Free-Living Microorganisms? A Biogeographical Analysis of Testate Amoebae from Polar Areas

A long-standing debate in microbial ecology is<br />the extent to which free-living microorganisms exhibit<br />cosmopolitan distributions. We use a comparison of testate<br />amoebae communities in cold &ldquo;polar&rdquo; locations (Arctic,<br />Antarctic, and Tibet) to investigate how a microorganism&rsquo;s<br />size affects its probability of having a cosmopolitan<br />distribution. We show that the probability a given taxa<br />being reported in all three locations increases as testate size<br />decreases. Likewise, excluding those testates found only in<br />Tibet, very small testates (&lt;20 &mu;m) are more likely to occur<br />in both the Arctic and Antarctic than in either of these poles<br />alone. Attempting to correct for phylogeny reduces the<br />number of statistically significant relationships&mdash;both because<br />of decreased sample size and potentially real<br />phylogenetic patterns, although some size-dependent effects were still apparent. In particular, taxa found in both<br />the Arctic and Antarctic poles were significantly smaller<br />than congeneric taxa found only in Tibet. This pattern may<br />in part be due to habitat effects, with the Tibetan samples<br />being more likely to have come from aquatic sites which<br />may be more suitable for larger taxa. Overall, our analysis<br />suggests that, at least within testate amoebae, a cosmopolitan<br />distribution becomes increasingly common as median<br />taxon size decreases