Winter marine heatwaves alter bacterial carbon uptake in <i>Macrocystis pyrifera</i> ecosystems

BACKGROUND AND AIMS: Warming and marine heat waves (MHWs) are responsible for global declines in kelp forest ecosystems. Research to date focuses on impacts of temperature increase based on thermal tolerances of habitat-forming kelp species. Winter MHWs do not tend to surpass the thermal maxima of macroalgae, providing the opportunity to examine impacts in the absence of large-scale biomass loss, and in conjunction with other abiotic stressors. The transformation of organic carbon by heterotrophic marine bacteria, a key component of carbon cycling and foodweb productivity in kelp forests is strongly modulated by temperature. In this study, the metabolic responses of Macrocystis pyrifera heterotrophic bacterial communities to winter MHW conditions were examined in conjunction with hydrogen peroxide (H2O2) stress, a known inhibitor of bacterial activity. METHODS: Carbon uptake rates of planktonic and surface-associated (biofilm) microbial communities from M. pyrifera forests were assessed with exposure to H2O2 treatments alone, and in tandem with short-term heat-spike experiments simulating ambient, moderate, and extreme heatwave conditions in winter. KEY RESULTS: Carbon uptake rates of planktonic bacteria increased under extreme MHW conditions, with no significant response to increasing H2O2 concentrations. Conversely, M. pyrifera biofilm communities showed a significant decrease in carbon uptake rates at high H2O2 exposure compared to the intermediate treatment regardless of temperature regime applied. CONCLUSIONS: In winter, ambient water temperatures are limiting planktonic bacterial carbon incorporation rates in kelp forests, and MHW events stimulate increased carbon turnover. Increasing bacterial metabolism during winter may affect kelp forest ecosystem services through altering turnover and maintenance of organic carbon within kelp forests and annual carbon cycling patterns. We suggest that temperature is not limiting bacterial carbon uptake at the algal surface, and other stressors are driving carbon uptake patterns in M. pyrifera biofilms.