Enzymatically hydrolyzable amino acids in North Sea sediments and their possible implication for sediment nutritional values

The nutritional value of peptidic material in five North Sea sediments that differ in organic matter quality has been investigated with an enzymatic approach measuring the digestibility (rate constant k of proteolysis by protease) and the proportion of bioavailable fraction of amino acids (%EHAA-i:THAA). The bioavailable fraction in sediments varied between 14-50% EHAA-i:THAA and was generally lower than in potential source organisms such as algae (40-43%), plankton (~80%) and bacteria (57-72%). The rate of proteolysis of the amino acids varied among stations (k = 0.3-3.1 h-1) with a systematic decrease when going from labile to more refractory organic matter. The concentration of both the total and the enzymatically hydrolyzable amino acid pool increased toward the north where fine-grained sediments accumulate whereas the relative contribution of the labile fraction (% EHAA-i:THAA) decreased. Differences were found between the amino acid composition of the EHAA and the THAA pool in sedimentary organic matter, but these differences were minor in source organisms. Reasons for the selective liberation behavior of protease-k toward peptidic material in sediments include (A) specific cleavage pattern of protease-k preferring certain bounds (B) selection against amino acids protected by structural matrixes (holds for glycine and methionine) (C) protection by adsorptive binding to sediment particles (basic amino acids arginine, histidine, methionine). It is argued that arginine and histidine play an important role in deposit feeder nutrition since they are deficient in the sedimentary food source and therefore may be limiting growth. Their strong adsorption affinity, low susceptibility toward cleavage and occurrence in structural organic matter make them poorly available to deposit feeders. The relative content of basic amino acids also co-varies systematically with increasing nutritional quality