Control of microbial growth on eelgrass (Zostera marina L.: Spermatophyta) by leaf-derived metabolites

The role of minor components of the leaves ofZostera marina L. in altering the activity of micro-organisms directly and indirectly by affecting amphipod grazers was investigated in laboratory experiments. Water-soluble extracts of eelgrass leaves inhibited the growth of many marine bacteria and a micro-alga (Platymonas sp.) at concentrations equivalent to as little as 10 μg dry wt. of leaf.ml−1. The potency of leaf extracts was higher in young, actively metabolizing tissue than in older leaves, and was higher in leaves collected in summer during rapid growth than in winter. Water-soluble inhibitors may explain the low biomass of epiphytes on actively growing leaves. Phenolic acids, previously identified in eelgrass leaves, may account for part of the antimicrobial activity. Three free phenolic acids inhibited the test micro-organisms at concentrations as low as 2 μM; caffeic acid was more potent than either protocatechuic or gentisic acid. Extracts of young leaves also inhibited grazing by amphipods on dead leaves. The time required for leaching of soluble inhibitors may account for the delay observed between the loss of leaves from the plants and the onset of grazing. Thus, interactions among the biotic components of the detritus ecosystem may be significantly modified by minor components of the macrophyte tissue.