Everything, It Appears, Is Not Everywhere
The world’s oceans are biological soups, repositories of unseen, single-celled organisms that dominate the planet’s nutrient cycles and dictate its atmospheric composition. But given how important these microbes are to a functioning planet-wide ecosystem – and the rapid rate of environmental changes currently being inflicted upon such ecosystems – we know dangerously little about how community structure works. Why are certain organisms where they are, and what are the consequences of their distributions?
These are the guiding questions for microbial ecologists as they attempt to understand how single-celled organisms are distributed across our planet. It’s a daunting task, and with an estimated 3.6 x 1030 cells in the ocean to catalog, it would be helpful if more generalized rules of diversity could be developed. That way, you could collect basic data from your environment of interest – temperature, chemical concentrations, or ocean current speeds, for example – toss them into your unified theory of diversity, and produce a reliable description of numbers, types, and behaviors of likely microbial constituents.
Of course, it’s never going to be quite that easy; many of the fundamental tenets of microbial diversity are still very much up for debate. A recent study by Woo Jun Sul and his colleagues at the Marine Biological Laboratory represents a key result in the continued refinement of these generalized rules.
One of the most enduring principles of microbial ecology over the years has been Dutch scientist Lourens Baas Becking’s oft-cited (and oft-mistranslated) pronouncement that “everything is everywhere, but the environment selects.” It’s a pretty self-explanatory hypothesis, reflecting the notion that microbial species are not limited by dispersion, and that a site’s species profile results from winnowing down a comprehensive list of organisms based on environmental parameters. In other words, the reason we don’t see psychrophilic (cold-loving) microbes inside hydrothermal vent chimneys isn’t because they can’t get there, but rather because they can’t handle the heat.
Sul examined the Bass Becking hypothesis by comparing a null model in which there were no checks on dispersal with actual data gleaned from the International Census of Marine Microbes project. 4.23 million microbial gene sequences from 277 sites across the world’s oceans clustered into 65,545 distinct bins of diversity, forming the basis for one of the few rigorous looks at large-scale diversity gradients.
Next, the researchers examined the overlap in microbial community structure at increasingly distant, increasingly selective habitats. If Bass Becking holds, they contend, then organisms well suited to a demanding environment would have the same universal access to it regardless of its distance from a similar, already-colonized site. Instead, they found that the communities diverged as the distance between the sites increased, suggesting that something is inhibiting global dispersal forces.
Among other findings, the report also shows that the diversity of marine waters decreases as you move pole-ward from the equator. But, Sul writes, the trend of this decrease is more severe in the southern hemisphere than in the north, “suggesting that the tropics may serve as a barrier to bacterial dispersal.”
The team concludes that, in addition to environmental factors, dispersion limiting factors like constrained ocean currents and “physical barriers such as the landlocked nature of the Arctic Ocean” play key roles in governing microbial diversity.
Sul’s revised proclamation: “everything is not everywhere, and the environment selects.”