The Mystery 40%

Mike Z got me to review a rather interesting small grant proposal today. For years now he's been looking at what the smaller end of the plankton community are up to. In oligotrophic regions of the world ocean (e.g. the subtropical gyres) the typical view is "not much", but there are are hard-core of bacterioplankton eking a living there.

In earlier work, Mike has found that ~60% of these bacteria are "conventional" [*] autotrophic bacteria performing photosynthesis, and have a high genome size. The remaining 40% are classed as low genome size, and have genomes that are trimmed right back to what people believe are the bare essentials. This is probably an evolutionary response to oligotrophy, since an organism's genome can be a not-inconsiderable portion of its cellular budget. Now, of the 40%, around 60% are identifiable as members of the SAR11 clade, and they're relatively well-characterised. However, the remaining 40% (or 16% of total bacterioplankton) are something of a mystery. Hence Mike's proposal.

Speaking to Mike, one idea is that these bacteria are using light in simple proteorhodopsin photosystems (i.e. not photosynthesis) to enable them uptake organic material. Rather than respire this material for energy, it is used directly to build cellular structure. So unlike most organisms, these would operate on an extremely tight energy budget that relies on the availability of macromolecules (amino acids, polypeptides) that can be used directly instead of going through a catabolic/anabolic loop.

Anyway, all news and exciting to me. I'm sure it's true for many scientists, but everyone else's work always seems more interesting than my own!

[*] That is, "conventional now" - 30 years ago people knew practically nothing about them.