Hot Springs Ecology
In the previous section, we examined the adaptations that allow certain groups of microorganisms to live in severe hot springs environments. This discussion necessarily emphasized the unmistakable hostility of these conditions for most other organisms. It is natural, then, to picture a hot spring as a mere scattering of die-hard microbes, clinging to a lonely existence and isolated from the entire biological community by their rather unwelcoming neighborhood. But hot springs, like other ecosystems, are home to rich, interdependent communities. Far from an empty and biologically impoverished wasteland, hot springs host species from every domain of life and each resident is part of an intricate and dynamic network of relationship.
While it is often useful for scientists to separate an organism from its natural setting to simplify its study, it is important to realize that no species exists in isolation. Understanding the interplay of a living species with its living and non-living environment is the goal of ecology. Microbial ecology carries these same principles down in scale; just as biologists view a strangler fig or carpenter ant as components of a larger rainforest system, so too should we examine Sulfolobus, Cyanidium and all other thermophilic microorganisms in the context of their own unique ecosystem.
Physical Ecology
The non-living, or abiotic, factors that define a hot spring ecosystem include temperature, oxygen content, nutrient abundance, pH and salinity, among others. These factors can vary widely even within a single thermal feature. Picture nearly boiling and highly acidic water emerging from the ground saturated with dissolved minerals. As it flows out and mixes with cool and relatively pure runoff water, gradients are formed by diminishing temperature, acidity and mineral content. These gradients overlap with still others to form a complex and variable physical environment. This spatial complexity is reflected in the differing communities that form in different parts of a thermal pool. To take an example from acidic springs, shifts in the water temperature completely shuffle the roster of microorganisms present. At high temperatures (above 60°C) sulfur metabolizing archaeans dominate the scene. Further down the temperature gradient, iron oxidizing bacteria become more abundant and, as the water cools even more, photosynthetic algae and cyanobacteria form the community’s backbone. We can actually trace this particular gradient with the naked eye; sulfur compounds yield yellow tint; iron oxidizers forms unmistakable reddish-brown streaks and photosynthesizers form dense green microbial mats.
Rather than meekly accepting the constraints of abiotic forces, some organisms actively change the physical conditions that govern their habitat. Our old friend Sulfolobus provides us yet another example. Because one of the products of a sulfur oxidizing metabolism is sulfuric acid, Sulfolobus actually lowers the pH of an already acidic environment. At low densities, colonizing cells act like cooperative pioneers, creating a habitat better suited to their collective needs. However, at higher densities, the mutually beneficial pioneer town becomes a cut-throat metropolis as members of the population compete for resources and increasing acidity limits further growth.
Discussing microbial ecology forces us to examine the idea of microenvironments. Consider the fact that a 1 mm distance surrounding a typical prokaryote is equivalent to a half-mile radius around a human! Imagine how the physical circumstances could differ between a dense, shaded forest and a sunny, open field a half-mile apart. For a microbe, such diverse environments may only span a few micrometers but they can provide a multitude of radically different niches. Small gradients in oxygen and nutrient concentrations, temperature and moisture levels can form between and within grains of sand! At the scale of prokaryotic life, this heterogeneity means astounding diversity is possible with mere centimeters.
Trophic Interactions
To understand an ecological community, we must also appreciate the interactions between the various residents. These biotic factors are the living context that shape a habitat. Whether boreal forest or open ocean, grassland or desertscape, all ecosystems share a common biological framework built upon certain roles played its members. Ecologists often define an organism’s role in terms of trophic levels. These designations, including producers, consumers and decomposers, may be familiar to you but the trophic levels of an exclusively microbial habitat have their own unique character.
Primary producers are the foundation of a biological community for they fix carbon from the atmosphere and build organic compounds. Without this initial step, which makes carbon available in an organic form, the macromolecules that build every living cell could not exist. In most familiar habitats, plants take up the role of primary production, using the energy of sunlight to build carbon compounds, the process of photosynthesis.
In acidic thermal pools, where plants cannot survive, photosynthesis is carried out by both algal species and cyanobacteria. Often forming thick green mats or filamentous streamers, these groups colonize areas open to sunlight with relatively mild temperature and pH regimes. In regions of extreme temperature and acidity, photosynthetic organisms can no longer cope with conditions but primary production still occurs. Here it the process of chemosynthesis, using energy from inorganic ions, that allows primary producers to create organic compounds. Chemosynthesis is a uniquely prokaryotic process; no plants, protozoa, algae or any other eukaryote can carry out this form of metabolism. With this in mind, we can see how fundamentally different microbial ecosystems can be from those of the visible world: the sun is no longer the sole energy supplier for these environments. Instead, energy may enter the system from a variety of chemical sources, pouring in from subterranean vents and even released by the breakdown of surrounding rock.
With organic molecules available to the hot spring ecosystem, a whole other class of organisms can take up residence. Consumers and decomposers utilize carbon compounds for their energetic and structural needs and, in many macroscopic systems, can only colonize a habitat with primary production already underway. In a spring habitat however, a significant portion of organic carbon may enter the system from the outside. Wind and surface runoff caries decaying organic matter into the spring and provides a decomposing microbes with a nutrient source. Amazingly, organisms such as Sulfolobus can play duel roles in hot springs habitats. With a flexibility found only in the microbial world, Sulfolobus can adjust its metabolism with the availability of nutrient sources, shifting from primary producer to consumer/decomposer and back with ease.
Population Limitations
One of the most important questions that ecologists can ask is “What limits the growth of a population?” We have looked briefly at the idea of competition within a population. Now, with an understanding of the basic trophic levels, we can examine interactions between different microbial populations. In most microbial systems, consumers from the eukaryotic domain, mostly protozoa, carryout significant reduction of bacterial populations, an activity known as grazing. However, the extreme nature of hot spring environments creates yet another unique situation. At the highest temperatures, eukaryotic life is not present at all and thus the food chain is considerably truncated. What then limits these microbial populations? One possibility is the limitation of nutrient resources. A microbial population can only grow as long as its limiting resource is available in adequate quantity. But increasing evidence points to another possibility: prokaryote populations often suffer high mortality due to viral predation.
These viruses, called bacteriophage or simply phage, fill most aquatic habitats in densities several orders of magnitude higher than that of resident bacteria. In other words, for each prokaryotic cell in a given habitat, we might find hundreds of individual viral bodies, or virions. With these numbers in mind, we can see what an important ecological role phage may play. Not only do they limit the growth of prokaryote populations, the destruction of host cells provides a nutrient source for decomposers in the form of dissolved organic material. Recent investigations into the viruses of Sulfolobus have shown that viral abundance is matched by unexpected diversity - several new viral families have been found a parasite of just this single organism!
Microbial Mats
Along with an understanding of the interactions possible within and between microbial populations, we need some picture of the spatial relationships in a hot spring system. The clearest examples come from floating islands of biodiversity called microbial mats. Though only a few centimeters in depth, mats support multiple trophic levels in neatly stratified layers- a condition that makes them ideal for ecological investigations. A typical thermal pool mat exists in moderate temperature and pH regimes, allowing a variety of resident organisms. The upper-most layer is highly exposed to sunlight and thus consists of photosynthetic cyanobacteria. The oxygen yielded by their metabolism filters into underlying layers and consequently this stratum may be filled with aerobic bacteria and archaea. The respiration of these mat residents can rapidly deplete oxygen and deeper layers may be completely anoxic. Often a distinctive purplish layer forms in the anaerobic interior of a microbial mat where purple sulfur bacteria reside. These organisms utilize sulfide as an electron donor and thus benefit from the activities of associated sulfate-reducing bacteria. Microbial mats provide each of these groups with a relatively stable island in a sea of constantly changing environmental conditions. With such neatly defined ecological arrangements, mats yield a text-book example of a microbial community.
The relative roles of resource availability and viruses in limiting populations and the spatial arrangements of microbial communities are just a few of the questions addressed by current research in the park. Hot spring ecology is a rapidly developing but still relatively new field. Scientist are even now refining techniques for identifying the organisms present; complete characterization of the ecosystem, with an understanding of nutrient cycling, complimentary metabolisms, and overall community structure is still a long way off. Yet developments in molecular and genetic techniques are bringing possibility of systems level understanding ever closer. These tools and the ways in which researchers utilize them are the subject of the next section.
Our tour of life in Lassen Volcanic National Park continues with a look at: