Feeding behaviour of a benthic species (Ammonia tepida) under oxic and anoxic conditions: TEM-NanoSIMS correlation

More and more marine areas are subjected to depleted-O2 concentration, mainly due to eutrophication induced by human activities. These phenomena affect benthic ecosystems, in particular continental shelves and coastal areas where the renewal of the bottom waters is low and/or the organic matter flux is high. Some species of benthic foraminifera are resistant to this hypoxic/anoxic events, surviving weeks to months without oxygen. One of the species known to survive to such extreme condition is the benthic one Ammonia tepida. However the metabolic process by which it succeed in it remains unknown. One hypothesis is that this species would be able to lower its metabolism under oxygen depleted conditions, until the return of better conditions. The aim of the present study is to understand the feeding behaviors of A. tepida under anoxia. For this purpose a laboratory experiment involving incubation with living A. tepida was carried out under controlled oxygen concentrations. The individuals were fed with a pulse of 13C-labeled diatoms (Navicula sp.) and then incubate in oxic or anoxic conditions during 28 days. Along the incubation time, observation of the cell ultrastructure with Transmission Electronic Microscope (TEM) and detection in the cell of the isotopic 13C signal using nanoSIMS, were made. The nanoSIMS (nanoscale Secondary-Ion Mass Spectrometry) is an analytical technique that allows to visualize the incorporation and transfer of isotopically labeled compounds in organisms; thus in our case to follow the ingestion of labeled-diatoms and further transfer of carbon in the cytoplasm of the foraminifera. According to the results acquired in this study, in both conditions the foraminifera directly integrate the diatoms in their cytoplasm during the first hours of the incubation. Then, in oxic conditions, these diatoms are quickly fully degraded, within 3 days, and the 13C-labeled compounds are transferred in the organelles of the cytoplasm. Whereas in anoxia, only a fraction of the diatoms are degraded and the transfer to the foraminifera cytoplasm is weak. These results are confirmed by the δ13C of the foraminifera, measured in bulk by mass spectrometry (GC-MS). In anoxia the δ13C is slightly increasing the first 24h, meaning that the 13C content of the cell increased, and then it remains stable until the end of the experiment. This leads us to assume that the foraminifera stop, or at least strongly lower their metabolic rate under anoxic condition.