Nitrogen cycling and bacterial community structure of sinking and aging diatom aggregates

ABSTRACT: Sinking phycodetrital aggregates can contribute to anaerobic nitrogen turnover as they may represent oxygen-depleted microbial hot spots in otherwise oxygenated waters. However, the dynamics of anaerobic nitrogen cycling during the long descent of aggregates through oxic or hypoxic waters are unknown. Thus, model aggregates prepared from the diatom Skeletonema marinoi were allowed to age for 4 d at high and low ambient O2 levels (70 and 15% air saturation, respectively), and changes in nitrogen transformations and microbial community structure were followed. At both O2 levels, denitrification and dissimilatory NO3– reduction to NO2– (DNRN) were the most important processes of aggregate-associated anaerobic nitrogen cycling. However, at 70% air saturation, rates of anaerobic N cycling were lower and decayed towards 0 after an early rise, whereas at 15% air saturation, they remained constantly high at average production rates of 0.66 nmol N2-N aggregate (aggr.)–1 h–1 and 0.26 nmol NO2– aggr.–1 h–1. At both O2 levels, but more pronouncedly at 70% air saturation, the microbial community underwent succession as expressed by an increase in (1) relative abundance of specific bacterial taxonomic units; (2) bacterial diversity; and (3) prokaryotic abundance. Probably, a higher carbon oxidation rate at high ambient O2 level progressively selected for microbes capable of using complex carbon polymers. Taken together, the occurrence of anoxic aggregate centers may be ephemeral at high ambient O2 levels, but persistent at low ambient O2 levels, indicating that sinking phycodetrital aggregates can remain sinks for bioavailable N in the oceans for several days, especially in O2‑depleted settings