Humans have long been fascinated with the extraordinary diversity of life on Earth. Not only is the sheer diversity of living creatures intriguing, but there are also striking patterns in their distribution over space and time. Most of what we know about the origin, maintenance and distribution of biodiversity stems from research on plants and animals. It is as important but increasingly difficult to examine the patterns in richness and community composition of diverse communities such the hyper-diverse communities of micro-organisms. Bacteria may be one of the most abundant and species-rich groups of organisms, and they mediate many critical ecosystem processes. Despite their incredible abundance and ecological importance, past practical and theoretical constraints have limited our ability to document patterns of bacterial diversity and to understand the processes that determine these patterns. However, recent advances in molecular techniques that allow more thorough detection of bacteria in nature have made it possible to examine such patterns and processes. Here, I extend the approaches and questions of traditional ecology to outstanding questions in aquatic microbial ecology.
First, I reviewed recent studies of the distribution of free-living bacterial diversity and compared our current understanding to what is known about patterns in plant and animal diversity. From these recent studies a preliminary picture is emerging: bacterial diversity may exhibit regular patterns, and in some cases these patterns may be qualitatively similar to those observed for plants and animals.
Second, the species-area relationship (SAR) has been observed in a number of organisms in many different ecosystem types across the globe and is thought to be universal for all taxa. The SAR describes the spatial distribution of species richness, has served as a useful tool for exploring other patterns of biodiversity, and was fundamental to the development of another of ecology’s “laws,” MacArthur and Wilson’s equilibrium theory of island biogeography. While the SAR has been shown repeatedly for plant and animal species on both islands and mainlands regardless of ecosystem type, it has not been observed for bacteria, and relatively little is known about the spatial distribution of microbes. I used a nested sampling design to determine if bacteria exhibit a species-area relationship, or more generally a taxa-area relationship, over the scale of centimeters to hundreds of meters in salt marsh sediments. I used cloning and sequencing to estimate bacterial diversity. I found that bacterial communities located close together were more similar than communities located farther apart and showed, for the first time, that bacteria do exhibit a taxa-area relationship. Environmental heterogeneity helped to explain this pattern, whereas aboveground plant composition did not. Finally, I found that bacterial richness in the salt marsh sediments was related to salinity, ammonia and sulfate.
Third, primary productivity is a key determinant of biodiversity patterns in plants and animals but has not previously been shown to affect bacterial diversity. I examined the relationship between productivity and bacterial richness in aquatic mesocosms designed to mimic small ponds. I observed that productivity could influence the composition and richness of bacterial communities. I showed that, even within the same system, different bacterial taxonomic groups could exhibit different responses to changes in productivity. The richness of members of the Cytophaga-Flavobacteria-Bacteroides group exhibited a significant hump-shaped relationship with productivity, as is often observed for plant and animal richness in aquatic systems. In contrast, I observed a significant U-shaped relationship between richness and productivity for alpha-proteobacteria and no discernable relationship for beta-proteobacteria. I showed, for the first time, that bacterial diversity varies along a gradient of primary productivity and thus make an important step toward understanding processes responsible for the maintenance of bacterial biodiversity.