Algal organic matter alters protistan community structure and assembly processes in coastal sediments

Diatom blooms are a global ecological perturbation that releases algal organic matter (AOM), significantly affecting coastal ecosystems by altering microbial community dynamics. AOM, derived from algal cell lysis, may serve as a nutrient source influencing protistan communities. However, the effects of AOM on protistan ecology, including the community structure and assembly processes, remain largely unexplored in coastal sediments. In this study, we investigated the impact of AOM on the protistan community structure using macrogenomic analysis and high-throughput sequencing. The results revealed significant shifts in the protistan diversity (alpha and beta diversity) and community composition. Phototrophs and consumers were the primary functional groups affected, with their relative abundances significantly regulated by AOM, highlighting its functional-level impacts. Moreover, AOM influenced also the protistan community assembly, increasing the proportion of deterministic processes and altering the dynamic succession within the protistan co-occurrence network. Diatom blooms act as ecological filters, reducing diversity while promoting the dominance of specific functional groups. This study bridges the gap in understanding the AOM's role in shaping the ecological succession of protists in coastal sediments, offering valuable insights into the broader ecological impact of marine diatom blooms.