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 of protein and carbohydrate pools in deep-sea sediments in order to estimate the organic fraction potentially bioavailable for benthic consumers. Sediment 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. Protein and carbohydrate concentrations released enzymatically were highest after 2 h of incubation for both organic pools. The release of enzymatically hydrolysed 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 utilising alpha-amylase, beta-glucosidase, Proteinase K and lipase instead of only alpha-amylase and beta-glucosidase. Hydrolysed proteins and carbohydrates accounted for a small fraction of the total protein and carbohydrate pools (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 carbohydrate 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 higher in deeper sediment layers. These results indicate that the 2 classes of organic compounds might play different roles in different sediment layers with important trophodynamic implications for benthic organisms living at different sediment depths.