Abstract
BackgroundA key characteristic of eutrophication in coastal seas is the expansion of hypoxic bottom waters, often referred to as ‘dead zones’. One proposed remediation strategy for coastal dead zones in the Baltic Sea is to mix the water column using pump stations, circulating oxygenated water to the sea bottom. Although microbial metabolism in the sediment surface is recognized as key in regulating bulk chemical fluxes, it remains unknown how the microbial community and its metabolic processes are influenced by shifts in oxygen availability. Here, coastal Baltic Sea sediments sampled from oxic and anoxic sites, plus an intermediate area subjected to episodic oxygenation, were experimentally exposed to oxygen shifts. Chemical, 16S rRNA gene, metagenomic, and metatranscriptomic analyses were conducted to investigate changes in chemistry fluxes, microbial community structure, and metabolic functions in the sediment surface.ResultsCompared to anoxic controls, oxygenation of anoxic sediment resulted in a proliferation of bacterial populations in the facultative anaerobic genus Sulfurovum that are capable of oxidizing toxic sulfide. Furthermore, the oxygenated sediment had higher amounts of RNA transcripts annotated as sqr, fccB, and dsrA involved in sulfide oxidation. In addition, the importance of cryptic sulfur cycling was highlighted by the oxidative genes listed above as well as dsvA, ttrB, dmsA, and ddhAB that encode reductive processes being identified in anoxic and intermediate sediments turned oxic. In particular, the intermediate site sediments responded differently upon oxygenation compared to the anoxic and oxic site sediments. This included a microbial community composition with more habitat generalists, lower amounts of RNA transcripts attributed to methane oxidation, and a reduced rate of organic matter degradation.ConclusionsThese novel data emphasize that genetic expression analyses has the power to identify key molecular mechanisms that regulate microbial community responses upon oxygenation of dead zones. Moreover, these results highlight that microbial responses, and therefore ultimately remediation efforts, depend largely on the oxygenation history of sites. Furthermore, it was shown that re-oxygenation efforts to remediate dead zones could ultimately be facilitated by in situ microbial molecular mechanisms involved in removal of toxic H2S and the potent greenhouse gas methane.
Highlights
A key characteristic of eutrophication in coastal seas is the expansion of hypoxic bottom waters, often referred to as ‘dead zones’
In a coastal anoxia dead zone remediation scenario, oxygenation of the sediment surface was followed by an increase in bacterial populations affiliated with the genus Sulfurovum and RNA transcripts attributed to sulfur cycling that would potentially prevent H2S from ascending into the water column
This was indicated by lower amounts of RNA transcripts for methanogenesis when compared to the anoxic controls
Summary
A key characteristic of eutrophication in coastal seas is the expansion of hypoxic bottom waters, often referred to as ‘dead zones’. Dissolved oxygen in the oceans has declined in the past five decades, and the decrease is accelerated by an increase in temperature In conjunction with this decline, an expansion of oxygen-minimum-zones in the oceans has been observed. The provision of nutrients from eutrophication events fuels phytoplankton blooms that eventually die, and a portion of the biomass sinks to the sediment surface where it is degraded by aerobic microbes [4]. These metabolic processes deplete the available oxygen [5] such that the sediments and the overlying bottom water become anoxic. Details on the molecular mechanisms regulating biogeochemical energy and nutrient cycles under fluctuating oxygen conditions remain largely unexplored
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