Marine Microbial Gene Abundance and Community Composition in Response to Ocean Acidification and Elevated Temperature in Two Contrasting Coastal Marine Sediments.

Ashleigh R Currie,Karen Tait,Beatriz De Francisco-Mora,A Mark Osborn,Helen Parry,Henrik Stahl,Natalie Hicks,Steve Widdicombe

doi:10.3389/fmicb.2017.01599

Abstract

Marine ecosystems are exposed to a range of human-induced climate stressors, in particular changing carbonate chemistry and elevated sea surface temperatures as a consequence of climate change. More research effort is needed to reduce uncertainties about the effects of global-scale warming and acidification for benthic microbial communities, which drive sedimentary biogeochemical cycles. In this research, mesocosm experiments were set up using muddy and sandy coastal sediments to investigate the independent and interactive effects of elevated carbon dioxide concentrations (750 ppm CO2) and elevated temperature (ambient +4°C) on the abundance of taxonomic and functional microbial genes. Specific quantitative PCR primers were used to target archaeal, bacterial, and cyanobacterial/chloroplast 16S rRNA in both sediment types. Nitrogen cycling genes archaeal and bacterial ammonia monooxygenase (amoA) and bacterial nitrite reductase (nirS) were specifically targeted to identify changes in microbial gene abundance and potential impacts on nitrogen cycling. In muddy sediment, microbial gene abundance, including amoA and nirS genes, increased under elevated temperature and reduced under elevated CO2 after 28 days, accompanied by shifts in community composition. In contrast, the combined stressor treatment showed a non-additive effect with lower microbial gene abundance throughout the experiment. The response of microbial communities in the sandy sediment was less pronounced, with the most noticeable response seen in the archaeal gene abundances in response to environmental stressors over time. 16S rRNA genes (amoA and nirS) were lower in abundance in the combined stressor treatments in sandy sediments. Our results indicated that marine benthic microorganisms, especially in muddy sediments, are susceptible to changes in ocean carbonate chemistry and seawater temperature, which ultimately may have an impact upon key benthic biogeochemical cycles.

Highlights

Coastal zones are under substantial pressure from multiple human induced stressors (Halpern et al, 2008), including increased atmospheric carbon dioxide levels
Macrofauna were removed from both sediment types using a 500 μm and a 1 mm mesh sieve in a seawater (UV treated; 1 μm filtered; salinity 35) bath and the sediment was left to settle for 24 h in large storage tanks to ensure retention of the fine particles
Observations showed that dissolved inorganic carbon (DIC) (Figure 2A) and AT (Figure 2B) were consistently higher in the muddy sediment compared to the sandy sediment across all treatments, and sediment was highly significant for both AT (p < 0.001) and DIC (p < 0.001)

Summary

Introduction

Coastal zones are under substantial pressure from multiple human induced stressors (Halpern et al, 2008), including increased atmospheric carbon dioxide (atmCO2) levels. Since the start of the industrial revolution, the ocean has taken up approximately 25–30% of total human CO2 emissions (Sabine and Tanhua, 2010), resulting in perturbations to ocean carbonate chemistry and a reduction in pH. This “ocean acidification” (OA) affects the equilibrium of the ocean carbonate system, increasing bicarbonate (HCO3−) and hydrogen ions (H+) and decreasing pH and carbonate (CO32−) concentration of the seawater (Zeebe and Wolf-Gladrow, 2001). Ocean warming can alter various ecosystem functions and associated services, influence changes to community structure (e.g., Hiscock et al, 2004; Mousing et al, 2014), and enhance carbon and nitrogen fluxes between phytoplankton and heterotrophic bacteria, indicating increased temperature may benefit mutualistic relationships of certain species (ArandiaGorostidi et al, 2017)

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Discussion

Conclusion