Competition studies on the uptake of dissolved organic nutrients by bivalve larvae (Mytilus edulis) and marine bacteria

Abstract

We studied the competition between larval mussels (Mytilus edulis L.) and naturally occurring populations of marine bacteria (Menai Straits, North Wales) for dissolved glycine and glocose. Laboratory reared larvae were placed in natural seawater containing populations of bacteria ranging from 2x10⁵ to 7x10⁵ ml^-1 and the rates of uptake of ¹⁴C-labeled substrates into an equal biomass of larvae and bacteria were determined. The larvae always accumulated dissolved nutrients at rates comparable to those of the equivalent biomass of bacteria. When uptake experiments were conducted with larvae and bacteria separately, the uptake rates were not greatly different from those observed when tested together. Direct counts of bacteria showed no significant increase or decrease in bacterial numbers during the 100 min time-course of any experiment, showing that entry of ¹⁴C-labeled substrate into the larvae did not occur by ingestion of bacteria. It was shown that, although larvae have a lower affinity for substrate (higher K _t) than bacteria, larvae also have a higher maximum uptake rate (V _max). Effective larval uptake in the presence of bacteria relies on the high values of V _max. It is invalid to assume that one transport system cannot compete with another solely because one has a lower K _t.