Microbial production and degradation of γ-aminobutyric acid (GABA) in the abalone larval settlement habitat
Abstract By producing or degrading gamma-aminobutyric acid (GABA), microbes may affect the settlement of abalone (Haliotis) larvae. GABA was not detectable in extracts of crustose red algae (CRA) which are the preferred settlement substratum for Haliotis larvae. The CRA surfaces and their associated biofilms did not produce GABA from glutamate, and GABA production from putrescine required gabaculine (transaminase inhibitor). The GABA-degrading activity on the CRA surfaces was removed by surface-sterilisation. It ranged from 0.45 to 1.12 nmol cm−2 h−1 with insignificant seasonal variation. Removal of grazing molluscs from CRA-covered stones led to the growth of a visible biofilm and markedly increased GABA degradation in 2 weeks. Within a few days of placing clean, sterile glass surfaces in the settlement environment, biofilms with GABA-degrading activity developed, and after 11 weeks incubation the GABA degradation rate on glass marbles was 0.37 nmol cm−2 h−1. GABA was not completely oxidised, and the products of GABA metabolism showed similar chromatographic behaviour to alanine and glutamate. In the typical New Zealand Haliotis settlement habitat, GABA is not likely to play a significant role as an inducer of abalone larval settlement because on the preferred settlement surfaces it is not readily produced from its precursors but it is easily degraded.