Ocean acidification reduces diatom and photosynthetic gene abundance on plastic in an coastal bay mesocosm experiment.

Discarded plastics are accumulating in the global ocean and posing threats to marine life. The plastisphere - the community colonizing plastic surfaces - profoundly influences the environmental behavior of plastic, affecting its degradation and entry into marine food webs. Ocean acidification (OA) due to anthropogenic CO2 emissions, is also threatening marine ecosystems, but the effect of OA on the structure and ecological functions of plastisphere communities remain poorly understood. Here, using a mesocosm experiment, we investigated the effects of OA on the plastisphere colonizing floating PET plastic bottles. The study was conducted using subtropical eutrophic coastal water from Southern China under two CO2 conditions: increased CO2 to 1000 μatm (HC) and ambient CO2 410 μatm (LC). Metagenomic sequencing of the plastic samples, after exposure for 32 days, showed striking changes in relative abundance of eukaryotes and bacteria caused by HC. There was a 75.3 % decrease in eukaryote read abundances at high CO2, most strikingly a 95.6% decrease in the relative abundance of diatoms. In addition, the relative abundance of genes involved in photosystem II light reactions and pigment synthesis decreased under high CO2 conditions. This suggests that OA could reduce the photosynthetic potential of the plastisphere. Shifts in plastisphere community structure and potentially diminished photosynthesis under OA could influence food chains within plastisphere, plastic degradation, transportation, and carbon cycling involving plastics. Overall, our results suggest that OA can alter the functional ecology of the plastisphere, with potential implications for marine biogeochemical processes and food web dynamics in subtropical eutrophic coastal water.