Abstract
Cool hydrothermal systems (CHSs) are prevalent across the seafloor and discharge fluid volumes that rival oceanic input from rivers, yet the microbial ecology of these systems are poorly constrained. The Dorado Outcrop on the ridge flank of the Cocos Plate in the northeastern tropical Pacific Ocean is the first confirmed CHS, discharging minimally altered <15°C fluid from the shallow lithosphere through diffuse venting and seepage. In this paper, we characterize the resident sediment microbial communities influenced by cool hydrothermal advection, which is evident from nitrate and oxygen concentrations. 16S rRNA gene sequencing revealed that Thaumarchaea, Proteobacteria, and Planctomycetes were the most abundant phyla in all sediments across the system regardless of influence from seepage. Members of the Thaumarchaeota (Marine Group I), Alphaproteobacteria (Rhodospirillales), Nitrospirae, Nitrospina, Acidobacteria, and Gemmatimonadetes were enriched in the sediments influenced by CHS advection. Of the various geochemical parameters investigated, nitrate concentrations correlated best with microbial community structure, indicating structuring based on seepage of nitrate-rich fluids. A comparison of microbial communities from hydrothermal sediments, seafloor basalts, and local seawater at Dorado Outcrop showed differences that highlight the distinct niche space in CHS. Sediment microbial communities from Dorado Outcrop differ from those at previously characterized, warmer CHS sediment, but are similar to deep-sea sediment habitats with surficial ferromanganese nodules, such as the Clarion Clipperton Zone. We conclude that cool hydrothermal venting at seafloor outcrops can alter the local sedimentary oxidation–reduction pathways, which in turn influences the microbial communities within the fluid discharge affected sediment.
Highlights
Ridge flank hydrothermal systems are globally widespread and responsible for over two-thirds of marine hydrothermal heat flux (Stein and Stein, 1994)
The hydrothermal cores had nitrate concentrations (∼30–40 μmol kg−1) similar to that measured in bottom seawater, a pattern previously interpreted as an indication of Cool hydrothermal systems (CHSs) advection (Wheat and Fisher, 2008)
Whereas many hot hydrothermal sediment microbial communities have been described in detail (Moyer et al, 1995; Teske et al, 2002; Cerqueira et al, 2015; Dowell et al, 2016), sediment communities impacted by discharge of cool, oxygen- and nitrate-containing basaltic formation fluids are less well understood
Summary
Ridge flank hydrothermal systems are globally widespread and responsible for over two-thirds of marine hydrothermal heat flux (Stein and Stein, 1994). A significant amount of this flux is proposed to be through low temperature fluids at cool hydrothermal systems (CHSs). CHS systems discharge fluid volumes that rival oceanic input from rivers, with an estimated 1014 kg year−1 of fluid flowing through just the ∼15,000 largest seamounts (Harris et al, 2004). This fluid flux facilitated by CHS results in the removal of riverine phosphate and affects global budgets for other solutes (Wheat et al, 2003, 2017). Core designation based on nitrate+nitrite concentrations, which were indicative of hydrothermal fluid flow through sediments (Wheat and Fisher, 2008). A dash (–) indicates that the value was not measured for this sample. ∗Nitrate+nitrite value from 2 cmbsf depth.
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