The influence of hydrothermal metal inputs on protistan and particle-associated bacterial metabolism in the Lau Basin of the tropical South Pacific Ocean

The tropical South Pacific Ocean contains oligotrophic surface waters with cyanobacteria and picoeukaryotes as dominant members of the phytoplankton community, which fuel primary production and the biological carbon pump. Low continentally-derived iron input can limit or co-limit phytoplankton growth in these waters, and subsurface hydrothermal vents may serve as important sources of iron and other bioactive metals. In the northeast Tonga-Fiji region off the Samoan Islands, hydrothermal activity (³He/⁴He) has been observed at ~1,700 and 2,500 m with the shallow helium plume hypothesized to be derived locally from the Lau Basin (Lupton et al. 2004; Geochemistry, Geophysics, Geosystems). The influence of such hydrothermal activity on biogeochemical metal cycling and microbial metal metabolism has yet to be investigated. Using seawater samples and microbial biomass collected along a meridional transect (17°N-15°S) beginning near the Hawaiian Islands (155°W) and terminating in the Tonga-Fiji area (175°W), we quantified dissolved metal concentrations and explored microbial metal metabolism through metaproteomics. We report here elevated iron and manganese concentrations consistent with the hydrothermal signal, and the abundance of metalloproteins in protists and particle-associated bacteria near the hydrothermal site. These results suggest that a recently identified hydrothermal vent system in the Lau Basin could be an important source of bioactive trace metals, influencing microbial physiology in surrounding seawater.