Kleptoplastidic benthic foraminifera from aphotic habitats: Assimilation (or not) of inorganic C, N, and S studied at sub-cellular resolution

The assimilation of inorganic compounds and their role in the foraminiferal metabolism compared to predation or organic matter assimilation is unknown. Here we investigate these processes with a study of the species Nonionellina labradorica (Dawson, 1860), a common kleptoplastic benthic foraminifer from Arctic and North Atlantic sublittoral regions, including aphotic zones of the Gullmar Fjord (Sweden). The objectives of this work were to (1) identify the origin of N. labradorica kleptoplasts, (2) assess their photosynthetic functionality for both oxygen production and conversion of inorganic carbon into photosynthates, and (3) investigate inorganic nitrogen and sulfate assimilation by N. labradorica. We used DNA barcoding to identify the origin of N. labradorica kleptoplasts and transmission electron microscope (TEM) imaging, correlated with nanometer-scale secondary ion mass spectrometry (NanoSIMS) isotopic imaging, to study 13C-bicarbonate, 15N-ammonium, and 34S-sulfate uptake. In addition, respiration rate measurements were determined to assess N. labradorica response to light. The partial 18S and 16S rDNA sequencing established that the kleptoplasts belonged to two species of Thalassiosira, a cosmopolitan planktonic diatom genus. TEM-NanoSIMS imaging revealed degraded cytoplasm in specimens exposed to light for 20 h and a complete absence of 13C assimilation. Oxygen measurements showed higher respiration rates under light than dark conditions, and no O2 production was detected. These combined results indicate that the photosynthetic pathways in N. labradorica are not functional. Under dark conditions, N. labradorica assimilated both 15N-ammonium and 34S-sulfate into its cytoplasm. Light exposure negatively impacted foraminiferal metabolism, ultimately causing specimen death. Our results suggest that foraminifera might have several ammonium or sulfate assimilation pathways, involving either the kleptoplasts or bona fide foraminiferal pathway(s) not yet identified.