In situ primary production in Mid-Ocean Ridges hydrothermal plumes: insights from biogeochemical modelling

Although the importance of hydrothermal plumes in the global ocean trace metal cycle is being increasingly recognized, their impact on the deep ocean carbon cycle is still poorly known. Hydrothermal subseafloor microorganisms have recently been emphasized as highly productive, but how much carbon is fixed within the actual plumes by microbial communities remains elusive.

Several studies have provided insights into the plume microbial communities and metabolisms, and numerous numerical models based solely on thermodynamics mixing approaches were used to estimate primary production and geochemical reaction rates in deep-sea hydrothermal systems. Two numerical studies have gone one step further by incorporating a kinetic component allowing to spatially resolve the geochemical reactions and microbial pathways within the hydrothermal plume or chimney.

Here, we present a spatially resolved new model for predicting hydrothermal plume from an integrated biogeochemical perspective: we aim at precisely characterizing the steep redox gradient produced by the mixing between the hydrothermal fluid and the ambient seawater. Focused on biotic related reduced compounds (e.g. Fe, H₂S, NH₄, H₂ or CH₄), it predicts abiotic transformation and potential microbial pathways in both buoyant and non-buoyant plume, to elucidate 1) the plume internal chemotrophic primary production and 3) the reactivity and dispersion of chemicals and their potential export. Including kinetics consideration, this model constitutes a useful tool for world-wide geochemists to access the impact of hydrothermal inputs on global biogeochemical cycles.

Applied to various hydrothermal sites along the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR), our model was able to reproduce accurately plume heights and chemical species distribution. Although there are no actual primary production measurements in hydrothermal plumes to compare to, it gives similar results than recent estimates of subseafloor carbon fixation rates, emphasizing its ability in predicting valid carbon fixation rates. Model outputs from MAR and EPR give plume primary production rates in the same order of magnitude than productive coastal ecosystems, suggesting that hydrothermal systems may have a significant impact on global carbon cycle, at a regional scale at least.