population genetics, evolution, and ocean ecosystems
I was trained as an Environmental Scientist long before I was at all interested in Microbes. So, I get excited when I come across microbial studies that are environmentally relevant. I get particularly nerd-cited when these studies take place in the ocean. A paper published in PNAS last week describes identifies what may be the key environmental factor distinguishing the evolution of microbial populations in the North Atlantic and North Pacific sub-tropical gyres.
The finding that populations of the abundant, widespread, and relatively well studied marine microbes Prochlorococcus and Pelagibacter (both) in an Atlantic study site had much higher frequencies of genes related to phosphorus (P) acquisition and metabolism than similar organisms in a Pacific study site was no surprise to the researchers. What was shocking was that virtually all of the genes with significantly different frequencies in the two sites were related to P use or uptake. This implies that reduced P concentrations in the Atlantic (relative to the Pacific) is the primary, and possibly sole, environmental factor driving the evolution of the P limited population. Environmental complexity generally prevents such complete correlations in studies comparing distant environments.
The methods in this study involved sampling water at three different depths at the two sites, isolating and culturing microbes of the two types and they sequencing the genomic DNA each population. Then they analyzed the variety in the sequences, and compared them to previously sequenced lab strains of each type, looking for genes that were present in some, but not all of microbes in the population. Most of these differences were due to random variation and neutral (not evolutionarily advantageous) evolutionary processes (genes are often passed around through horizontal gene transfer, but they not always advantageous). The genes of interest were those that were more abundant (statistically speaking) in one site or the other, because that indicates that that gene is conferring some evolutionary advantage in that environment, but not in the other. In Prochlorococcus, there were 29 such genes, and nearly all of them were more abundant in the Atlantic site, and were phosphorus-related. This pattern was confirmed with Pelagibacter. The conclusion is strengthened by the fact that these two types organisms are very different. Prochlorococcus is a photosynthetic cyanobacteria, where as Pelagibacter is a heterotroph
Microbial ecology is a relatively young science, and very little ecological theory has been tested with microbial populations. Studies like this one allow scientists to make predictions (that future work can support or contradict) about how evolution is working in microbial populations in natural environments. These types of studies on marine microbes are especially important because we know so little about these communities, and many of them are beginning to deal with changing environmental conditions. The paper concludes with the following statement.
In this way, population genomics of ocean microbes not only is a powerful tool for diagnosing environmental change, but also can illuminate the fundamental evolutionary processes underlying biological organization.
Coleman ML, & Chisholm SW (2010). Ecosystem-specific selection pressures revealed through comparative population genomics. Proceedings of the National Academy of Sciences of the United States of America PMID: 20937887