Biogeochemistry of hadal trenches: Recent developments and future perspectives

Hadal trenches with water depth ranging from 6000 to 11,000 m are the deepest biogeochemical province in the world's oceans. Due to technological challenges of sampling and observation at such extreme depths, these trenches are one of the least exploited habitats on our planet. Recent findings show high sedimentation rates, high biomass, intense microbial activity and chemosynthetic communities in hadal trenches, suggesting that they may play a more important role in global ocean biogeochemical cycles than previously recognized. Here we review the hadal biogeochemistry in four aspects: food supply and its effect on hadal life; hadal carbon cycle; microbial-mediated stable carbon isotope fractionation in biosynthesis of lipids under high hydrostatic pressures; and pollution in the hadal trenches. Our current knowledge of hadal biogeochemistry is rudimentary and many fundamental questions remain to be answered. However, with the rapid development of the full-ocean-depth exploration and sampling technologies, it is very likely hadal scientists will make significant breakthroughs in hadal biogeochemistry research in the coming decades.