Macrofauna regulate heterotrophic bacterial carbon and nitrogen incorporation in low-oxygen sediments

Oxygen minimum zones (OMZs) currently impinge upon >1 million km ² of sea floor and are predicted to expand with climate change. We investigated how changes in oxygen availability, macrofaunal biomass and retention of labile organic matter (OM) regulate heterotrophic bacterial C and N incorporation in the sediments of the OMZ-impacted Indian continental margin (540-1100 m; O 2 0.35-15 μmol l 1). In situ pulse-chase experiments traced 13 C: 15 N-labelled phytodetritus into bulk sediment OM and hydrolysable amino acids, including the bacterial biomarker D-alanine. Where oxygen availability was lowest (O 2 0.35 μmol l 1), metazoan macrofauna were absent and bacteria assimilated 30-90% of the labelled phytodetritus within the sediment. At higher oxygen levels (O 2 2-15 μmol l 1) the macrofaunal presence and lower phytodetritus retention with the sediment occur concomitantly, and bacterial phytodetrital incorporation was reduced and retarded. Bacterial C and N incorporation exhibited a significant negative relationship with macrofaunal biomass across the OMZ. We hypothesise that fauna-bacterial interactions significantly influence OM recycling in low-oxygen sediments and need to be considered when assessing the consequences of global change on biogeochemical cycles.