Carbon/nutrient balance in relation to biomass production and halogenated compound content in the red alga Asparagopsis taxiformis (Bonnemaisoniaceae)

Abstract

We tested how the availability of carbon and nitrogen determines both the production of Asparagopsis taxiformis (Delile) V. Trevis. and content of the two major halocarbons, bromoform and dibromoacetic acid. The halogenated secondary metabolites of Asparagopsis species are particularly interesting from an applied perspective due to their remarkable antimicrobial activity. Terrestrial ecologists named the relationship between resources and secondary metabolites as the carbon (C)/nutrient balance (CNB) hypothesis. This relationship was tested both in the laboratory, with a factorial analysis using different concentrations of total ammonia (TAN) and dissolved inorganic carbon (DIC), and in an integrated aquaculture system where TAN and DIC fluxes of fish effluent were manipulated. The total C/N content of A. taxiformis biomass cultivated in laboratory was highly significantly linearly related to the content of both halocarbons, as predicted by the CNB hypothesis. A. taxiformis cultivated at low levels of carbon and high levels of nitrogen (N) (lowest C/N ratio) had the lowest content in both halogenated metabolites. Increased availability of CO2 in the medium resulted in a general higher halocarbon content in the biomass, even though the effect was only statistically significant for bromoform at high levels of N. The farm experiments supported the results of the laboratory experiments. DIC fluxes had the highest effect on the production of both bromoform and biomass, as shown by multiple regression analysis. In A. taxiformis integrated aquaculture, C, rather than N, is the most important factor affecting the production of biomass and of valuable halocarbon secondary metabolites.