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Hypotheses on the role of the protistan rare biosphere in a changing world

Studies of protistan diversity have gained impetus in microbial ecology during the last decade due primarily to the widespread application of molecular approaches developed initially to study bacteria, archaea and viruses. These investigations have revealed many undescribed taxa and lineages of protists, and have indicated the presence of large numbers of rare taxa (the microbial rare biosphere) among the relatively few abundant species that dominate these assemblages in any particular environment. As a consequence of this incredible diversity, the species richness of most natural protistan assemblages is still poorly characterized. One question that has arisen from these studies is whether the taxa comprising the rare biosphere play significant roles in community func- tion, or merely represent inactive, moribund or dead individuals that have little or no ecological importance. We hypothesize that many of the protistan rare taxa can and do become dominant with changing environmental conditions, and that the ecological redundancy represented by these spe- cies indicates a highly adaptable microbial eukaryotic community that can maintain biogeochemical processes during significant and perhaps dramatic shifts in environmental forcing factors. If true, the structure of protistan assemblages (and presumably other microbial assemblages) may act to buffer the impact of changing environmental conditions by preventing wholesale changes in basic ecosys- tem processes. However, it is not clear whether many of the species that might dominate under altered environmental conditions (e.g. toxic or noxious species of protists) would support the same macrobial assemblages that existed previously in those environments.