Quantification of methanogenic Archaea within Baltic Sea copepod faecal pellets

Methane enrichments are frequently observed in the oxic upper water column of the central Baltic Sea during summer months. However, methane sources as well as the fate of methane produced in surface near waters still remain unclear. In the present study, we conducted ship-based grazing experiments to examine the presence of methanogenic archaea in copepod faecal pellets. We quantified bacterial and archaeal 16S rRNA and the mcrA gene and transcripts within copepod faecal pellets by using droplet digital polymerase chain reaction. We showed that the pellets (< 150-µm) harbour a small number of methanogenic archaea; however, mcrA transcripts indicating methanogenic activity were not detected. This suggests that copepod faecal pellets from the central Baltic Sea, similar to analogous data on copepod guts, harbour the potential but are an unlikely hotspot for methane production by methanogenic archaea.