The International Union for Conservation of Nature (IUCN) has identified ocean oxygen loss as a key marine environmental stressor, disrupting the delicate balance of marine life communities. Oxygen Minimum Zones (OMZs), characterized by dissolved oxygen levels below 20 μM, are primarily found in areas of high primary production, such as the Arabian Sea, Eastern Tropical South Pacific, Eastern Tropical North Pacific, and Bay of Bengal. The expansion of OMZs is driven by global warming and increased nutrient input from human activities. Coastal hypoxia, where seawater dissolved oxygen levels drop below 2 mg/L, negatively impacts essential marine ecosystem functions and adversely affects marine life, including fish and invertebrates [1]. In Chinese seas, seasonal hypoxia is frequently observed in the Changjiang Estuary, Pearl River Estuary, Yellow Sea, and Bohai Sea, particularly during the summer months.

Microorganisms, including bacteria and archaea, though tiny in size, are incredibly abundant and contribute significantly to the total biomass, playing a key role in driving material and energy flows in marine ecosystems. As ocean deoxygenation advances, these flows are expected to shift from higher trophic levels to microorganisms. The OMZs have long been recognized as biogeochemical hot spots and well-studied. Within OMZs, oxygen is effectively absent from sea water and life is dominated by microorganisms that utilize chemicals other than oxygen for respiration. Unlike the stable OMZs found in other regions, China's coastal waters experience dynamic and periodic fluctuations in oxygen levels that typically remain above the suboxic threshold, preventing the formation of permanent OMZs. However, our understanding of how bacterial communities respond to this coastal deoxygenation in China is still limited.
The Changjiang River has been a primary source of nutrients, particularly nitrogen and phosphorus, to the East China Sea, driven by both natural processes and human activities, including agriculture and urbanization. This influx of nutrients has intensified eutrophication, leading to seasonal hypoxia near the estuary. In 2022, a record-breaking global heatwave amplified these conditions, contributing to a hypoxic event in the East China Sea. This heatwave, driven by a combination of atmospheric and oceanic anomalies, such as enhanced solar radiation, weakened winds, and reduced ocean heat loss, resulted in stratification in the water column and worsened oxygen depletion. During this period, we conducted a survey in the Changjiang Estuary and adjacent coastal waters, collecting water samples to investigate the microbial community composition and organic matter dynamics under hypoxic conditions.

In our recent publication in Scientific Data, we reported the microbial communities of a spatiotemporal survey in the Changjiang Estuary and adjacent East China Sea. Taxonomic assignments of metagenome-assembled genomes recovered from this region, matched those of marine microbes common in coastal areas in general. Our previous study in this region revealed that, despite the presence of hypoxia, bacteria maintain a heterotrophic lifestyle and aerobic respiration, with microbial communities adapting to low oxygen levels rather than undergoing abrupt changes. This ecosystem possesses robust adaptive mechanisms that buffer against abrupt changes, even below the well-defined hypoxic threshold in marine ecosystem [2]. Moreover, eutrophication-induced surface blooms are common in the East China Sea, offering an ideal setting to study the impact of organic matter on bacterial communities. Additionally, our time-series sampling in this region revealed that free-living bacteria also exhibited greater temporal variability and sensitivity to the composition of organic matter molecules, compared to particle-associated bacteria [3].

Initiatives like the Bohai Sea restoration and the Ten-Year Fishing Ban on the Changjiang River have shown positive outcomes in recent years, resulting in significant improvements in water quality and pollution control. Bioremediation, a technique that utilizes microorganisms to decompose contaminants, offers a potential solution for further restoring the health of marine ecosystems.

Reference
1.Vaquer-Sunyer, R., & Duarte, C. M. (2008). Thresholds of hypoxia for marine biodiversity. Proceedings of the National Academy of Sciences of the United States of America, 105(40), 15452–15457. https://doi.org/10.1073/pnas.0803833105
2.Liu, S., Hou, C., Dong, C., Zhao, D., Chen, Q., Terence Yang, J. Y., & Tang, K. (2024). Integrated multi-omics analyses reveal microbial community resilience to fluctuating low oxygen in the East China sea. Environmental research, 261, 119764. https://doi.org/10.1016/j.envres.2024.119764
3.Liu, S., Chen, Q., Liu, L., Dong, C., Qiu, X., & Tang, K. (2024). Organic matter composition fluctuations disrupt free-living bacterial communities more than particle-associated bacterial communities in coastal waters. The Science of the total environment, 949, 174845. https://doi.org/10.1016/j.scitotenv.2024.174845