Do trophic strategies shape biogeography and environmental niches? Marine dinoflagellates as a case study

Marine unicellular eukaryotes (protists) exhibit a wide spectrum of trophic strategies ranging from specialists (strict phototrophy or strict phagotrophy) to generalist (mixotrophy). Generalist strategies enable flexibility in nutrient sources, which impacts biogeochemical cycles, energy fluxes in planktonic food webs as well as species biogeography. Dinoflagellates exhibit specialist and generalist trophic strategies, making them a key group for studying the ecological success of trophic traits from a biogeographical perspective. Yet, our understanding of what drives their biogeography remains limited although they are a major component of planktonic communities. Here, we combine one of the largest environmental genomics databases with state-of-the-art species distribution modelling to test whether trophic dinoflagellate specialists exhibit distinct spatial distributions and abiotic drivers compared to generalists. Based on field observations alone, we find that dinoflagellate species show similar abundance and evenness patterns, regardless of their trophic strategies. However, our models reveal differences in environmental niches at the trait level: mixotrophy is favoured in tropical oligotrophic regions whereas strict phagotrophy is favoured in the productive high-latitudes. At the species level, mixotrophs show similar responses across gradients of nutrient availability, whereas species responses to abiotic gradients are more divergent within strict phagotrophs. The latter pattern is consistent with a trait scenario of multiple evolutionary convergences. We show that trophic classification effectively explains the distribution patterns and environmental responses of generalists but is less effective in capturing the diverse responses of specialists that could result from other factors (evolutionary history, biotic interactions, cell size).