We investigated the response of heterotrophic bacterioplankton to the addition of macrophytic dissolved organic matter (DOM) and temperature in the central Red Sea by adding ca. 40 µmol C L^-1 of leachates obtained from seagrass and mangrove leaves to natural bacterial communities and incubating them at 3 temperatures spanning 6°C around the in situ value (25.5°C). Seagrass and mangrove DOM, important allochthonous sources in tropical oligotrophic regions, had distinct chemical characteristics compared to unamended seawater, with mangrove substrates containing comparatively more nitrogen and protein-like fluorescent DOM than seagrass. Specific growth rates increased approximately 2-fold in the seagrass and mangrove treatments (1.0 and 0.8 d^-1, respectively) relative to the seawater control (0.4 d^-1). The biomass of heterotrophic bacteria generally reflected µ changes, reaching maximum values of 16.8 and 17.3 µg C L^-1 in the seagrass and mangrove treaments, respectively, compared to just 2.6 µg C L^-1 in seawater. The increase in specific growth rates due to experimental warming followed the metabolic theory of ecology, mostly as a result of enhanced exoenzymatic activity, while cell size decreased as predicted by the temperature-size rule (mean -3% per °C increase). Although the labile nature of the specific seagrass and mangrove DOM leachates was clearly demonstrated, we conclude that Red Sea heterotrophic bacteria will have limited capability to increase their biomass as a consequence of future warming, even in the presence of high loadings of macrophytic DOM.

Primary Presenter: Xose Anxelu G. Moran, King Abdullah University of Science and Technology (xelu.moran@kaust.edu.sa)

Authors:

Maria Ll. Calleja,   ()

Federico Baltar,   ()

Luis Silva,   ()

Paloma Carrillo de Albornoz,   ()

Carlos M. Duarte,   ()

Christian Lønborg,   ()