What is the upper size limit for cosmopolitan distribution in free-living microorganisms?

A comparison of restate amoebae assemblages from the Arctic and Antarctic ( areas of similar habitat a maximum distance apart) is used to try and answe r the question 'What is the upper size limit for cosmopolitan distribution in free-living microbes?' Species restricted to either the Arctic or Antarc tic exhibited sizes up to 230 mum while the largest cosmopolitan species wa s 135 mum in size. Comparison of the restate assemblages using a multivaria te classificatory technique (TWINSPAN) also suggested more restricted distr ibution for the larger species. There was a negative relationship between s pecies size and number of sites at which it was recorded (r(s) = -0.261, P < 0.05), with all the more widespread species having a size of below 100 <m u>m. It is suggested that for restate amoebae cosmopolitan distributions be come common below 100-150 mum. This suggests that most species of restate ( indeed most free-living microbes) have low species richness because of lack of opportunities for allopatric speciation as most are below 100 Irm and s o geographical isolation is unlikely. It is suggested that if this is corre ct, only the largest free-living microbes (> 150 mum) are likely to be of c onservation concern because of their smaller ranges. However, I point out t hat currently different studies are giving very different answers to the qu estion, how ubiquitous and species rich are free-living microbes? The subje ct requires further work to try and reconcile these different results.