There are some exciting developments in the world of SAR11 microbiology, and I’d like to take a moment to summarize recent research I’ve been involved in. All three papers have come out this year in The ISME Journal.
The final main project of my postdoc with Steve Giovannoni was a characterization of a deep-water subclade of the larger SAR11 group using single-cell genomics and metagenomics. This was a collaboration between our lab and that of Ramunas Stepanauskas and Ed DeLong, and although it began at Oregon State, the work carried over (like many projects do) into my appointment at LSU. The main findings of the paper were that this “subclade Ic” dominated deep-water metagenomic reads compared to extant surface-originated genomes, and that a few gene-content specific variations could be identified that may distinguish these organisms from the surface strains, but that, on the whole, the main observable differences were in global genomic footprints such as expected average genome size, average coding region size, intergenic spacer size, and preferential amino acid substitutions. The deep water ecotype, subclade Ic, has obviously been evolving separately from the surface subclades for some time based on these accumulated variations and phylogenomic branch lengths, however, it also appears these organisms share a similar metabolic niche with the surface strains: they are predicted to be obligate aerobic organisms with a particular appetite for common metabolic intermediates found in the cellular milieu of most organisms: simple carboxylic and amino acids and C1 and methylated compounds.
Steve was also invited to write a Winogradski Review article in coordination with him recently being given the prestigious Jim Tiedge Award from ISME. Ben Temperton, who is also a former postdoc of Steve’s, and who worked closely with me on the subclade Ic study, above, was a co-author. Steve took the opportunity to review many of the observed variations between streamlining selection on free-living, small genomes like SAR11 and Prochlorococcus, and the small genomes of obligate intracellular symbionts like Rickettsia. Importantly, the review also pointed out that examples of streamlining selection are becoming more prevalent as data from single-cell genomics and assembled metagenomes continues to pour in. Indeed, the odd nutrient requirements that are sometimes a result of streamlining selection may be at the heart of the difficulty in cultivating many of these organisms in the lab.
Dovetailing nicely with the theoretical concepts brought up in Steve’s review was a third piece from our lab, a detailed study on an unusual vitamin requirement in SAR11 that was discovered by Paul Carini, currently postdoc-ing with Alyson Santoro. This work identified that members of many SAR11 subclades are unable to make the vitamin thiamine by typical synthetic means, because they lack a single gene in an otherwise complete pathway, thiC. As a result, they are unable to synthesize 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP), a direct precursor to thiamine. However, Paul also discovered that SAR11 has the capacity to use free HMP in media to relieve thiamine limitation, and identified a putative transporter for HMP in several SAR11 genomes. Additionally, HMP measurements from the Sargasso Sea and cultivation experiments pointed to a possible natural source of HMP from cyanobacteria, thus providing an explanation for how SAR11 can remain successful without the key thiC gene.
-jct
