Intracellular Isotope Localization in Ammonia sp. (Foraminifera) of Oxygen-Depleted Environments: Results of Nitrate and Sulfate Labeling Experiments

Abstract

Some benthic foraminiferal species are reportedly capable of nitrate storage and denitrification, however, little is known about nitrate incorporation and subsequent utilization of nitrate within their cell. In this study, we investigated where and how much 15N or 34S were assimilated into foraminiferal cells or possible endobionts after incubation with isotopically labeled nitrate and sulfate in dysoxic or anoxic conditions. After 2 weeks of incubation, foraminiferal specimens were fixed and prepared for Transmission Electron Microscopy (TEM) and correlative nanometer-scale secondary ion mass spectrometry (NanoSIMS) analyses. TEM observations revealed that there were characteristic ultrastructural features typically near the cell periphery in the youngest two or three chambers of the foraminifera exposed to anoxic conditions. These structures, which are electron dense and ~200–500 nm in diameter and co-occurred with possible endobionts, were labeled with 15N originated from 15N-labeled nitrate under anoxia and were labeled with both 15N and 34S under dysoxia. The labeling with 15N was more apparent in specimens from the dysoxic incubation, suggesting higher foraminiferal activity or increased availability of the label during exposure to oxygen depletion than to anoxia. Our results suggest that the electron dense bodies in Ammonia sp. play a significant role in nitrate incorporation and/or subsequent nitrogen assimilation during exposure to dysoxic to anoxic conditions.

Changes in ultrastructural features of the foraminifera Ammonia spp. in response to anoxic conditions: Field and laboratory observations

“…In a study of Ammonia sp. (phylotype T6) by Nomaki et al (2016), cells were observed to incorporate more 15 N-labeled nitrate and 34 S-labeled sulfate under dysoxic (in the study defined as O 2 = 0.1-22 μmol l − 1 ) than anoxic conditions, indicating the reduced activity of foraminifera in anoxic conditions. Additionally, LeKieffre et al (2017) have shown, using nanoSIMS analysis, that Ammonia sp.…”

“…The morphology of the bacterial cells also differed between specimens of Ammonia sp. (phylotype T6) collected during different seasons from the same field site in Tokyo Bay (Nomaki et al, 2014(Nomaki et al, , 2016. The ultrastructural observations, therefore, suggest that during anoxic exposure, Ammonia spp.…”

“…(phylotype T6) are actively involved in amino acid synthesis and other nitrogenassimilation processes, and might not be related to anaerobic respiration. In a study by Nomaki et al (2016), multiple electron dense bodies (Fig. 8) were reported to co-occur with the possible bacterial endobionts, both in dysoxic and anoxic incubations, and observed to align along the cell periphery.…”

“…The different protocols may have led to slight variations in image quality and appearance of the organelles (Table 2). Nomaki et al (2016). c LeKieffre et al (2017).…”

“…For imaging, 3 oxic and 3 dysoxic specimens were used. Further details of the sample location and experimental design of Experiments are outlined in Nomaki et al (2014) and Nomaki et al (2016), respectively. For Experiment 3, Ammonia sp.…”