Sulfated phenolic acids as readily activatable storage forms of antifouling agents in marine plants
Zosteric acid (ZS) has recently been discussed as a potent, natural biofilm inhibiting agent. Previously to this work have further sulfated metabolites of marine origin been detected, presumably structurally related to ZS. For identification of these unknown metabolites, pure standards were synthesized and the assumed compounds 4-(sulfooxy)benzoic acid (BS) and 4-(sulfooxy)-phenylacetic acid (AS) confirmed by UPLC-MS (Ultra high performance liquid chromatography-mass spectrometry) experiments. The structural relation of AS and BS to ZS encouraged to investigate, if these would exhibit similar properties as ZS and which structural elements define their activity. The combination of different bioassays and test organisms (Escherichia coli, Vibrio natriegens), UPLC-MS stability screening of the test compounds and monitoring of the impact on bacterial growth, provided a comprehensive overview of the activity of sulfated phenolic acids in comparison to their non-sulfated forms. Impacts of the test compounds were species dependent and influenced by the experimental setup. Still, some common conclusions could be drawn. In neither of the employed assays could the previously suggested antifouling activity of ZS be confirmed, nor any effect observed from AS and BS. Instead, the non-sulfated form of ZS, 4-hydroxy-cinnamic acid (Z), proved highly efficient to inhibit bacterial settlement. UPLC-MS measurements revealed sulfatase activity in V. natriegens, leading to an in situ release of the active, non-sulfated form Z into the bacterial medium. The non-sulfated forms of AS and BS (A, B) showed either only a short term effect on biofilm inhibition (B), or none (A). The influence of Z on bacterial growth was both concentration and species dependent. The common consent concerning the activity of ZS could be revised. The sulfate ester seems thus to constitute a storage form of the active compound Z.