Potential trace-chemical (Fe, Mn, CH4) limitation on bacterial growth in deep sea hydrothermal vent plumes along the Southern East Pacific Rise
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Global cycling of elements like carbon, nitrogen, and iron has a key role in maintaining the biosphere. These and other micro- and macro-nutrients undergo important reduction-oxidation and acid-base transformations in the environment. Biologically, iron (Fe) and manganese (Mn) are utilized by microbes as cofactors in many essential proteins and enzymes including nitrogenase, ferredoxin, peroxidase, cytochromes, and phosphotransferase. These elements (most notably Fe) can often limit microbial growth in large regions of the ocean due to their trace environmental concentrations, structural bioavailability, or competitive microbial uptake and utilization. This can impact key ecosystem processes, including chemosynthetic carbon fixation at hydrothermal vents, nitrogen species reduction, and metabolic electron transport. Here I used flow cytometry measurements (FCM) to quantify bacteria and archaea from hydrothermal vent plumes along the Southern East Pacific Rise in September-November 2021. I compared patterns in microbial abundance with total dissolvable Fe concentrations (predominantly Fe3+, including dissolved and labile particulate Fe) and other key chemicals like Mn and methane (CH4) at the same locations. I hypothesized that hydrothermal sites with higher chemical concentrations would also have higher microbial abundance. I found that bacterial abundance is positively related to trace-concentration of Fe below 400nM (r2Fe= 0.67), and that similar relationships exist with trace CH4 and Mn concentrations (r2CH4 = 0.55; r2Mn = 0.51). These findings suggest that microbial abundances in vent plumes can be partially explained by trace metal and methane distributions. Further research is required to disentangle whether these substrates covary with other biochemical controls on microbial growth and metabolism (like sulfide nutrients) in these dynamic environments.