Earlier today, I wrote about biofilms and the negative consequences they can have on human health during infection. Much discussion of biofilms centers around the interaction of pathogenic microbes and their abilities to form during infection. However, as I alluded to in the introduction, biofilms are important in many nonmedical aspects of life – they contaminate cargo ships, food processing centers (including your kitchen), as well as occur naturally in many environmental settings.
These environmental settings can be as benign as pond scum or as extreme as the hot springs of Yosemite or ever-dark caves. This introduces a really cool type of biofilm called a microbial mat. These microbial mats are layered communities of microbes that are interdependent on each other for waste removal and nutrient production (which can be the same reaction, depending on which microbial point-of-view you’re using). The ability of one species to tolerate sulfide, for example, allows colonization of sulfur springs – and the layered nature of the mat protects the upper levels from otherwise-toxic hydrogen sulfide as the lower layers oxidize it to sulfate.
Another neat aspect of these mats is that they can grow in really extreme places: places without oxygen, light, or many nutrients. Their ability to colonize earth’s extreme environments can give biologists clues about early formation of life – these mats serve as one of the oldest fossilized records of life, with Australian samples dating back nearly 3.5 billion years!
Now new research from a team of scientists has looked at a particularly extreme environment: a perennially ice-covered Antarctic lake. Despite its harsh conditions, the lake has a microbial mat growing throughout observed the oxygen gradient. The authors wanted to see how the microbial makeup of the mat differed as the available oxygen changed, and to see how this makeup was reflected in the macroscopic characteristics of the mat. Their findings are published in Applied and Environmental Microbiology.
The lake chosen, Lake Fryxell, was previously studied for other variables affecting its microbial mat. Because the colonized floor has a gradual incline, it offers a variety of niches into which different microbes have formed symbiotic communities. There, the researchers observed four mat morphologies (clockwise from upper left in photo): pinnacle, ridge-pit, or prostrate mats, and flocculent biomass. The mixture of these different mat types varied with depth, with pinnacle and ridge-pit mats dominant at shallower depths and prostrate matts and flocculent biomass dominant at greater depths.
Were these different morphologies due to compositional differences? This appears to be the case, as the makeup varied based not only on mat type but also depth. At greater depths, with lower oxygen and light diffusion, the ridge-pit mats were primarily composed of diatoms and sulfide-tolerant cyanobacteria Phormidium. At lesser depths, the pinnacle mats were primarpubily composed of Leptolyngbya, a cyanobacterium previously associated with pinnacle mat formation. Overall, the researchers were able to conclude that the macroscopic appearance of the mat is the result of different microbial community makeup and niche preferences.
Understanding the makeup of these communities helps us understand how microbial mats influence their environments – by using water as a reducing agent, for example, some cyanobacteria produce oxygen in otherwise oxygen-poor environments, facilitating growth of more aerobic species. But there’s a more exciting angle to this research as well (and a reason this research is partially funded by NASA) With the promising discovery of water, scientists are more optimistic than ever about finding life on Mars. Could this research be applied to help scientists identify most-likely environments where it might be hiding?
-- Julie Wolf