Microbial community structure and nitrogen cycling in limestone biofilms

Biofilms inhabiting rock surfaces were of particular interest for this study, due to their ubiquity in central Texas and possible role in nitrogen cycling. Biofilm samples from an ornamental pond in Austin, Texas were collected over the course of two and a half years. Microscopic analysis indicated that the general physical structure of the biofilm remained constant, but the organisms inhabiting the biofilm varied. Metagenomic analysis confirmed that taxonomic diversity in biofilm communities is dynamic and variable, but the predicted functional capacities appear to be relatively stable throughout the sampling time. Less than one quarter of the variation in the taxonomic community data is explained by climate variables, indicating that a combination of stochastic and deterministic factors may drive community assembly. Limestone biofilm communities may be colonized from guilds of organisms that assemble based on the functional requirements prescribed by environmental conditions and resource availability. Natural biofilm samples were collected from other distinct patches of limestone in green spaces near Austin. Natural biofilms were thicker and more complex than the ornamental pond biofilms, yet they were not significantly different from each other in terms of their taxonomic community composition. The functional diversity of the natural biofilms was nearly identical to that seen in the ornamental pond biofilms. Taxonomic and functional diversity in natural biofilms were strongly correlated and significantly dissimilar. A strong negative correlation between actinobacteria abundance and bacteroidetes abundance was found, indicating that these organisms might be competitors. Cyanobacteria abundance was positively correlated with both humidity and precipitation, indicating that cyanobacteria might require more water than other organisms in the biofilm community. Organisms within limestone biofilm communities were capable of fixing atmospheric nitrogen, but the rate of nitrogenase activity was highly variable throughout the sampling period. Nostoc was the most abundant and active nitrogen-fixing organism. The abundance of cyanobacteria capable of fixing nitrogen was prone to fluctuation, whereas the abundance of non-photosynthetic nitrogen fixers remained relatively constant. Nitrogenase activity in the light reflects a combined effort between cyanobacterial and other nitrogen fixers and a consortium of other nitrogen fixers may be solely responsible for nitrogenase activity in the dark.