Copepod carcasses as microbial hot spots for pelagic denitrification

Copepods are exposed to a high non-predatory mortality and their decomposing carcasses act as micro-niches with intensified microbial activity. Sinking carcasses could thereby represent anoxic microenviron-ment sustaining anaerobic microbial pathways in otherwise oxic water columns. Using non-invasive O2imaging, we document that carcasses of Calanus finmarchicus had an anoxic interior even at fully air-saturated ambient O2level. The extent of anoxia gradually expanded with decreasing ambient O2levels.Concurrent microbial sampling showed the expression of nitrite reductase genes (nirS) in all investigatedcarcass samples and thereby documented the potential for microbial denitrification in carcasses. The nirSgene was occasionally expressed in live copepods, but not as consistently as in carcasses. Incubations of sink-ing carcasses in15NO23amended seawater demonstrated denitrification, of which on average 34% 6 17%(n 5 28) was sustained by nitrification. However, the activity was highly variable and was strongly dependenton the ambient O2levels. While denitrification was present even at air-saturation (302 lmol L21), the averagecarcass specific activity increased several orders of magnitude to  1 nmol d21at 20% air-saturation (55 lmolO2L21) at an ambient temperature of 78C. Sinking carcasses of C. finmarchicus therefore represent hotspots ofpelagic denitrification, but the quantitative importance as a sink for bioavailable nitrogen is strongly depend-ent on the ambient O2level. The importance of carcass associated denitrification could be highly significantin O2depleted environments such as Oxygen Minimum Zones (OMZ).