Antibacterial compounds in estuarine submersed aquatic plants

Estuarine seagrasses often exist in nutrient-rich waters and thus might benefit from mechanisms to control exterior growth of epiphytic microorganisms. In this study, we tested extracts from three estuarine seagrass species, Potamogeton pectinatus L. (sago pondweed), Potamogeton perfoliatus L. (redhead grass) and Ruppia maritima L. (wigeon grass), for antibacterial activity. Methanolic extracts of axenic cultured plants were effective against all gram-positive bacteria tested, inhibiting growth of 12 species in the genera Micrococcus, Staphylococcus, Streptococcus, Bacillus, Aerococcus, Mycobacterium and Corynebacterium. Some gram-negative species in the genera Vibrio, Listonella and Pasteurella were also sensitive. Other gram-negative bacteria were resistant. Antibacterial activity was detected in field-collected plants in spring, but appeared absent in plants collected in fall. These seagrass species thus appear to possess an antibacterial agent. It has not been identified and may consist of one or multiple compounds. The antibacterial agent in P. pectinatus was released into the water column in vitro at concentrations sufficient to inhibit or kill sensitive bacterial species. The agent was stable, with an in vitro half-life of 12 days at 25 °C and 6.6 days at 37 °C. It is possible that antibacterial production has ecological effects upon the bacterial and faunal communities associated with seagrass ecosystems.