How does plastic pollution influence primary productivity of large rivers? A study in the Lower Mekong River

Once introduced into aquatic ecosystems, plastic debris undergoes rapid biofouling, resulting in the formation of a man-made ecosystem known as the 'plastisphere.' This plastisphere comprises various microbial communities, including bacteria, single-celled algae, and fungi. The impact of these plastic substrates and their associated epiplastic community on ecological processes and biogeochemical cycles has been largely overlooked. Although limited evidence exists, it has revealed potential processes that could affect ecological functioning. To address this knowledge gap, we conducted a study investigating the microalgal and bacterial communities thriving on plastic debris collected from three rivers in the Lower Mekong River Basin (Cambodia). We incubated different plastic polymers for 30 days and characterised the plastisphere community by high-throughput Illumina sequencing of the hypervariable V5−V6 regions of 16S rRNA gene and the V4-V5 hypervariable region of the 18S rRNA. We assessed chlorophyll-a concentration as an indicator of autotrophic organism biomass and examined the effects on ecosystem production (gross, respiration, and net) by measuring oxygen dynamics in light-dark bottle incubations. Additionally, we monitored changes in the chemical species of nitrogen, phosphorus, and organic carbon. Our findings revealed limited growth of the microalgal community and a dominance of the bacterial one, with populations predominantly consisting of organisms from the families Nocardiaceae, Comamonadaceae, and Rhodobacteraceae. We also identified the presence of potential bacterial pathogens. Across all sites, we observed the dominance of net bacterial heterotrophic activity, which led to a reduction in dissolved oxygen levels. These results suggest that plastic debris can have far-reaching effects, potentially altering metabolic traits within aquatic ecosystems.