From river to ocean: Connectivity and heterogeneity of aquatic ecosystems depicted by planktonic microeukaryotes

Understanding how the diverse habitats incubated a high diverse biosphere is a core question for biodiversity conservation. Community assembly of organisms in connected but different habitats could provide insight into this issue. As an important group of plankton, microeukaryotes are widely distributed in aquatic habitats and are sensible to ecological conditions. However, how the distribution of planktonic microeukaryotes from river to ocean are shaped by habitat connectivity and heterogeneity remains unclear. In order to assess the connectivity and heterogeneity of aquatic habitats, environmental DNA sequencing was used to profile the planktonic microeukaryotic communities in typical habitats throughout the major rivers, estuaries, coastal waters, and ocean of China, covering a geographic distance of 3530 km. The hypothesized "freshwater species pool (FSP)" and "marine species pool (MSP)" of planktonic microeukaryotes were confirmed by their significant dissimi-larities in community composition. The planktonic microeukaryotic communities completed turnover of FSP-pattern and MSP-pattern within salinity range of 5-15 in estuaries of the river-to-ocean continuum. In this scale covering fresh and salty waters, the community assembly of planktonic microeukaryotes was driven pri-marily by habitat filtering rather than salinity nor geographic distance. Four types of habitat, i.e., river, estuary, tidal zone, and ocean, were outlined by their planktonic microeukaryotic communities, which depicted the habitat heterogeneity in this continuum. As to the constructive species in these habitats, 98.1 % of them were shared between at least two types of habitat and 41.3 % of them shared among all habitats. These shared constructive species depicted the habitat connectivity of aquatic ecosystems from river to ocean. The findings suggest that the assessment of planktonic microorganism assemblages is a potential approach to indicate habitat diversity in connected aquatic ecosystems.