Specialist and generalist utilization of phosphorus forms by aquatic microbes: a mechanism for maintaining microbial diversity?

Background/Question/Methods   A key goal of ecology is to understand the factors that allow for the coexistence of species in diverse communities. While microbial communities are arguably the most diverse on earth, few studies have investigated how this diversity is maintained. Resource competition theory states that species coexistence can be constrained by the number of limiting resources. Phosphorus (P) is an essential and frequently limiting resource for microbes. It is often viewed as a single resource, but in reality, P is found in many forms, some of which may be more or less available to certain microbes. Thus, the coexistence of microbial species may be influenced by their ability to specialize on different sources of P. To determine whether bacteria from different communities can partition P form, we first isolated 30 bacterial isolates from three lakes differing in trophic status. In the laboratory, we measured growth rates for each isolate on each of eight P sources (inorganic phosphate, phytate, 2-aminoethyl phosphonate, phenyl phosphonate, adenosine-5'-triphosphate (ATP), guanosine diphosphate (GDP), 3'-5'-cyclic adenosine monophosphate (cAMP), and tripolyphosphate) that varied in their biochemical structure.

Results/Conclusions   Bacteria varied in their ability to use diverse P forms. Several strains were generalists, capable of utilizing nearly all of the P sources. In contrast, a few strains were specialists and had the ability to degrade only two P sources. The specialist strains were restricted to growing on phytate and inorganic phosphate. In addition, the biochemical structure of the P compound affected resource utilization. For example, strains that were able to degrade ATP could also access P from GDP and the nucleotide derivative cAMP, though with varying success. This finding suggests that P form partitioning within nucleotide resources may be more subtle than partitioning among other forms of organic P. Lastly, our results suggest that bacteria from an oligotrophic, mesotrophic, and eutrophic lake differed in the proportion of P-use specialists and generalists. These results suggest that microbes are able to partition the P pool, though some communities more so than others, and that this diversity of P-utilization abilities may contribute in part to the diversity we see in natural ecosystems.