Heavy metals and organic enrichment affect greenhouse gas production pathways in sediment microbial communities

Estuaries, ports and harbours are being subjected to increasing anthropogenic pressure from industrial, urban, and agricultural run-off, and shipping and leisure activities. Contaminants entering these waterways bind to... [ view full abstract ]

Estuaries, ports and harbours are being subjected to increasing anthropogenic pressure from industrial, urban, and agricultural run-off, and shipping and leisure activities. Contaminants entering these waterways bind to particles in the water and eventually settle into the sediments, where they have the potential to affect the microbial and benthic invertebrate communities. The microbial communities play an important role in the regulation of the climate through biogeochemical cycles which control the production of several greenhouse gases. It is therefore of global importance to understand the impact of contaminants on the gas production in affected sediments. Our knowledge so far is based on measures of overall physicochemical sediment output, over quite limited locations and sediment strata types. However, metatranscriptomic surveys can provide a measure of the whole community function at a specific time point using measurements of mRNA levels. We have used metatranscriptomics to measure the gene-level response of a sediment microbial community to varying levels of heavy metal contamination and nutrient enrichment, with a focus on nitrogen, sulphur, methane and carbon metabolisms. In this designed ecological experiment, nutrient enrichment affected all gas production metabolisms that we investigated. Generally, the changes to gene expression in gas production pathways imply that enrichment would result in a greater production of green house gases such as nitrous oxide (N20), hydrogen sulphide (H2S) and methane (CH4). In addition, nutrient enrichment effects on gene expression would likely result in the accumulation of toxic intermediate products. Heavy metal contamination only affected a small number of genes. Therefore, the impact on the gas production genes was not very clear. However, we found that heavy metal contamination generally led to increased CH4 production. Overall, our results further understanding of the changes in greenhouse gas production from sediment communities in the presence of contaminants. Our findings carry implications for climate change models and for ecosystem management since microorganisms are key players in biogeochemical cycles and have the potential to regulate a substantial part of the global greenhouse gas production. The metatranscriptomic methods enable the acquiring of rapid information for making predictions over large spatial scales and ranges of sediment strata type.