Microplastic pellets do not affect heat stress-induced night-time respiration in the brown alga <i>Fucus serratus</i>

Brown macroalgae like Fucus serratus are key ecosystem engineers in intertidal environments and are increasingly threatened by anthropogenic pollution and global change. This study examined how the combined effects of conventional and biosourced microplastic (MP) pellets and thermal stress affect the night-time respiration of F. serratus. Respiration rates were assessed after algae were exposed to a combination of aerial temperature treatments (6 h at 25°C as control or 35°C as a heat stress) and five immersed MP treatments (6 h exposure to control seawater, polypropylene, polypropylene with a biological matrix, polylactic acid or biopolyester solutions; n = 5 per treatment) in darkness. Our results showed a significant increase in respiration following heat stress, independent of MP exposure, suggesting a sustained metabolic stress-repair response and may indicate a cost to carbon balance and long-term tolerance. However, no significant effect of MP or interaction between stressors was detected. These findings suggest short-term resilience of F. serratus to MP exposure under the tested conditions. In an era of global change, further research on the combined and long-term effects of multiple stressors, including MP, on key physiological processes across seasons and species is needed to clarify their ecological consequences for intertidal macroalgal communities.