A nutritional role for amino acid transport in filter-feeding marine invertebrates

SYNOPSIS. Filter-feeding invertebrates are exposed to a small but continuous source of potential foodstuffs in the form of dissolved free amino acids (DFAA). Studies with two genera of bivalve molluscs, Mytilus and Modiolus, indicate the existence of carrier-mediated transport processes for DFAA, and that these processes are capable of a net accumulation of substrate from external solution. The kinetics of DFAA transport in bivalve gill, the primary organ for uptake, reveal these processes to be well-adapted for the efficient utilization of substrate at the extremely low concentrations (approximately 1 μM) characteristic of the water column: Michaelis constants for uptake are in the micromolar range, resulting in transport rates that are a significant fraction of the maximum capacity of the system. Comparison of potential rates of DFAA accumulation to rates of oxidative metabolism suggest that DFAA can serve as an important nutritional supplement to these animals. Future work should emphasize new analytical procedures, including the use of high performance liquid chromatography, to identify levels of DFAA in specific environments and rates of net DFAA uptake into organisms from these habitats.