Cable bacteria control iron-phosphorus dynamics in sediments and water column at two sites of a Coastal Hypoxic Basin

Phosphorus is an essential nutrient for life. The release of phosphorus from sediments is critical in sustaining phytoplankton growth in many aquatic systems and is pivotal to eutrophication and the development of bottom water hypoxia. Conventionally, sediment phosphorus release is thought to be controlled by changes in iron oxide reduction driven by variations in external environmental factors, such as organic matter input and bottom water oxygen. Here, we show that internal shifts in microbial communities, and specifically the population dynamics of cable bacteria, can also induce strong seasonality in sedimentary iron-phosphorus dynamics. Field observations in a seasonally hypoxic coastal basin demonstrate that the long-range electrogenic metabolism of cable bacteria leads to a dissolution of iron sulfides in winter and spring. Subsequent oxidation of the mobilized ferrous iron with manganese oxides results in a large stock of iron-oxide-bound phosphorus below the oxic zone. In summer, when bottom water hypoxia develops and cable bacteria are undetectable, the phosphorus associated with these iron oxides is released, strongly increasing phosphorus availability in the water column. Future research should elucidate whether formation of iron-oxide-bound phosphorus driven by cable bacteria, as observed in this study, contributes to the seasonality in iron-phosphorus cycling in aquatic sediments worldwide磷是生命活动必需的营养元素。沉积物中磷的释放对多数水生系统维持浮游植物生长至关重要，同时也是引发富营养化（eutrophication）及底层水体缺氧发生的关键环节。传统认知中，沉积物磷释放被认为由外部环境因子变化驱动的氧化铁还原过程所调控，例如有机质输入与底层水体含氧量波动。本研究发现，微生物群落的内部动态变化，尤其是电缆细菌（Cable Bacteria）的种群动态，同样可诱导沉积铁磷循环呈现显著的季节性特征。对一处季节性缺氧沿海盆地的野外观测显示，电缆细菌的长距离产电代谢会在冬、春两季促使硫化铁溶解；随后，被活化的亚铁与锰氧化物发生氧化反应，在含氧层下方形成大量铁氧化物结合态磷。至夏季，当底层水体出现缺氧且无法检测到电缆细菌时，这些铁氧化物结合的磷便会释放，大幅提升水体中的磷可利用性。未来研究需进一步阐明，本研究中观测到的由电缆细菌驱动的铁氧化物结合态磷形成过程，是否会推动全球水生沉积物中铁磷循环的季节性变化。