In the previous update, we visited the coast and observed how tidal erosion can fuel the growth of microbial mats and planktonic communities within shallow marine habitats and how organic matter produced within these environments can then be washed into deeper waters, forming large depositional habitats along continental shelves and slopes. Within these environments, chemoautotrophs form the base of a highly-productive ecosystem largely independent from sunlight. In some cases, these ecosystems radically alter the chemistry of the ocean and sediment around them, paving the way for widespread euxinia, for new niches, and for greater ecological complexity.
*NOTE: While it's true these deep marine habitats are mostly sustained by light-independent metabolisms (also known as dark chemoautotrophy, dark heterotrophy, and dark fermentation), some organisms might make use of infrared light produced by other microbes as excess metabolic heat. There is a precedent for this, given that some slow-growing green sulfur bacteria have been found to use infrared light in a similar manner around hydrothermal vents. However, whether or not the meager amount of infrared light generated by microbes is sufficient for photosynthesis is something that will have to be investigated in the future.
Meanwhile, within freshwater lakes and inland seas, microbes are also modifying the environment around them. These habitats are known as depositional basins and tend to favor the growth of benthic communities similar to those covered in the previous updates, complete with sulfur cycles that are somewhat analogous to those found in the ocean. However, unlike marine depositional habitats, the basins derive much of their detritus from other sources aside from tides. The most important of which are rivers.
*NOTE: On Earth, sulfur is a limiting nutrient within freshwater environments and is far more abundant in marine habitats due to the presence tidal erosion. As a result, most lakes and ponds tend to contain very little biogenic sulfur cycling (With that said, some sulfur oxidizing and reducing organisms do exist within ponds and lakes. For instance, Beggiatoa can typically be found in freshwater sediments, but are generally less prolific than those in marine samples). However, by studying Leeuwenhoek's host star (Tau Ceti), we can infer that the planet's interior has a magnesium-silicon ratio of 7:4 (For context, Earth has a ratio of 9:7). Because of this, Leeuwenhoek's interior would favor the development of the mineral olivine, rather than pyroxene. As a consequence, the mantle also contains far more free ferric iron, which serves as an excellent electron acceptor. Under these oxidizing conditions, sulfur dioxide would become the most abundant sulfur species in the mantle and would be readily outgassed into the atmosphere during volcanism. Since Leeuwenhoek is currently experiencing an episode of extreme tectonic activity within the northwestern hemisphere, this would result in sulfur dioxide accumulating on the surface and precipitating as sulfite (a highly coveted electron acceptor for microbial metabolism), thus explaining why marine and freshwater ecosystems on Leeuwenhoek tend to be so similar.
Disclaimer : While this planet is meant to serve as a generalized model of how oxygenated biospheres may develop in the absence of complex life, the unique sulfur cycling on Leeuwenhoek is an example of how stochastic processes (i.e. star formation, planetary composition, plate tectonics, etc.) can result in very different outcomes for microbially-dominated worlds. In the future, I hope to expand upon this idea by discussing worlds that have traveled along different developmental paths, with less active plate tectonics, more metallicity, or that are towards the end of their lifespans.
Since rivers facilitate continuous erosion, they are also able to supply microbes with essential nutrients. All the communities along the sediment have to do is hold on tight and let dissolved ions and organics suspended in the currents wash over them.
Over time, the edges and bottoms of active rivers can become highly productive ecosystems dominated by photosynthetic mats. As this occurs, primary producers fix carbon and nitrogen from the atmosphere and provide communities downstream with detritus.
Upon reaching a lake or pond, dissolved carbon and organic particles in the river will gradually sink to the bottom, filling the basin with a thin dusting of detritus. Depending on the amount of productivity in the river, the sediments of these basins can become saturated with organic material, developing carbon-rich strata ranging from a few millimeters to several meters in depth.
Additionally, lakes and inland seas can acquire organics from other sources as well. For instance, if a body of water is in close proximity to the coast, it can become inundated with sea spray (tiny, aerosolized droplets of seawater). Within this fine mist, organics produced by coastal microbial mats and planktonic communities can become concentrated and delivered further inland by passing winds. Alternatively, glaciers in the upper latitudes can weather rock into sediment known as glacial till. When the glaciers melt during the summer months on Leeuwenhoek, this material is delivered down stream, along with catalytic ions liberated from the rocks. As a result, seasonally frozen lakes and brine pools can also become more productive over time.
(Above) A diagram displaying the nutrient cycling that takes place within a coastal lake in Antarctica. Since many bodies of water in Antarctica receive very little nutrient input from terrestrial plants and animals, they serve as excellent models for how nutrients might become concentrated within lakes on Leeuwenhoek.
Regardless of its origin, this organic material serves as a growth medium for a host of heterotrophic and chemoautotrophic organisms. If present in great enough numbers, these microbes will go on to rapidly deplete the sediment and detrital layers of oxygen. However, from this point onward, mixing regimes begin to play an important role in determining the ultimate fate of the basin ecosystem.
For example, freshwater basins and inland seas where winds and storms are present throughout much of the year support continuous mixing. This results in the basins becoming well aerated and causes the anoxic zone to retreat further down into the sediment. Meanwhile, lakes with less frequent turnover will remain stratified for much of the year, containing oxygen within their upper layers and hypoxic or anoxic conditions within their underlying waters. However, periodic disturbances can lead to these stratified lakes becoming aerated, resulting in alternating cycles of oxygen rich and oxygen-poor conditions.
In addition, lakes within Leeuwenhoek's temperate environments experience mild winters due to the planet's slight axial tilt of 13.4 degrees. During which, oxygen-rich surface waters cool and sink to the bottom, aerating the oxygen-poor waters below. Eventually, the newly-oxygenated bottom layer will gradually return to being deprived of oxygen as microbes continue to feast on the underlying detritus.
However, the fate of most stagnant bodies of water is far less eventful. Without a sufficient means of accumulating detritus, isolated bodies of water remain poor in nutrients and organic carbon. As a result, their entire water column remains permanently aerobic, with little to no change in dissolved oxygen levels.
In contrast, the most interesting bodies of water from an ecological perspective are the permanently stratified lakes, inland seas, and sink holes. These environments support massive deposits of organic matter which rapidly deplete oxygen levels as they gradually decompose. However, unlike the other examples mentioned prior, these bodies of water experience meromixis; a condition in which the bottom of the water column becomes enriched in salts. This saltwater can enter coastal lakes as seawater percolates through porous rock or can wash in during coastal storms. Alternatively, dolomite (a mineral made by microbes on Earth through passive biomineralization) commonly occurs on Leeuwenhoek and can form massive deposits. As a consequence, groundwater that comes into contact with the dolomite can become saturated with salts and contaminate nearby bodies of water (Because of this, ponds and lakes that reside within continental interiors or on mountains can still develop meromixis, despite being entirely land-locked).
*NOTE : Along with salts, meromictic lakes also obtain large quantities of sulfate from seawater and groundwater contaminated by dolomite. As a consequence, these environments also harbor more pronounced sulfur cycles, rivaling those of typical freshwater basins of Leeuwenhoek.
Since saltwater is more dense than freshwater, the two layers remain unable to mix, which keeps the water beneath the halocline (the transitional zone from fresh to salt water) permanently deprived of oxygen. As a result, meromictic lakes, seas, and sink holes house two radically different environments separated by a thin veil. The upper environment is known as the mixolimnion and supports oligotrophic conditions due to the fact that most organic material entering the basin sinks into the layer beneath and becomes trapped within convection cells. As a result, the water is clear and sparsely populated in comparison to the environment below. For this reason, these environments appear clear, perhaps with a slight greenish or turquoise tint. This unusual color is due to calcite particles passively precipitated by plankton and microbial mats.
(Top) In New York's "green lake", surface waters are oligotrophic and support low population densities of cyanobacteria and other photosynthesizers. However, during the summer months,
Synnecococcus (a common planktonic cyanobacteria) populations increase and cause seasonal "whiting events" in which calcite is precipitated into the water column. In addition, benthic mats of cyanobacteria trap this calcite within their extracellular matrix and grow above it in order to keep themselves from being buried. Over time, this process can create microbial reefs known as "microbialites" (which are the pedestal-like formations shown in the image above). (Bottom) This is a sign from the nearby park, displaying the composition of the lake.
In contrast, the waters beneath the halocline are known as the monimolimnion and contain higher levels of dissolved organic matter. In addition, disturbances along the surface from storms, tides, and passing winds fuel weak convection cycles, enabling dissolved nutrients to be cycled throughout the anoxic waters of the monimolimnion (This is further aided by microbes which liberate inorganic ions from detritus during decomposition and return them to the water column through a process known as remineralization.)
The transitional zone between these two habitats is known as the oxic-anoxic interface and serves as a gathering place for large communities of microbes. These organisms go on to form planktonic blooms known as "meromictic plumes" or "microbial plates". The reason for this attraction stems from the lack of organics and electron donors in the mixolimnion and poor availability of electron acceptors in the monimolimnion.
By straddling these two distinct habitats, microbes can effectively combine the best of both environments; gathering oxygen from the surface while also extracting carbon in deeper waters and reduced gases (i.e. hydrogen gas, hydrogen sulfide, methane, etc.) made by microbes deep within the sediment.
Since these gases tend to react rapidly with oxygen, they are only attainable in large concentrations along the oxic-anoxic interface and within deeper environments. As a result, microbes that respire oxygen and rely upon these energy sources must carefully regulate their depth, lest they travel to close too the surface and starve or drift too far down and suffocate.
In order to maintain their close proximity to the oxic-anoxic interface, these organisms must constantly swim in order to prevent themselves from sinking or regulate their buoyancy through the use of gas vacuoles. However, while both strategies are energy intensive, they offer microorganisms an opportunity to combine hydrogen sulfide oxidation with the reduction of oxygen, which enables them to attain energy yields that are unrivaled by other competing forms of metabolism. For this reason, aerobic sulfide oxidizers are able to rapidly grow and reproduce, becoming the most dominant organisms along the interface.
Additionally, this form of energy metabolism can be made more efficient through the utilization of light. As a result, many of the sulfide-oxidizing organisms in this environment are photosynthetic, relying upon sunlight to excite electrons harvested from sulfide so that they can produce more ATP and fixed carbon / nitrogen.
(Above) On the "Bacterial Realms" youtube channel, I posted a clip from a nature documentary describing the plumes of aerobic sulfur bacteria in Alat Lake (the video is linked in the citations). Since most of the red and blue light entering the lake is absorbed by calcite granules and water molecules within the first few meters of the mixolimnion, very little of it makes it's way to the oxic-anoxic interface. As a consequence, this habitat favors organisms that preferentially make use of the abundant green-yellow light for energy and reflect red and blue light. This in turn causes them to appear purple when brought up to the surface or illuminated. (Bottom) Meanwhile, microbes that are close to the surface of the water and those that are on dry land are exposed to higher concentrations of yellow and green light, which leads pigments capable of absorbing these wavelengths to experience a high level of energy flux (otherwise known as "noise"). Since sudden changes in light availability are difficult for microbes to adapt to in real-time and put them at risk of suffering from phototoxicity, most photosynthesizers prefer to absorb wavelengths that exhibit more predictable levels of flux. Because of this, cyanobacteria in these habitats (such as the
Synnecococcus elongatus shown here) typically absorb red and blue light, while reflect the yellow-green portion of the visible spectrum. As a result, they appear green to the naked eye.
NOTE : Since Leeuwenhoek orbits a sun-like star (in other words, a G-type main sequence star) and supports lakes with similar optic / geochemical properties, it's likely that meromictic environments will be partitioned into a similar assortment of "photic niches", with microbes in the mixolimnion predominantly making use of blue and red light and those in deeper waters making use of green / yellow, thereby turning them purple. However, there are still exceptions. For instance, green sulfur bacteria and green non sulfur bacteria on Earth have specialized light harvesting antennae known as chlorosomes which allow them to capture incredibly small amounts of blue and red light, while occasionally making use of infrared. By doing so, these organisms are able to live in environments that are far too dark for other photosynthesizers to survive within. However, there is no reason for why chlorosomes had to evolve within a lineage of green photosynthesizers. In contrast, a lineage of purple photosynthesizers on Leeuwenhoek had developed similar structures, thereby enabling them to monopolize dimly-lit environments before there green counterparts had a chance to expand their ecological range.
Meanwhile, all of this planktonic activity produces even more detritus, attracting a fair amount of organisms with heterotrophic metabolisms. By consuming and breaking down organics within the plumes, inorganic ions (such as phosphorous and molybdenum) are released. This in turn makes them available to the plume's primary producers, which allows for even more growth.
In addition, the swimming of trillions of microorganisms along the interface alters the flow of the water, causing most of nutrients and dead microbes within the plumes to remain suspended, rather than sinking to the bottom of the basin. This process is known as bioconvection and plays an important role in locally concentrating important resources within the plumes, thereby boosting their productivity even further (This is a trend that has been commonly observed on Earth within similar environments).
Bioconvection also mediates many of the ecological interactions within the plume. For instance, active swimmers will often use bioconvection to "push" photosynthesizers dependent upon gas vacuoles out of environments where oxygen, light, and sulfide are found in the greatest abundance, thereby outcompeting them. In contrast, heterotrophs that are unable to maintain buoyancy or swim can ride these convection currents as a means of staying aloft and remaining close to their food source (i.e. the waste products of the purple photosynthesizers).
NOTE : In most meromictic lakes on Earth, a single genus or a few species of purple sulfur bacteria tend to be considerably more dominant than the rest. This is presumably due to the fact that these bacteria produce the most torque while swimming and are able to push all of their competitors out of their territory. However, there are also examples where other defensive strategies enable some purple sulfur bacteria to acquire the upper hand. For instance, within the South Andros Black Hole in the Bahamas, a few lineages of purple sulfur bacteria contain carotenoids that purposefully convert photic energy into metabolic heat. While this leads to less efficient energy conversion during photosynthesis, this heat raises the ambient water temperature to 36 degrees Celsius (96.8 degrees Fahrenheit), which is above the limit at which most purple sulfur bacteria can tolerate. In a sense, they transform the lake into a solar cooker and boil their enemies to death! (Since the role metabolic heat plays in microbial competition and altruism is something that is rarely discussed in the the literature, it is something I definitely plan to speculate on in my book.)
Swimming chemotactic and phototactic organisms (organisms that swim in response to chemical or light) can also serve as a mode of transport for a host of other organisms. By attaching directly to the surfaces of flagellated microbes, small parasites, commensals, and mutualists can hitch a ride. However, since the addition of a passenger would increase the amount of drag the host experiences while swimming, it would have to expend more energy in order to move, thereby leading to a decline in reproductive fitness. As a result, many hitch-hikers are either "selfish" (readily jumping from host to host as they die from exhaustion or from stress due to parasitism) or take on the role of altruists and try to limit their impact on the host cell. In doing so, many of them have reduced their cell diameters to minimize drag and have cycles of cell division that are synched up with their hosts.
Additionally, many provide important ecological services and common goods in order to make up for the toll they take on the host's ability to swim. Some for instance, provide antimicrobial substances and protect their hosts from competitors, others offer the host the ability to make use of novel carbon sources or offer fixed nitrogen. Additionally, many share biomolecules, producing amino acids, nucleotides, and fatty acids that would normally be energetically costly for the host cell to make on their own.
In some cases, these relationships can even result in the host and epibiont becoming completely reliant upon one another for survival (for context, watch the video on the "Black Queen Hypothesis" down below.) Epibionts can also develop complex systems of chemical communication, manipulating their hosts chemotactic apparatus to steer them towards conditions that are favorable for their own growth.
*NOTE : As covered in Microcosmos Entry #1, this "steering" behavior has evolved multiple times within meromictic lakes on Earth between non-motile green sulfur bacteria and motile heterotrophs. In these relationships, the green sulfur bacteria encourage their hosts to swim towards sulfide and light in exchange for vital metabolites. Meanwhile, the host's avoidance of oxygen provides both with added protection from oxidative stress.
However, one of the most common and widespread consortia found in meromictic plumes are formed between aerobic sulfide oxidizers and dissimilatory sulfate reducers (those that respire sulfate and release it into the environment as sulfide. This is not to be confused with assimilatory sulfate reduction, in which the sulfate is reduced and converted into reactive sulfur species for use in constructing biomass.) In this relationship, photosynthetic sulfide oxidizers produce sulfate as a waste product and the sulfate reducers convert it back into sulfide in exchange for exudate (a slurry of dissolved organics produced by the sulfide oxidizers offer as a form of "payment".) This relationship enables the consortia to create microenvironments with higher sulfide concentrations than the surrounding water, thereby providing both partners with faster rates of energy production and growth. As a result, sulfur-recycling consortia have proven to be surprisingly successful within meromictic plumes on Leeuwenhoek.
*NOTE : Back on Earth, close-knit relationships between sulfate reducers and sulfide oxidizers are commonplace and have evolved on numerous occasions. As a result, they can be found all over the world and within a wide assortment of habitats, from salt marshes to the microbial plumes of meromictic lakes. Because of this, it's likely that sulfur-rich environments on Leeuwenhoek will tend to favor similar partnerships.
(Above) A planktonic consortium collected from Lake Cadagno in switzerland, involving
Lamprocystis (A purple sulfur bacterium / The larger cells) and
Desulfocapsa (A sulfate-reducing deltaproteobacteria / the smaller rod-shaped cells).
NOTE : To help differentiate my fictional microorganisms and their communities from one another, I decided to nickname these consortia "planktonic sulfureta", in reference to the term that was once used to describe sulfide-rich environments (such as geothermal springs and intertidal microbial mats).
Similar consortia can also play an important role in ecosystems along the sediment, assuming that the photic zone manages to penetrate all the way to the bottom. In these cases, these microbes can also form colorful patches along the sediment-water interface, ranging from dark purple to pastel pink in color.
As a consequence of their dependence upon light, many of these microbes within these mats will engage in chemotaxis and climb over one another in an attempt to outcompete their neighbors for sunlight, forming small "pinnacles". In addition, gas bubbles from sulfidic and methanic sediments below can become trapped within these mats. This causes them to distend and form stalagmite-like spires that rise towards the surface due to the buoyancy of the gasses contained within.
(Above) Pinnacles and spires formed by sulfide-oxidizing microbes (observed
within the Lake Huron Sinkhole)
(Above) Purple cyanobacteria and Beggiatoa collected from benthic mats in
the Lake Huron sinkhole.
Zooming out, lakes, ponds, and other inland basins are exceptionally diverse. Given that they vary tremendously in terms of geochemistry, ecology, and nutrient cycling, it's hard to pin down a universal model for how they should function from an ecological perspective. However, this serves as a small slice of what freshwater habitats might be like on an alien planet (For now, this is where are story will be left off. However, I'll be posting some illustrations of the organisms that inhabit the meromictic basins at some point in the near future!)
Citations :
Video Discussing the "Black Queen Hypothesis" : https://www.youtube.com/watch?v=AhEyhju_Iks
Documentary Footage of a Meromictic Lake : https://www.youtube.com/watch?v=10sBKxBdVfg
https://www.sciencedirect.com/science/article/abs/pii/S0048969720363634
https://genome.jgi.doe.gov/portal/chlph/chlph.home.html
https://journals.asm.org/doi/10.1128/mbio.00052-22
https://pubmed.ncbi.nlm.nih.gov/17906940/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10756677/
https://www.researchgate.net/publication/324795723_Low-Light_Anoxygenic_Photosynthesis_and_Fe-S-Biogeochemistry_in_a_Microbial_Mat
https://www.semanticscholar.org/paper/Novel-Bacterial-Diversity-in-an-Anchialine-Blue-on-Gonzalez/96bc659a0ed7d3539cba52d9880a68af6a4b9445
https://www.nature.com/scitable/knowledge/library/rock-water-microbes-underwater-sinkholes-in-lake-25851285/
