Shifts in microbial communities driven by the replacement of seagrasses by benthic macroalgae may exacerbate heatwave effects in eutrophicated coastal lagoons

ABSTRACT Climate change is expected to increase both the frequency and intensity of marine heatwaves, prolonging periods of extreme sea surface temperatures. These events can disrupt stratification, reduce oxygen availability, and alter nutrient cycling, ultimately reshaping marine community structure and function. Impacts are likely to be particularly severe in coastal lagoons, which could compromise their biodiversity and ecosystem services. In this study, we simulated a marine heatwave under controlled mesocosm conditions to investigate its influence on microbial communities in sediments colonized by the native seagrass Cymodocea nodosa and the opportunistic seaweed Caulerpa prolifera in the eutrophicated Mar Menor coastal lagoon. Community shifts and ecologically relevant taxa were assessed by using 16S rRNA gene metabarcoding and compositional data analysis, using complementary statistical approaches combining log ratio analysis and the glmnet algorithm. Our results show that sediments colonized by different macrophyte species harbor distinct microbial assemblages with different functional capacities. While sediments associated with the invasive macroalga Caulerpa prolifera were enriched in sulfate-reducing bacteria (mostly Desulfobacterota and Desulfosarcinaceae ), Cymodocea nodosa colonized sediments showed higher abundances of sulfur-oxidizing taxa such as Thiotrichaceae . During marine heatwaves, Cymodocea nodosa sediments exhibited an increase in sulfate reducers coupled with a decline in sulfur oxidizers that favored sulfide accumulation. These findings suggest that marine heatwaves may alter sediment microbial sulfur cycling in vegetated coastal systems, with potential implications for ecosystem resilience. Our results also highlight the importance of considering microbial processes when evaluating the responses of coastal lagoons to increasingly frequent and intense marine heatwaves. IMPORTANCE Coastal lagoons are among the ecosystems most vulnerable to thermal anomalies; however, the responses of sediment-associated microbial communities remain poorly understood. Here, we assess the impact of simulated marine heatwaves on sediment microbiota associated with the seagrass Cymodocea nodosa and the macroalga Caulerpa prolifera under controlled laboratory conditions. Heatwave exposure led to the accumulation of sulfur compounds and pronounced shifts in microbial community composition, indicating altered biogeochemical functioning. These microbial responses suggest that marine heatwaves could potentially contribute to conditions that have been associated with dystrophic events in eutrophicated lagoons. Our findings provide evidence that future marine heatwaves could alter the microbial composition of coastal lagoon sediments and highlight the urgent need to incorporate microbial processes into ecosystem monitoring and management frameworks.