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 μchanges, 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.