| Yhteenveto: | Microbes drive global biogeochemical cycles and ecosystem functions, thus
providing essential ecosystem services. Yet, most microbial species are rare. While there is evidence that rare species make significant contributions to
ecosystem functions, the distribution of rare microbial taxa remains
uncharacterised, especially in terms of rare species’ functional traits. Identifying patterns of rarity and quantifying functional rarity in microbes is important for defining other theoretical frameworks, identifying areas or species in need of conservation, and predicting environmental change and ecosystem response. Here, I determined (1) latitudinal gradients and hotspots of rarity for marine microbes (bacteria) and (2) whether rare taxa could provide different functions to more abundant taxa. Microbes have been hypothesised to follow similar ecological patterns as macroorganisms, and if this is the case, I hypothesised that most rare taxa should be found at the middle latitudes and the tropics, and a portion of the rare biosphere should also be functionally rare (i.e. possess distinct and unique functional traits). Utilising sequence data from the Tara Oceans expedition and a bioinformatics pipeline, I found that rarity correlates with latitude on a hump-shaped pattern, with the highest incidence of rare taxa found at middle latitudes. Functional distinctiveness did not show substantial variation between communities, but the functional diversity of rare taxa appears to be greater than that of more abundant taxa. Furthermore, phylogenetic clustering of functional distinctiveness was identified in phyla that are involved in major biogeochemical cycles. These results indicate areas that may be significant hotspots of biodiversity and important for ecosystem function, but also susceptible to ecosystem change and disturbance and thus need more attention. Altogether, these results give novel insights into latitudinal patterns of rarity and functional rarity in microbial communities and call for further studies with more extensive sampling to understand the effects of anthropogenic activities and climate change on microbial communities and ecosystem response.
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