Fifteen important questions in the spatial ecology of diatoms

Recently, studies about the spatial distribution of microbial species and communities have increased substantially. Freshwater diatoms are used as one of the most popular model taxa in such macroecological studies, but the most exciting patterns in the spatial ecology of diatoms have not been reviewed. Here, we identify 15 important questions about freshwater diatom ecology, review the major findings, and suggest novel research avenues. We found that diatoms do not support classical latitudinal diversity gradient, species-area relationship or Rapoport's rule. We found some, albeit inconsistent support for an elevational diversity gradient, Bergmann's rule and niche conservation. Diatoms do support a positive interspecific abundance-occupancy relationship like nearly all other taxa. With regards to metacommunity dynamics, diatom communities are typically structured by species sorting, but neutral dynamics or mass effects are also evident at large or small spatial scale, respectively. Taken together, this review suggests that diatoms follow some classical biogeographical patterns similarly to larger taxa, but patterns are typically weaker than for macro-organisms. This may result because diatoms are typically relatively strongly controlled by local factors and often exhibit efficient stochastic dispersal processes both at small and large scales. More research is needed to build a more comprehensive view of diatom species-area relationships and latitudinal gradients in diversity, for example. In the future, well-replicated macroecological field experiments, diatom studies with intercontinental or global coverage, trait-based approaches, and DNA-based species identification would be fruitful avenues to shed more light into the spatial ecology of diatoms.