The 16S rDNA microbiome of the Arctic foraminifera Neogloboquadrina pachyderma is comprised of hydrocarbon-degrading bacteria and a diatom chloroplast store

Abstract. Neogloboquadrina pachyderma is the only true polar species of planktonic foraminifera. It therefore plays a crucial role in the calcite flux, and in reconstructions and modelling of seasonality and environmental change within the high latitudes. The rapidly changing environment of the polar regions of the North Atlantic and Arctic Oceans poses challenging conditions for this (sub)polar species in terms of temperature, sea-ice melt, calcite saturation, ocean pH and contraction of the polar ecosystem. To model the potential future for this important high latitude species, it is vital to investigate the modern ocean community structure throughout the annual cycle of the Arctic to understand the inter-dependencies of N. pachyderma. We use 16S rDNA metabarcoding and TEM to identify the microbial interactions of N. pachyderma during the summer ice-free conditions in Baffin Bay. We demonstrate that the N. pachyderma diet consists of diatoms and bacteria. The core microbiome is defined as the 16S rDNA amplicon sequencing variants (ASVs) found in 80 % of individuals investigated. This core microbiome consists of two diatom chloroplast ASVs and seven bacterial ASVs and accounts for, on average, 50 % of the total ASVs in any individual. The bacterial ASVs represent hydrocarbon-degrading bacteria, including those found routinely in the diatom phycosphere. On average the two chloroplast ASVs compose 40 % of the core microbiome. Significantly, an average of 55.7 % of all ASVs in any individual are of chloroplast origin. TEM highlights the importance of diatoms to this species, conclusively revealing that chloroplasts remain undigested in the foraminiferal cytoplasm in very high numbers, comparable to those observed in kleptoplastic benthic foraminifera. Diatoms are the major source of kleptoplasts in benthic foraminifera and other kleptoplastic groups, but this adaptation has never been observed in a planktonic foraminifer. Further work is required to understand the association between N. pachyderma, diatoms and their chloroplasts in the pelagic Arctic realm. It may confer an advantage to this species for survival in this extreme habitat, but it could also become compromised by the rapidly changing climate.