Potential for Sulfate Reduction in Mangrove Forest Soils: Comparison between Two Dominant Species of the Americas.

Melike Balk,Joost A Keuskamp,Hendrikus J Laanbroek

doi:10.3389/fmicb.2016.01855

Abstract

Avicennia and Rhizophora are globally occurring mangrove genera with different traits that place them in different parts of the intertidal zone. It is generally accepted that the oxidizing capacity of Avicennia roots is larger than that of Rhizophora roots, which initiates more reduced conditions in the soil below the latter genus. We hypothesize that the more reduced conditions beneath Rhizophora stands lead to more active sulfate-reducing microbial communities compared to Avicennia stands. To test this hypothesis, we measured sulfate reduction traits in soil samples collected from neighboring Avicennia germinans and Rhizophora mangle stands at three different locations in southern Florida. The traits measured were sulfate reduction rates (SRR) in flow-through reactors containing undisturbed soil layers in the absence and presence of easily degradable carbon compounds, copy numbers of the dsrB gene, which is specific for sulfate-reducing microorganisms, and numbers of sulfate-reducing cells that are able to grow in liquid medium on a mixture of acetate, propionate and lactate as electron donors. At the tidal locations Port of the Islands and South Hutchinson Islands, steady state SRR, dsrB gene copy numbers and numbers of culturable cells were higher at the A. germinans than at the R. mangle stands, although not significantly for the numbers at Port of the Islands. At the non-tidal location North Hutchinson Island, results are mixed with respect to these sulfate reduction traits. At all locations, the fraction of culturable cells were significantly higher at the R. mangle than at the A. germinans stands. The dynamics of the initial SRR implied a more in situ active sulfate-reducing community at the intertidal R. mangle stands. It was concluded that in agreement with our hypothesis R. mangle stands accommodate a more active sulfate-reducing community than A. germinans stands, but only at the tidal locations. The differences between R. mangle and A. germinans stands were absent at the non-tidal, impounded location.

Highlights

Mangrove species inhabiting tropical and subtropical coastal zones are adapted to tidal influences on soil temperature, water content and salt concentration, and to varying degrees of anoxia (Alongi, 2008)
Based on the generally accepted idea that Rhizophora soils are more reduced than Avicennia soils, we hypothesized that sulfate reduction traits such as community size and activity would be more manifest in soils from Rhizophora stands than in soils from Avicennia stands
Potential steady state reduction rates, copy numbers of the dsrB gene, and numbers of culturable cells were all significantly higher values in samples obtained from A. germinans stands than in samples from R. mangle stands, at least at the tidal locations Port of the Islands and South Hutchinson Island

Summary

Introduction

Mangrove species inhabiting tropical and subtropical coastal zones are adapted to tidal influences on soil temperature, water content and salt concentration, and to varying degrees of anoxia (Alongi, 2008). Avicennia and Rhizophora, Potential Sulfate Reduction in Mangroves both mangrove genera with a global distribution, developed different mechanisms to adapt to stress factors imposed by the prevailing tidal regime. Due to these differences, Avicennia and Rhizophora generally form mono-specific stands at distinct positions in the tidal zone. Since the redox status in the root zone is determined by the balance between oxygen-producing and consuming processes, the more oxidized zones with Avicennia are likely due to a higher oxygen to labile carbon ratio released from the roots of this mangrove species compared to Rhizophora species

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Results

Conclusion