Chemical Defence of a Seagrass against Microfoulers and Its Seasonal Dynamics

Featured Application The development of new antifouling methods gives increasing attention to biomimicry, which could provide nontoxic or less toxic solutions for the deterrence of unwanted settlers. Natural mechanisms against fouling are often highly dynamic and future applications may need to mimic not only defensive compounds, but also their dynamics in order to exploit the full potential of biomimicry. We here explore the seasonal dynamics of chemical defences against microfouling in eelgrass and the drivers of these dynamics. In marine environments bacterial microfoulers are an important determinant for the settlement of algal and animal macrofoulers. At the same time fouling is usually subject to seasonal fluctuation. Additionally, the seagrass Zostera marina is prone to microfouling, although this marine spermatophyte is known to be chemically defended against bacterial settlers. Spermatophytes are often capable of induced or activated defences against biological enemies such as pathogens or herbivores, but it is still unknown whether they can fine-tune their antifouling-defence according to settlement pressure. We therefore assessed the seasonality of bacterial settlement pressure, defence against microsettlers and concentrations of a previously identified defence compound, rosmarinic acid, on surfaces of Z. marina. All examined variables peaked in summer, while they tended to be lower in spring and autumn. The seasonality of defence activity and rosmarinic acid surface concentration was positively correlated with the seasonal fluctuation of fouling pressure, which suggests that Z. marina can adjust its defence level to the relatively high bacterial fouling pressure in summer. Besides of biotic factors the seasonal change of environmental factors, such as nitrogen supply, and in particular temperature, also affected the defence level, either directly or through indirect effects on the microbial settlers.