Aerobic Methane Production in the Mediterranean Sea.
Supervisors: Jana Milucka, Gaute Lavik, Judith van der Giessen (Max Plank, Bremen)
The crescent need for understanding methane (CH4) biogeochemical cycling has led to the discovery of a mechanism for aerobic CH4 production in the ocean, the C-P lyase pathway. Here, organic P sources, such as methylphosphonate (MPn), are utilized to overcome phosphate (PO3−4 ) limitation. In this context, the Mediterranean Sea, a region chronically limited in PO3−

4 and enriched in genes encoding MPn synthesis and C-P lyase enzymes, could be a possible hotspot for this pathway. In this work, it was demonstrated that microbial communities in the Mediterranean Sea produce CH4 in oxic conditions, as a consequence of metabolizing MPn through the C-P lyase pathway. Offshore, MPn-driven CH4 production rates decreased with depth due to weaker PO3−4 limitation in deeper waters. In the coast, higher rates of MPn-driven CH4 formation were observed in Posidonia oceanica meadows compared to offshore stations, one of the hypothesis is that the high nutrient uptake capacity of P. oceanica might deprive the surrounding waters from PO3−4 , promoting CH4 production as a consequence of more C-P lyase activity. In experiments with excess PO3−4 addition, MPn-driven CH4 production was sustained, suggesting that microbial communities might have become adapted to metabolize MPn as their primary P source. This result implies that MPn-driven CH4 production could be prevalent in periods of nutrient replenishment, such as winter mixing. Additionally, in situ MPn concentrations were determined to be below 200 pM, therefore it is suspected that (1) MPn might be removed from the environment as soon as it is released from dying or grazed cells, thereby not being accumulated and/or (2) MPn might be utilized directly from polysaccharide-MPn esters. To further evaluate the importance of this pathway in a future where nutrient limitation in surface waters is expected to increase due to stratification it is crucial to (1) quantify the bioavailable MPn, either as free or part of organic matter; (2) understand the role of P. oceanica meadows in this pathway; (3) identify microorganisms performing the C-P lyase pathway in this region; and (4) investigate the spatio-temporal variability of this pathway in more regions of the Mediterranean Sea.