Changes in the microbial community associated with the large benthic foraminifera Cycloclypeus carpenteri, along a depth gradient

Cycloclypeus carpenteri is one of the deepest living large benthic foraminifera. It has an obligatory relationship with diatom photosymbionts, and, in addition, houses a diverse prokaryotic community. Variations in the eukaryotic and prokaryotic endobiotic community composition might be key in allowing Cycloclypeus to occur in low light environments. We assessed the variability of the prokaryotic and eukaryotic communities associated with Cycloclypeus along a depth gradient from 50 to 130 m at two locations in the Federated States of Micronesia (Northwest Pacific) by metabarcoding of the 18S V9 rRNA region for eukaryotes and the 16S V3-V4 rRNA region for prokaryotes. We observed a single foraminiferal operational taxonomic unit (OTU), as well as a single dominant diatom OTU that was abundant in all sequenced specimens. Both the prokaryotic and the eukaryotic endobiotic communities (excluding the dominant diatom) changed with water depth and associated irradiance levels. We observed a distinct change in the prokaryotic community composition around 90–100 m water depth at Pohnpei, equivalent to ∼1% surface radiation. This change in microbial communities in the Cycloclypeus holobiont suggests a potential role of the associated microbial communities in accommodating differences in (micro)- habitat, although we cannot exclude that the prokaryote community is to a large extent driven by their community composition in the ambient environment.