Deposit-feeding of Nonionellina labradorica (foraminifera) from an Arctic methane seep site and possible association with a methanotroph

Several foraminifera are deposit feeders that consume organic detritus (dead particulate organic material with entrained bacteria). However, the role of such foraminifera in the benthic food web remains understudied. Foraminifera feeding on methanotrophic bacteria, which are C-13-depleted, may cause negative cytoplasmic and/or calcitic delta C-13 values. To test whether the foraminiferal diet includes methanotrophs, we performed a short-term (20 h) feeding experiment with Nonionellina labradorica from an active Arctic methane-emission site (Storfjordrenna, Barents Sea) using the marine methanotroph Methyloprofundus sedimenti and analysed N. labradorica cytology via transmission electron microscopy (TEM). We hypothesised that M. sedimenti would be visible post-experiment in degradation vacuoles, as evidenced by their ultrastructure. Sediment grains (mostly clay) occurred inside one or several degradation vacuoles in all foraminifers. In 24 % of the specimens from the feeding experiment degradation vacuoles also contained bacteria, although none could be confirmed to be the offered M. sedimenti. Observations of the apertural area after 20 h incubation revealed three putative methanotrophs, close to clay particles, based on bacterial ultrastructural characteristics. Furthermore, we noted the absence of bacterial endobionts in all examined N. labradorica but confirmed the presence of kleptoplasts, which were often partially degraded. In sum, we suggest that M. sedimenti can be consumed via untargeted grazing in seeps and that N. labradorica can be generally classified as a deposit feeder at this Arctic site.