column.
Within deep waters, slowly-sinking detrital snow and dissolved organic matter could serve as the basis for planktonic communities. Unlike planktonic microbes in aerated, coastal waters, those within the anoxic portions of the water column would make use of nitrate as their preferred acceptor.
However, nitrate reducers might not be the only organisms in the water column. Although nitrate is a thermodynamically superior electron acceptor to sulfate, sulfate-reducers can still persist within the interiors of detrital snow particles where nitrate has been depleted. This would afford them some protection from their nitrate reducing neighbors, allowing them to coexist. Additionally, if sulfide is present (either in the environment or within particles colonized by sulfate reducers), planktonic communities would readily use it due to its status as a high-energy electron donor.
NOTE : This is a commonly described phenomenon within anoxic environments on Earth, but this begs an interesting question as to why these microenvironments are dominated by sulfate reducers and not those that use iron or manganese. Perhaps the reason lies in the abundance of sulfate in the water column (relative to iron and manganese) or perhaps these metal reducers just haven't been observed yet within detrital particles. For now, I'm inclined to agree with the latter and will probably include them on Leeuwenhoek when doing some of my illustrations for the book.
Lastly, in sulfide-rich, hypoxic (low-oxygen) environments, we may find microaerophiles that supplement their diets with hydrogen sulfide. This would provide them with an energetic boost, thereby offering them a bit of a competitive edge over their less metabolically-versatile neighbors.
Anyways, this is where we'll end for now. Tune in next time for a deep-dive into how depositional environments in enclosed bodies of water differ from those in the ocean.
Citations :
https://sci-hub.st/10.1017/s0025315400044945
https://www.nature.com/articles/s41467-023-37272-8
https://www.researchgate.net/publication/350127413_Long-distance_electron_transfer_in_a_filamentous_Gram-positive_bacterium
https://www.researchgate.net/publication/283730886_Biophysical_basis_for_convergent_evolution_of_two_veil-forming_microbes
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8359478/
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