Hydrolytic ectoenzyme activity associated with suspended and sinking organic particles within the anoxic Cariaco Basin

Ectohydrolase activities of suspended microbiota were compared to those associated with sinking particles (sed-POM) retrieved from sediment traps deployed in the permanently anoxic Cariaco Basin. In shore-based assays, activities of aminopeptidase, β-glucosidase, chitinase and alkaline phosphatase were measured in samples obtained from oxic and anoxic depths using MUF- and MCA-labeled fluorogenic substrate analogs. Hydrolysis potentials for these enzymes in the seston varied widely over the nine cruises sampled (8 Nov 1996–3 May 2000) and among depths (15–1265 m); from <10 to over 1600 nM d−1 hydrolysate released, generally co-varying with one another and with suspended particulate organic carbon (POC) and particulate nitrogen (PN). Hydrolytic potentials, prokaryotic abundances and POC/PN concentrations in sinking debris were 400–1.3×107 times higher than in comparable volumes of seawater. However when normalized to PN, hydrolytic potentials in sediment trap samples were not demonstrably higher than in Niskin bottle samples. We estimate that PN pools in sediment trap samples were turned over 2–1400 times (medians=7–26x) slower by hydrolysis than were suspended PN pools. Median prokaryotic growth rates (divisions d−1) in sinking debris were also ∼150 times slower than for bacterioplankton. Hydrolytic potentials in surface oxic waters were generally faster than in underlying anoxic waters on a volumetric basis (nM hydrolysate d−1), but were not significantly (p>0.05) different when normalized to PN or prokaryote abundances. Alkaline phosphatase was consistently the most active ectohydrolase in both sample types, suggesting that Cariaco Basin assemblages were adapted to decomposing phosphate esters in organic polymers. However, phosphorus limitation was not evident from nutrient inventories in the water column. Results support the hypothesis that efficiencies of polymer hydrolysis in anoxic waters are not inherently lower than in oxic waters.