Environmental association of heterotrophic micro-eukaryotes in the varying biogeochemical regimes of the Arabian Sea, resolved via high-throughput sequencing

The diverse physicochemical conditions prevailing in the Arabian Sea are expected to result in marked spatial variations in heterotrophic flagellate (HF) and ciliate communities. Here, we report the environmental association of heterotrophic micro-eukaryotes, particularly the heterotrophic flagellates and ciliates, based on 18S rRNA gene survey in the region. High-throughput next-generation sequencing, using the V4 eukaryotic-specific primer, was employed to study the composition of these communities associated with low-O2 waters in both coastal and offshore settings. Canonical correspondence analysis (CCA) revealed a preference of the heterotrophic flagellates for nitrate- and nitrite-rich zones. Notably, the heterotrophic nanoflagellate genus Monosiga showed a strong positive correlation with NO3−, which suggests its potential denitrifying capability. Shannon’s entropy analysis revealed a higher HF diversity in the hypoxic waters of the open ocean (depth 103 m), whereas ciliates were more diverse at oxygenated coastal stations. The estuarine waters exhibited a low diversity of both ciliates and flagellates. The UPGMA clusters of heterotrophic flagellates and ciliates in suboxic waters of the open ocean oxygen minimum zone were distinct from those found at other sites. Overall, CCA revealed the important relationship between nitrite, nitrate, salinity and chlorophyll a, which could be important factors for the partitioning of different ecological niches for specific HF and ciliate communities in the Arabian Sea. The community of heterotrophic protists that can adapt to varying biogeochemical regimes has been identified.