Activity of microbial dark carbon fixation along the gradient of hydrothermal plume inflow by subtropical shallow-water vents

Abstract

Dark carbon fixation is a significant component of marine primary production. Although non-photosynthetic chemoautotrophic dark carbon fixation has been well-documented in aphotic deep-sea hydrothermal vents, shallow-water hydrothermal systems generate distinctive chemical conditions that may also support chemoautotrophic activity within well-lit water columns. To investigate dark carbon fixation associated with hydrothermal vent plumes in a euphotic environment, we conducted inorganic 14C incubation experiments under varying light conditions using surface and mid-layer waters collected along three transects near Kueishantao Islet in northeastern Taiwan-an area characterized by active shallow-water hydrothermal venting. The findings revealed that surface and mid-layer waters influenced by hydrothermal vent plumes, characterized by lower pH (7.6-7.8), reduced dissolved oxygen concentrations (143-180 mu M), and elevated methane levels (358-920 nM), exhibited high chlorophyll-specific primary productivity (PB) even under low irradiance (E), including in the dark (14-49 mg C mg Chl- 1 h- 1). No apparent saturation was observed in the PB-E relationship under these conditions. However, waters from the northern and southern transects and from the offshore area east of the vent, which were weakly influenced by hydrothermal vent plumes (pH 7.9-8.1, O2 180-211 mu M, CH4 6.8-128 nM), exhibited no detectable PB in the dark and showed saturation of PB with increasing irradiance. These observations suggest that the reductive chemical environment and elevated methane availability associated with shallow-water hydrothermal venting may promote chemoautotrophic dark-carbon fixation. By elucidating the interactions between hydrothermal activity and dark carbon fixation in euphotic coastal waters, this study enhances our understanding of marine carbon cycling and its broad biogeochemical implications.