Phosphorus Species in Deep-Sea Carbonate Deposits: Implications for Phosphorus Cycling in Cold Seep Environments

Junlie Zhou,Shun Chen,Xiaotong Peng,Hengchao Xu,Kaiwen Ta,Jiwei Li,Mengran Du

doi:10.3390/min10070645

Abstract

Phosphorus (P) is an important nutrient for biological communities in cold seeps. However, our knowledge on the source, species, and cycling of P in cold seep environments is limited. In this study, the concentration, species, and micro to nanometer scale distribution of P in seep carbonates were examined at three deep-sea cold seeps in the South China Sea and East China Sea. The Ca-P accounts for the largest proportion of P—followed by detrital-P, Fe-P, organic-P, and exchangeable-P. The distribution patterns of Ca-P, detrital-P, and organic-P in the seep carbonates differ from one another, as shown by elemental mapping with NanoSIMS and scanning electron microscopy. The covariation of P with Ca and C reveals that Ca-P co-precipitates with Ca-carbonate, which is linked to the process of sulfate-driven anaerobic oxidation of methane. Organic-P is also observed within biofilm-like organic carbon aggregates, revealing the microbial enrichment of P by fluids in the process of anaerobic oxidation of methane. P with a granulated morphology was identified as detrital-P derived from deep sediments. Most importantly, it is evident that Ca-P is positively correlated to the Fe content in all the seep carbonates. This indicates the likelihood that the dissolved P in cold-seep fluids is released primarily from Fe oxides through Fe-driven anaerobic oxidation of methane in deep sediments. These processes associated with different species of P may have significant implications for P geochemical cycling and anaerobic oxidation of methane impelled by Fe and sulfate reduction in cold seep environments.

Highlights

Phosphorus, an important nutrient that controls primary productivity [1,2], is cycled throughout various environments
These results are consistent with the analysis of major chemical composition (Table S1) that show that the carbonate deposits from Site E have high SiO2 (38.8–55.8%), Al2 O3 (6.6–10.3%), and CaO (12.5–19.9%) contents, whereas those from the SCS are characterized by higher CaO content (42.8–47.9% and 19.9–47.3% at Haima and Site F, respectively) and lower SiO2 content (5.4–12.2% and 7.9–36.0% at Haima and Site F, respectively)
The partition of P species in the carbonates illustrates that a large portion of the P in the fluids precipitate in the form of Ca-P within Ca-carbonate minerals associated with sulfate-driven anaerobic oxidation of methane

Summary

Introduction

Phosphorus, an important nutrient that controls primary productivity [1,2], is cycled throughout various environments. All organisms require P in addition to C, energy sources, and electron acceptors and donors to synthesize nucleic acids, fatty acids, and for energy reactions. This includes those communities of organisms that are found around cold seeps, which are among the most widespread chemosynthesis-based ecosystems in the deep-sea [3]. Sulfate reducing bacteria, partners of anaerobic oxidation of methane consortia [4], can accumulate significant amounts of P in their cells during the anaerobic oxidation of methane [5]. Minerals 2020, 10, 645; doi:10.3390/min10070645 www.mdpi.com/journal/minerals consortia [4], can accumulate significant amounts of P in their cells during the anaerobic oxidation of methane [5]

Methods

Results

Conclusion