Efficient recycling of nutrients in modern and past hypersaline environments

Y Isaji,N O Ogawa,T Yoshimura,M Roveri,S Lugli,A Makabe,J Kuroda,T Shibuya,V Manzi,N Ohkouchi,A Santulli,H Kawahata,F J Jiménez-Espejo

doi:10.1038/s41598-019-40174-9

Abstract

The biogeochemistry of hypersaline environments is strongly influenced by changes in biological processes and physicochemical parameters. Although massive evaporation events have occurred repeatedly throughout Earth history, their biogeochemical cycles and global impact remain poorly understood. Here, we provide the first nitrogen isotopic data for nutrients and chloropigments from modern shallow hypersaline environments (solar salterns, Trapani, Italy) and apply the obtained insights to δ15N signatures of the Messinian salinity crisis (MSC) in the late Miocene. Concentrations and δ15N of chlorophyll a, bacteriochlorophyll a, nitrate, and ammonium in benthic microbial mats indicate that inhibition of nitrification suppresses denitrification and anammox, resulting in efficient ammonium recycling within the mats and high primary productivity. We also suggest that the release of 15N-depleted NH3(gas) with increasing salinity enriches ammonium 15N in surface brine (≈34.0‰). Such elevated δ15N is also recorded in geoporphyrins isolated from sediments of the MSC peak (≈20‰), reflecting ammonium supply sufficient for sustaining phototrophic primary production. We propose that efficient nutrient supply combined with frequent bottom-water anoxia and capping of organic-rich sediments by evaporites of the Mediterranean MSC could have contributed to atmospheric CO2 reduction during the late Miocene.

Highlights

Present-day and past hypersaline environments are unique and extremely important settings that affect biology, climate, and global geochemistry[1,2,3]
On the basis of insights obtained from modern solar salterns, we sought to reconstruct the nitrogen cycle during the peak of the massive evaporation event in the late Miocene, the Messinian salinity crisis, by measuring the nitrogen isotopic compositions of geoporphyrins from an ancient Sicilian deposit (Fig. 1G,H)
We propose that the 15N-enriched ammonium was produced by mechanisms similar to those discovered for the solar salterns: the suppression of nitrification under extreme salinity and the release of NH3(gas) during the arid season (Fig. 5)

Summary

Introduction

Present-day and past hypersaline environments are unique and extremely important settings that affect biology, climate, and global geochemistry[1,2,3]. Nitrogen has an active redox cycle comprised mainly of biological processes (e.g., nitrate and ammonium assimilation, N2-fixation, nitrification, denitrification, anaerobic ammonium oxidation [anammox]) It strongly influences photosynthetic primary productivity, and the entire biological community, by modifying the amount and chemical species of the bioavailable nitrogenous nutrients[9]. The concentrations and nitrogen isotopic compositions of nitrate, ammonium, and chloropigments allowed us to trace the major processes occurring inside the microbial mats and surface brine. On the basis of insights obtained from modern solar salterns, we sought to reconstruct the nitrogen cycle during the peak of the massive evaporation event in the late Miocene, the Messinian salinity crisis, by measuring the nitrogen isotopic compositions of geoporphyrins from an ancient Sicilian deposit (Fig. 1G,H). The degree of evaporation of the surface brine in each pond is calculated from the molar concentrations of Mg2+ in the seawater and brine samples as follows (Table S1): DEMg

Methods

Findings

Conclusion