The Social Life of Phytoplankton

i-fac553c2c999c1b81fc89dcfc4c3c259-Ocean bloom (2).jpg
Some of the best moments in my job as a Weizmann science writer are the times when a scientist I'm interviewing slips in a finding that shifts my understanding of how the world works. Not long ago, for instance, I was speaking with a researcher about his work on phytoplankton. Now, the fact that phytoplankton release about half of all the free oxygen on the planet should be an eye-opener to anyone, and a reminder of the importance of ocean health. (But that wasn't the surprise.)

Dr. Assaf Vardi researches the chemicals that phytoplankton produce to communicate. Of course, all sorts of bacteria have been shown to communicate, some of them in quite sophisticated ways, so the fact that phytoplankton send each other chemical signals shouldn't be too big a surprise. It does, however, put a new spin on the social life of phytoplankton (the word means "plant drifters"), especially when you contemplate the fact that their messaging system can affect the whole aquatic food chain. Phytoplankton are the very bottom of the chain, on the one hand, so their numbers count, but they can also multiply out of control, choking off waterways or forming toxic "red tides," on the other. (In fact, Vardi thinks that the toxins produced by red tides may be a kind of "infochemical.")

Here's the bit made me raise my eyebrows: In both phytoplankton and the viruses that infect them, Vardi found genes for apoptosis - cell suicide. Why is this surprising? Apoptosis is a basic mechanism - though quite a complex one - known from multi-celled organisms, in which badly damaged cells kill themselves for the greater good of the whole. Why would single-celled organisms need a suicide option? Vardi has yet to figure out exactly what's going on in the phytoplankton, but it does imply that those mindless little drifters are evolved to the point where some of them will take a hit for their buddies. I don't know about you, but that one certainly gives me pause.

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Just because there are genes of apoptosis it doesn't mean that phytoplankton actually exhibits that function. From a wider point of view Dr. Vardi's observations provide more proof that we are all "connected" somehow.

I love hearing from scientists outside my own research area. I recently spoke to a final year PhD student about diatoms; his enthusiasm for something we rarely hear about was incredible.

If apoptosis genes were expressed in an individual you'd have to be incredibly lucky to find that individual since it would sink pretty quickly.

It's probably a form of kin selection. As members of the same bloom are highly related a virus entering the population could be catastrophic to the population. Suicide would reduce the chances of that virus spreading and allow the common genes to be inherited....

Hi Derek,

You are off course correct - finding the genes themselves doesn't mean PCD is actually used/activated. But further research by Dr. Vardi (see: "Viral Glycosphingolipids Induce Lytic Infection and Cell Death in Marine Phytoplankton") showed that purified glycosphingolipids caused E. huxleyi cells to activate the programmed cell death pathway..
Cells mixed with the GSL showed compromised photosynthetic efficiency of PSII and the more common sign of PCD - elevated in vivo caspase activity after 48 hours. Furthermore, massive cell lysis occured only in the cells treated with the GSL.

This research should be continued. From what I've read just now, phytoplankton plays a huge role in the oceanic world. They reproduce and serve as a food for some sea creatures but when they multiply greatly they'll be also become a harm.