Enzymatically hydrolysed protein and carbohydrate pools in deep-sea sediments: estimates of the potentially bioavailable fraction and methodological considerations

In this study we developed existing protocols for the enzymatic digestion o f protein and carbohydrate pools in deep-sea sediments in order to estimate the organic fraction potentially bioavailable for benthic consumers. Sedim ent samples were collected in an abyssal locality (Porcupine Abyssal Plain, 4800 m depth) of the NE Atlantic in September 1996, and March and October 1997, and vertically divided into 5 layers down to a depth of 15 cm. Protei n and carbohydrate concentrations released enzymatically were highest after 2 h of incubation for both organic pools. The release of enzymatically hyd rolysed proteins from the deep-sea sediments was significantly higher using Tris+EDTA and sonication instead of a simple sediment homogenisation in Na -phosphate buffer. Hydrolysed carbohydrate concentrations were higher utili sing alpha-amylase, beta-glucosidase, Proteinase K and lipase instead of on ly alpha-amylase and beta-glucosidase. Hydrolysed proteins and carbohydrate s accounted for a small fraction of the total protein and carbohydrate pool s (on average 15 and 13%, respectively), indicating that total protein and carbohydrate concentrations do not represent the actual available fraction of the sedimentary organic matter in this deep-sea environment. Hydrolysed protein pools changed significantly between sampling periods whereas carboh ydrate pools appeared to be more conservative both spatially and temporally . Hydrolysed protein and carbohydrate concentrations were characterised by different vertical patterns and carbohydrate concentrations were much highe r in deeper sediment layers. These results indicate that the 2 classes of o rganic compounds might play different roles in different sediment layers wi th important trophodynamic implications for benthic organisms living at dif ferent sediment depths.