Processing of 13C-labelled diatoms by a bathyal community at sub-zero temperatures

ABSTRACT: The carbon (C) budget in bathyal permanently cold sediments was assessed by means of a pulse-chase experiment in the deep Faroe-Shetland Channel (FSC). The food pulse was simulated by adding 500 mg C m–2 of the 13C-labelled marine diatom Chaetoceros radicans to sediment cores retrieved from 1080 m in the FSC. The fate of the tracer was followed over 6 d into the dissolved inorganic C pool (DI13C) as well as the bacterial and faunal (metazoan macrofauna and meiofaunal-sized nematode) biomass. After 3 d of incubation, 14.9 and 0.81 mg C m–2 of the algal C was recovered from bacterial and faunal biomass, respectively, while only 3.8 mg C m–2 was respired. Respiration was the dominant tracer pathway after 6 d of incubation (44 mg C m–2). Bacterial tracer uptake did not increase significantly between Days 3 and 6. The tracer recovered from metazoan fauna at the end of the experiment constituted 3.2% (2 mg C m–2) of the total processed C, with meiofauna contributing only ~1% to the total metazoan uptake. The bacterial response was characterised by varying bacterial growth efficiency (BGE). During the first half of the experiment, low respiration and high bacterial uptake of the 13C-labelled substrate resulted in particularly high BGE, while the opposite was observed in the second half of the incubation. We postulate that the high BGE at the start of the experiment represents the absorption and metabolism of the readily available labile components of the added organic matter (OM). The decrease in BGE possibly corresponds to the initiation of the energetically costly hydrolytic processes necessary for the consumption of more recalcitrant OM