Spora and Gaia: How Microbes Fly with Their Clouds

WD Hamilton is my favorite scientist of the 20th century. He may not have had the largest impact, nor may he have had the most awards, but his relentless creativity is an inspiration to us all. This week's citation classic is an example of his wide ranging interests.

WD Hamilton and TM Lenton. 1998. Spora and Gaia: How Microbes Fly with Their Clouds. Ethology, Ecology and Evolution 10:1-16.

In the paper, Hamilton and Lenton hypothesize that some microbes use chemical induction of water condensation (ice nuclei) to enhance their own dispersal between habitats. The bacteria are supposed to create ice nuclei by releasing a gas called dimethyl sulfide (DMS). But this is an example of a cheater-prone trait. Non-producers can reap the benefits of DMS production without paying the energetic costs. So why are microbes producing DMS?
The idea that algae might produce DMS to get themselves into the air occurred to Hamilton first. 'Tim had mentioned that DMSP has a possible function as an antifreeze,' he recalls. 'Now why would a cell in a tropical ocean need antifreeze? Perhaps they sometimes end up high in the air, shot up there by a waterspout. Or maybe there are other ways they could go. Convective energy created by cloud formation would help them.' Flying high, the algae would be exposed to very low temperatures. Idle speculation rapidly led to the formation of a theory that beautifully explains why algae produce DMS. 'Seldom have I had a run of reading where so many papers were relevant or connected and nothing contradicted my ideas," says Hamilton. "I felt certain that there was something interesting here.'

As the Hamilton and Lenton write, "Dispersal is extremely important to life, indeed for self or progeny, it can be considered an organism's third priority after survival and reproduction". Thus microbes may be creating wind, clouds, rain, and snow to carry them around the globe. A great synopsis of the article appeared in the New Scientist, when the paper was first published.

When I first heard about these ideas, I thought them sort of fuzzy, cute and entirely without substance.

But some recent reports are suggesting otherwise. An article by Brent Christner et al. in today's issue of Science shows that ice nucleators are microbial in origin.

"Despite the integral role of ice nucleators (IN) in atmospheric processes leading to precipitation, their sources and distributions have not been well established. We examined IN in snowfall from mid- and high-latitude locations and found that the most active were biological in origin. Of the IN larger than 0.2 micrometer that were active at temperatures warmer than -7C, 69 to 100% were biological, and a substantial fraction were bacteria. Our results indicate that the biosphere is a source of highly active IN and suggest that these biological particles may affect the precipitation cycle and/or their own precipitation during atmospheric transport."

One of the coolest aspects (to me) is that one of the organisms responsible may be Pseudomonas syringae, an organism I use in my own lab. Although it remains unclear which microbes may be most responsible for snowfall or rainstorms, one leading candidate is the plant pathogen Pseudomonas syringae, which infects wheat, corn and other crops. It is a major pest—and the target of genetic modification—because it causes immediate crop damage if the temperatures drop below freezing.

The reason I find this interesting is that I study P. syringae's phage: the cystoviridae. A recent report found that "phages isolated from single clovers were not consistently more similar to each other than to phages isolated from sites across the country [i.e. opposite coasts] or from other previously isolated phages... These data are thus consistent with frequent continent-wide migration in the Cystoviridae."

Olivia Judson writes about Life and Clouds.
A synopsis of Christner et al.'s work is available at Science Daily.
Photo from Nicholas T