Diversity and assembly processes governing abundant microbial eukaryotic Communities along environmental gradients in the West Philippine Sea

Pagaduan, Jeno Rey R.

Microbial eukaryotic communities (MECs) are key players in marine trophic interactions and global biogeochemical cycles. However, despite their significance, studies on marine microbial eukaryotic diversity and the processes that affect their community structures remain limited especially in the context of Philippine waters. In this study, the diversity of MECs in distinct habitats of West Philippine Sea (WPS) and potential drivers shaping their community structure were investigated and identified. Samples were collected from different habitats including an embayment influenced by riverine and anthropogenic inputs (Manila Bay), a seagrass bed (Bolinao), and offshore reef ecosystems (Pagasa Island and Sabina Shoal). Profiles of the dominant members of the eukaryotic microbial communities were generated by high-throughput sequencing of the V4 region of the 18S rRNA gene. Physicochemical parameters and beta diversity indices clustered the samples into three distinct groups (MB, BOL, and OFFSHORE). Microbial communities of each habitat correlated with the measured environmental parameters, consistent with the deterministic rule of community assembly. In particular, nutrient levels were found to be the most important driver of community structuring. Further, the investigated habitats also represented environmental gradients based on nutrient levels— MB with high nutrient levels, BOL with intermediate levels, and OFFSHORE with low nutrient levels. Alpha diversity of MECs generally decreased along an increasing nutrient gradient. In contrast, phylogenetic clustering, inter-OTU connections, and proportion of autotrophic OTUs increase with nutrient levels. Analysis of co-occurrence patterns also identified potential food web interactions of detected OTUs, approximating their role in transfer of matter and energy. This study identified habitats in WPS with distinct physicochemical characteristics and provided a baseline community structure of microbial eukaryotes in these sites. Observed patterns in their community structure and community-level interactions along a nutrient gradient sheds light on the potential alterations that may occur in MECs in spite of shifting nutrient regimes.