Genetic Diversity of Benthic Microbial Eukaryotes in Response to Spatial Heterogeneity of Sediment Geochemistry in a Mangrove Ecosystem

Little is known about the diversity, community composition, or distribution of benthic microbial eukaryotes in organic carbon (OC)-rich mangrove sediments. We hypothesized that the distribution of microeukaryotes was related to the spatial heterogeneity of mangrove sediment geochemistry due to tidal zonation and the rhizosphere effect. A range of geochemical properties of surface sediments were characterized, and alpha and beta diversities of microeukaryotes in a mangrove ecosystem were investigated using MiSeq sequencing of 18S rRNA genes. We found that the sequence proportions of Chlorophyta and Bacillariophyta were notably high in the datasets. Both operational taxonomic unit (OTU) richness and microeukaryotic community structure (MCS) were significantly different between the OC-rich upper tidal zone and the sulfate- and ammonium-rich lower tidal zone, indicating a strong response of microeukaryotic diversity to tidal zonation. The zonewise community differences were characterized by distinct shifts in the proportions and OTUs of chlorophytes, diatoms, and fungi. However, neither OTU richness nor MCS was significantly different between near-root and bulk sediments, though several geochemical parameters varied. Similarly, the assemblages of fungi showed a pattern of tidal zonation. Overall, variations in MCS in mangrove sediments were mainly driven by the quantity and quality of organic matter, grain size, and concentration of sulfate.