Abstract
After oxygen, sulfate is the most important oxidant for the oxidation of organic matter in mangrove forest soils. As sulfate reducers are poor competitors for common electron donors, their relative success depends mostly on the surplus of carbon that is left by aerobic organisms due to oxygen depletion. We therefore hypothesized that sulfate-cycling in mangrove soils is influenced by the size of net primary production, and hence negatively affected by mangrove degradation and exploitation, as well as by carbon-exporting waves. To test this, we compared quantitative and qualitative traits of sulfate-reducing communities in two Saudi-Arabian mangrove stands near Jeddah, where co-occurring differences in camel-grazing pressure and tidal exposure led to a markedly different stand height and hence primary production. Potential sulfate reduction rates measured in anoxic flow-through reactors in the absence and presence of additional carbon sources were significantly higher in the samples from the non-grazed site. Near the surface (0–2 cm depth), numbers of dsrB gene copies and culturable cells also tended to be higher in the non-grazed sites, while these differences were not detected in the sub-surface (4–6 cm depth). It was concluded that sulfate-reducing microbes at the surface were indeed repressed at the low-productive site as could be expected from our hypothesis. At both sites, sulfate reduction rates as well as numbers of the dsrB gene copies and viable cells increased with depth suggesting repression of sulfate reduction near the surface in both irrespective of production level. Additionally, sequence analysis of DNA bands obtained from DGGE gels based on the dsrB gene, showed a clear difference in dominance of sulfate-reducing genera belonging to the Deltaproteobacteria and the Firmicutes between sampling sites and depths.
Highlights
Mangrove forests, which are confined to tropical and subtropical coast lines, are known to be highly productive ecosystems with the capacity to efficiently trap suspended material from the water column (Kristensen et al, 2008)
Sulfate reduction rates based on differences in sulfate concentrations in the inflow and the outflow of the flow-through reactor (FTR), declined in the first days of the measurement in the noncarbon-amended, surface samples from South Corniche and reached a steady state after 4 days (Figure 1A)
Lower levels of steady state sulfate reduction rates, lower numbers of the sulfate reduction-specific dsrB genes and lower quantities of culturable sulfate-reducing microorganisms in the surface layer from South Corniche compared to the upper layer from Thuwal suggested that sulfate reduction is less important at the first few millimeters of the grazed site
Summary
Mangrove forests, which are confined to tropical and subtropical coast lines, are known to be highly productive ecosystems with the capacity to efficiently trap suspended material from the water column (Kristensen et al, 2008). The production of above- and belowground litter as well as root exudates provide significant inputs of organic carbon to mangrove soils (Alongi, 1998). Depending on the environmental conditions, this organic carbon is oxidized by a range of aerobic and anaerobic microorganisms using a variety of electron acceptors, of which oxygen is the most preferred oxidant for thermodynamic reasons (Laanbroek, 1990). Aerobic respiration and sulfate reduction are considered to be the major pathways of mangrove-derived carbon degradation with a global share of 40–50% each (Kristensen et al, 2008). Due to the presence of oxygen, sulfate reduction is often suppressed in the upper few millimeters of the soil, but underneath this oxic zone, sulfate reduction generally increases with depth (Kristensen et al, 1991, 2000, 2011)
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