Iron-sulfur coupling controls phosphorus fluxes and burial in sediments from coastal to deep seas

Abstract

Sediment controls the recycling and sequestration of phosphorus (P), an essential nutrient in marine ecosystems. Understanding sediment P cycles helps constrain the P budget and predict how ecosystems respond to environmental variability. This study investigates and compares P cycling in sediments of various systems including typical coastal to deep seas, a lagoon (Pinqing lagoon), a mangrove swamp (Mai Po nature reserve), and deep-sea methane seeps (Haima seep). By characterizing the porewater and solid sediment geochemistry of P and related elements including iron (Fe), sulfur (S), and oxygen (O), the work aims to decipher the mechanistic controls of sediment P cycling under diverse regimes. We find that sediment P recycling is higher in coastal compared to deep seas, because organic matter availability controls Fe and sulfate reduction, affecting P fluxes and sequestration. However, such control is nonlinear, which is demonstrated in the sediments of Pinqing Lagoon, where orders of magnitude differences in P effluxes are observed within a single system despite the similar organic matter supplies. This disproportional control is likely due to the organic matter reactivity. Moreover, our results suggest that the classic organic-matter driven Fe-S-P coupling can be further complicated: we find unexpectedly low P recycling in the organic-rich anoxic sediments of a mangrove pond because organic matter fuels denitrification and affects P sequestration. On the other hand, when organic matter is scarce, alternative electron donors may also control P release: in the sediment at deep-sea methane seeps, Fe-S-P coupling can be promoted by anaerobic oxidation of methane, leading to P fluxes as surprisingly high as coastal sediments. These findings demonstrate the complexity of coupled C-Fe- S cycles in controlling the P cycle and highlight several crucial directions to explore in the future.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Jiying LI (Supervisor)