Biodiversity loss and climate change simultaneously threaten marine ecosystems, yet their interactions remain largely unknown. Ocean acidification severely affects a wide variety of marine organisms and recent studies have predicted major impacts at the pH conditions expected for 2100. However, despite the renowned interdependence between biodiversity and ecosystem functioning, the hypothesis that the species’ response to ocean acidification could differ based on the biodiversity of the natural multispecies assemblages in which they live remains untested. Here, using experimentally controlled conditions, we investigated the impact of acidification on key habitat-forming organisms (including corals, sponges and macroalgae) and associated microbes in hard-bottom assemblages characterised by different biodiversity levels. Our results indicate that, at higher biodiversity, the impact of acidification on otherwise highly vulnerable key organisms can be reduced by 50 to >90%, depending on the species. Here we show that such a positive effect of a higher biodiversity can be associated with higher availability of food resources and healthy microbe-host associations, overall increasing host resistance to acidification, while contrasting harmful outbreaks of opportunistic microbes. Given the climate change scenarios predicted for the future, we conclude that biodiversity conservation of hard-bottom ecosystems is fundamental also for mitigating the impacts of ocean acidification.

Global change is determining the expansion of marine oxygen-depleted zones, which are hot spots of microbial-driven biogeochemical processes. However, information on the functioning of the microbial assemblages and the role of viruses in such low-oxygen systems remains largely unknown. Here, we used the marine Rogoznica Lake as a natural model to investigate the possible consequences of oxygen depletion on virus-prokaryote interactions and prokaryotic metabolism in pelagic and benthic ecosystems. We found higher bacterial and archaeal abundances in oxygen-depleted than in oxic conditions, associated with higher heterotrophic carbon production, enzymatic activities and dark inorganic carbon fixation (DCF) rates. The oxygen-depleted systems were also characterized by higher viral abundance, production and virus-induced prokaryotic mortality. The highest DCF relative contribution to the whole total C production (> 30%) was found in oxygen-depleted systems, at the highest virus-induced prokaryotic mortality values (> 90%). Our results suggest that the higher rates of viral lysis in oxygen-depleted conditions can significantly enhance DCF by accelerating heterotrophic processes, organic matter cycling, and hence the supply of inorganic reduced compounds fuelling chemosynthesis. These findings suggest that the expansion of low-oxygen zones can trigger higher viral impacts on prokaryotic heterotrophic and chemoautotrophic metabolism, with cascading effects, neglected so far, on biogeochemical processes.

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