Feeding experiments of the seep-associated foraminifer Nonionellina labradorica with a marine methanotroph from the Arctic

Foraminifera on the seafloor are known to have species-specific feeding habits. Among those are deposit feeders, eating organic detritus and bacteria. Little is known about the feeding habits of foraminifera from Arctic seep environments. That is, in particular, of interest as variable δ¹³C values in the tests of foraminifera have been suggested to be partly linked with a diet rich in bacteria, themselves lighter in δ¹³C values. As there is little information on the ecology of the foraminifer Nonionellina labradorica (Dawson, 1860), this study examined feeding habits on bacteria and compared them to in situ collected specimens, using Transmission Electron microscopy (TEM). As bacterial food, the marine methane-oxidizing bacterium Methyloprofundus sedimenti was chosen, which is an important representative of methanotrophs in the marine environment near methane seeps. Sediment samples containing living N. labradorica specimens collected in close vicinity(approx. 5 m) from an active methane seep in Storfjordrenna, Barents Sea (382-m water depth).  We performed a feeding experiment on N. labradorica (n=17 specimen), which were incubated in the dark at in situ temperature. Specimens were fed at the beginning of the experiment, except the un-fed controls, and incubations terminated after 4, 8 and 20 h. After fixation in epoxy resin the ultrastructure of all specimens and their food vacuoles was observed and compared using a TEM. All examined specimens were living at the time of fixation, based on observation of intact mitochondrial membranes. In all specimens, inorganic detritus was preserved inside food vacuoles. Closer observation of food vacuoles also revealed that in addition to inorganic debris, such as clay, occasionally bacteria were visible. This led us to conclude that our N. labradorica can  generally be classified as a deposit feeder, which is rather a generalist than a specialist. Regarding uptake of M. sedimenti, the timing of the experimentation seemed to be critical. We did not observe methanotrophs preserved in the resin at the 4 and 8 h incubations, but found two putative methanotrophs near the apertural region after the 20-h incubation. After closer observation, we could identify one of those two putative specimen as the menthanothroph M. sedimenti near the foraminiferal aperture, based on presence of a typical type I stacked intracytoplasmic membrane (ICM) and storage granules (SC). We concluded that N. labradorica may ingest M. sedimenti via “untargeted grazing” in seeps. Further studies must examine the exact relationship between diet and δ¹³C in foraminiferal test on several different paleo-oceanographically relevant species.