Saturday, 10 October 2009

Not so autotrophic now

We had an interesting inaugural seminar in yesterday's Friday Seminar Series. MVZ was in the barrel, as was I, since I was on introducing duty through my new membership of the Series' organising committee. He gave a quite revelatory (and impressively brief) overview of some work he's done on less-than-autotrophic behaviour of phytoplankton. After starting with an introduction that took in the evolutionary history of prokaryotes and the endosymbiosis that led them to eukaryotes, we jumped to a conundrum in the nutrient and carbon budgets in the modern North Atlantic. Basically, eukaryotic algae are doing a fair fraction of the carbon fixation, but are only responsible for a small fraction of phosphate uptake. Given that there should be parity between these, something is up. Bactivory, apparently, is what that "something" turns out to be. By carefully labelling prokaryotic cells by providing them with an isotope-labelled amino acid, MVZ was able to show that, within hours, this label wound up in some interesting places. Even card-carrying autotrophs like the coccolithophorids have a sideline in getting phosphate in a less-than-first-principles manner. The idea is that, in highly nutrient stressed environments like the oligotrophic gyres, bacteria provide an excellent source of phosphate, and other useful molecules to boot. That said, my favourite phytoplankton, the diatoms, are still hardcore-autotrophs, with no whiff of bactivory on their part (at least in the open ocean), but this probably stems from their adaptation to high nutrient environments (plus their frustules would probably impede dining). In the environments that MVZ studied, it turns out that eukaryotic phytoplankton are as important at grazing bacteria as bona fide protistan microzooplankton. Quite a turn up for the books [*], although pleasing to see that, despite billions of years of autotrophic life, the eukaryotes are hanging on to their phagotrophic machinery ... just in case.

[*] Actually, as MVZ pointed out, people have known for a long time (probably for almost as long as they've known about microplankton) that eukaryotic phytoplankton engage in grazing activity. What's new here is that the scale of this has been established. I certainly wouldn't have expected algae to be such enthusiastic players in the grazing business. It's definitely something for future generations of ecosystem models ...

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