It's a question we ask about many recently discovered bacteria: What, exactly, do the Verrucomicrobia do in the environment? Since their discovery, representatives of the phylumA study in mBio this week features a shotgun metagenomics approach to characterizing "Spartobacteria baltica", an abundant - but fluctuating - resident of the Baltic Sea. Clues in the reconstructed genome indicate the bacterium uses a set of pathways typical of many aerobic heterotrophic organisms, but there are also hints that it shares a close association with phytoplankton.
A common phylotype
Samples from the Baltic Sea show that a single 16S phylotype of Spartobacteria (which, my research tells me, was not named for my alma mater's mascot) is one of the dominating bacteria during the summer months. Using samples rich in this particular type, the team employed 454 pyrosequencing and assembled the ≥2kb contigs into a full genome of what they're calling "Spartobacteria baltica".
In addition to the usual metabolic suspects involved in heterptrophic metabolism, "Spartobacteria baltica" possesses 23 genes encoding glycoside hydrolases, enzymes that degrade polysaccharides. The genes represent 13 different glycoside hydrolase families, a hint that they can tackle a range of different substrates, including cellulose, mannan, xylan, chitin, and starch.
And where can an aquatic bacterium get a good supply of polysaccharides? From phytoplankton.
A close association with phytoplankton
Phytoplankton are a major source of polysaccharide substrates in marine environments, and work in freshwater habitats has found Spartobacteria attached to filamentous algae. What's more, previous work in the Baltic Sea revealed a pronounced seasonal fluctuation in the abundance of the "Spartobacteria baltica" 16S gene, and the abundance peaked in July, at a time that roughly coincides with blooms of filamentous cyanobacteria.
So it appears that "Spartobacteria baltica" comes and goes with the seasons, possibly feeding off polysaccharides provided in decaying phytoplankton blooms. These are some interesting ideas, considering this is the first aquatic Spartobacterium for which we have a genome sequence.
The authors write that further work is needed to investigate these seasonal fluctuations and to possibly provide more direct links between aquatic Spartobacteria and phytoplankton blooms.