An interdisciplinary approach to assess the functional diversity of free-living microscopic eukaryotes

Abstract

Free-living microbes are arguably ubiquitously dispersed. Molecular phylogeographies have been used to assess microbial distributions but can be difficult to interpret. Combinations of molecular and ecophysiological data have, however, proven useful in assessing the influence of dis- persal, local adaptation and historical contingency on distributions. A logical extension of this approach is to develop molecular markers for ecophysiological traits, which would allow the assess- ment of adaptations in large numbers of environmental isolates without requiring extensive cultur- ing. To assess this approach, we compared enzyme activity and gene expression of Na + /K + ATPase in response to salinity, and compared Na + /K + ATPase mRNA sequences, in 3 Brachionus plicatilis (Rotifera) sibling species. B. plicatilis siblings display different salinity tolerances, which in turn influence their distributions. Na + /K + ATPase is an important component of salinity tolerance and a potentially useful marker for ecophysiological variation. In all cases, Na + /K + ATPase enzyme activity and gene expression increased in response to salinity (5 to 50‰) and paralleled growth rate differ- ences: highest enzyme activity/gene expression occurred in the sibling species that displayed high- est growth. However, sequence variation in Na + /K + ATPase mRNAs was minor (~4%) and did not match gene expression patterns; thus, differences between siblings in Na + /K + ATPase occurred as a result of differential expression of highly similar gene transcripts. While we were not able to develop functional molecular markers for salinity tolerance in B. plicatilis, we highlight that the application of phylogenetic and functional markers will be a powerful tool for assessing the distributions of free- living microorganisms.