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Marine Ecology Progress Series

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MEPS 126:177-184 (1995)  -  doi:10.3354/meps126177

Seasonal variation in the relative utilization of dietary protein for energy and biosynthesis by the mussel Mytilus edulis

Kreeger DA, Hawkins AJS, Bayne BL, Lowe DM 

Mussels Mytilus edulis were collected from Whitsand Bay in southwest England during April, July and September 1993 and January 1994, and fed a defined diet of prekilled microalgae with one of 2 types of mixed protein/carbohydrate microcapsules. Both capsule types were comprised of the same w/w proportion of 15N-labeled protein and nonlabeled carbohydrate. However, Types 1 and 2 contained trace amounts of [14C]carbohydrate and [14C]protein, respectively. Defecation, excretion, respiration and incorporation of 14C and 15N isotopes were measured to describe the mussel's relative utilization of dietary carbohydrate (14C from Type 1), protein-C (14C from Type 2) and protein-N (15N from Types 1 and 2). In each experiment, mussels ingested similar quantities of capsule Types 1 and 2. However, the 14C-assimilation efficiency for carbohydrate (16 to 20%) was significantly greater than the 14C-assimilation efficiency for protein (7 to 15%). In contrast, protein-N was always assimilated with greater efficiency (25 to 82%) than either carbohydrate or protein-C. The difference in assimilation efficiency between protein-N and protein-C indicates that a large proportion of amino acids from dietary protein were hydrolyzed, with amino-N being preferentially assimilated. The relatively poor assimilation of amino-C indicates that mussels were not nutritionally limited by energy since amino-C generated by the energy-intensive digestion process was not fully catabolized in the tricarboxylic acid cycle (i.e. not respired). The conservation of protein-N relative to protein-C was particularly noticeable in July when rates of ingestion and ammonia excretion were lowest, <1% of absorbed [15N]protein was excreted, and [15N]protein assimilation was 5 times more efficient than [14C]protein assimilation. Further, rates of protein synthesis were greatest in July (71.2 mg d-1), compared with April, September and January (39.2, 58.9 and 20.5 mg d-1). These findings suggest that seasonal variation in dietary protein utilization in M. edulis is governed chiefly by changing anabolic demands for amino-N, rather than by requirements for energy or essential amino acids.

Protein . Carbohydrate . Mytilus edulis . Microcapsule . Assimilation

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