Shallow Hydrothermal Fluids Shape Microbial Dynamics at the Tagoro Submarine Volcano (Canary Islands, Spain).

Shallow underwater hydrothermal systems are often overlooked despite their potential contribution to marine diversity and biogeochemistry. Over a decade after its eruption, the Tagoro submarine volcano continues to emit heat, reduced compounds, and nutrients into shallow waters, serving as a model system for studying the effects of diffuse hydrothermal fluids on surface microbial communities. The impact on both phytoplankton and bacterial communities was examined through experimental manipulations mimicking dilution levels up to ~100 m from the primary crater of Tagoro. Chlorophyll a concentration doubled in the presence of hydrothermal products, with peak levels detected about a day earlier than in controls. Picoeukaryotes and Synechococcus cell abundances moderately increased, yet small eukaryotic phytoplankton (≤ 5 μm) predominated in the hydrothermally enriched bottles. Dinoflagellates, diatoms, small green algae and radiolarians particularly benefited from the hydrothermal inputs, along with phototrophic and chemoautotrophic bacteria. Our results indicate that hydrothermal products in shallow waters enhance primary production driven by phototrophic microbes, potentially triggering a secondary response associated with increased organic matter availability. Additionally, protistan grazing and parasitism emerged as key factors modulating local planktonic communities. Our findings highlight the role of shallow submarine hydrothermal systems in enhancing local primary production and element cycling.