Impact of wild fish faeces to deep sea dissolved organic carbon as revealed by laboratory incubations

Abstract

Wild fish faeces play an important role in mediating marine organic carbon cycle, but the faeces released dissolved organic matter (DOM) and its potentials in contribution to refractory DOM is unclear. The mixed wild fish (including mesopelagic fish) faecal samples from the northern South China Sea were collected and incubated up to 75 days using the waters collected at 800 m depth of the same site, to reveal the stability of the faeces released DOM. Our result showed that the faeces had a good DOM-release potential and dissolved organic carbon (DOC) was up to 150 μM in the first day and reached its maximum (172 μM) on day 3, which was in comparison with the control group (DOC <70 μM). In our incubations, the faecal samples contributed net DOC of 110 μM, in which 42 μM persisted till the end of the 75 days dark incubation. This meant the faeces-released DOC had a refractory fraction of 38% over the incubation. Total hydrolysable amino acids concentration significantly dropped, and amino acids molecular indicator (DI, AA C yield and D/L ratio) changes over the incubation indicated the fast transformation of DOM from labile to degraded stage. Network analysis was applied to the amino acids and our result showed that D-form amino acids were almost positively coupled with L-form amino acids. Glycine (in mol%) showed consistent accumulation along with DOM degradation. Also, glycine was at the key node position of the amino acids network, showing negative relation with all other amino acids enantiomers in both faeces group and control group. Our results proved that glycine had the potential in tracing the refractory fraction over the DOM degradation.