Pollutant-degrading microbes emit more greenhouse gases than captured
Mangrove forests are among the most valuable ecosystems on earth, supporting a diverse assemblage of flora and fauna along tropical coasts including tree-dwelling mammals, fish and aquatic plants. They also act as a buffer to shield coastal populations from tropical storms and tsunamis, and capture millions of tons of carbon every year. Yet in industrial regions, these ecosystems have been inundated with toxic waste and domestic sewage. Although mangrove forests are highly resilient to such contaminants due to their natural capacity to filter water, a new study in Scientific Reports has demonstrated that the sheer volume of pollution appears to be overloading the ecosystem, altering the structure of the microbial community in the underwater sediment. These microbes play a key role in capturing carbon and methane from the atmosphere, and provide nutrients to mangrove roots - services which are now being degraded by the continuous stream of pollutants.
The researchers compared microbial communities in pristine and contaminated areas of two mangrove forests located in the northern and southern parts of Hainan Island in China: Dongzhai harbour and Sanya city. The forest in Dongzhai harbour is among the largest in China and was the first to be designated as a nature reserve in the country. However, excess nutrients from the local aquaculture industry have led to severe eutrophication in the offshore area of the reserve, leaving only the region closer to the coast uncontaminated. To the south, the forest on the coast of Sanya city has been heavily polluted by domestic sewage and industrial waste, whereas the offshore area remains relatively pristine.
The research team extracted and sequenced microbial DNA from sediment samples taken from each region in order to identify the types of microbe present. They also analysed the abundances of genes related to processing of methane and nutrients such as nitrogen and phosphorus, as well as resistance to heavy metal toxicity.
The results showed polluted sediments contained higher levels of genes associated with processing sulphates, which release carbon dioxide when broken down. They also harboured more genes linked to breaking down chemicals such as methanol into methane, another greenhouse gas. However, there was little difference in the abundance of genes associated with carbon fixation between contaminated and pristine sediments. The researchers also measured the volume of methane emitted from the sediment, and found that polluted sediment released methane, whereas samples taken from pristine forests continued to capture the gas from the atmosphere.
These findings indicate that the continuous inundation of mangrove forests with pollutants has stimulated the microbial community to release more carbon dioxide and methane into the atmosphere, while unpolluted ecosystems have continued to sequester these greenhouse gases.
The researchers also examined changes in nutrient cycling, and found that genes related to nitrogen assimilation and denitrification were more abundant in polluted sediments. Industrial waste and domestic sewage contain high levels of nitrogen-rich chemicals such as ammonia, which sediment bacteria can break down and release as inert nitrogen gas, helping to remove pollutants from the water without contributing further to climate change, as pure nitrogen is not a greenhouse gas. The mangrove trees are also likely to be using these nitrogen-rich pollutants as a source of nutrients. A previous study in Brazil showed similar results, suggesting that this bioremediative effect is occurring in polluted mangrove sediments worldwide.
This study highlights an urgent need to address pollution along tropical coasts on a global scale, to help restore these valuable ecosystems to their natural state and mitigate future greenhouse gas emissions.
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Journal reference: Li, Y., Zheng, L., Zhang, Y., Liu, H., & Jing, H. (2019). Comparative metagenomics study reveals pollution induced changes of microbial genes in mangrove sediments. Scientific Reports, 9(1), 5739.