Substrate availability may limit the response of tropical bacterioplankton biomass to warming

The response of heterotrophic bacterioplankton to the addition of macrophytic dissolved organic matter (DOM) and temperature was investigated in the Red Sea. We added 40 μmol C L−1 of leachates obtained from seagrass and mangrove leaves to natural bacterial communities, incubated them at three temperatures (25.5°C found in situ plus 3°C below and above that value) and monitored the microbial and biogeochemical responses over 4 d. 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 (μ) increased twofold 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 treatments, respectively, compared to just 2.6 μg C L−1 in seawater. The increase in μ values due to experimental warming followed the metabolic theory of ecology, mostly because 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 tropical heterotrophic bacteria may have limited capability to increase their biomass as a consequence of future warming, even in the presence of high loadings of macrophytic DOM.