Toxin and Growth Responses of the Neurotoxic Dinoflagellate Vulcanodinium rugosum to Varying Temperature and Salinity

Vulcanodinium rugosum, a recently described species, produces pinnatoxins. The IFR-VRU-01 strain, isolated from a French Mediterranean lagoon in 2010 and identified as the causative dinoflagellate contaminating mussels in the Ingril Lagoon (French Mediterranean) with pinnatoxin-G, was grown in an enriched natural seawater medium. We tested the effect of temperature and salinity on growth, pinnatoxin-G production and chlorophyll a levels of this dinoflagellate. These factors were tested in combinations of five temperatures (15, 20, 25, 30 and 35 °C) and five salinities (20, 25, 30, 35 and 40) at an irradiance of 100 µmol photon m−2 s−1. V. rugosum can grow at temperatures and salinities ranging from 20 °C to 30 °C and 20 to 40, respectively. The optimal combination for growth (0.39 ± 0.11 d−1) was a temperature of 25 °C and a salinity of 40. Results suggest that V. rugosum is euryhaline and thermophile which could explain why this dinoflagellate develops in situ only from June to September. V. rugosum growth rate and pinnatoxin-G production were highest at temperatures ranging between 25 and 30 °C. This suggests that the dinoflagellate may give rise to extensive blooms in the coming decades caused by the climate change-related increases in temperature expected in the Mediterranean coasts.

Coupled Ge/Si and Ge isotope ratios as geochemical tracers of seafloor hydrothermal systems: Case studies at Loihi Seamount and East Pacific Rise 9°50’N

Germanium (Ge) and Silicon (Si) exhibit similar geochemical behaviour in marine environments but are variably enriched in seafloor hydrothermal fluids relative to seawater. In this study, Ge isotope and Ge/Si ratio systematics were investigated in low temperature hydrothermal vents from Loihi Seamount (Pacific Ocean, 18°54’N, 155°15’W) and results were compared to high-temperature vents from the East Pacific Rise (EPR) at 9°50’N. Loihi offers the opportunity to understand contrasting Ge and Si behaviour in low temperature seafloor hydrothermal systems characterized by abundant Fe oxyhydroxide deposition at the seafloor. The results show that both Ge/Si and δ74/70Ge in hydrothermal fluids are fractionated relative to the basaltic host rocks. The enrichment in Ge vs. Si relative to fresh basalts, together with Ge isotope fractionation (Δ74/70Ge fluid-basalt up to 1.15 ‰ at EPR 9°50’N and 1.64 ‰ at Loihi) are best explained by the precipitation of minerals (e.g. quartz and Fe-sulfides) during higher temperature seawater-rock reactions in the subsurface. The study of Fe-rich hydrothermal deposits at Loihi, largely composed of Fe-oxyhydroxides, shows that Ge isotopes are also fractionated upon mineral precipitation at the seafloor. We obtained an average Ge isotope fractionation factor between Fe-oxyhydroxide (ferrihydrite) and dissolved Ge in the fluid of -2.0 ± 0.6 ‰ (2sd), and a maximum value of -3.6 ± 0.6 ‰ (2sd), which is consistent with recent theoretical and experimental studies. The study of a hydrothermal chimney at Bio 9 vent at EPR 9°50’N also demonstrates that Ge isotopes are fractionated by approximately -5.6 ± 0.6 ‰ (2sd) during precipitation of metal sulfides under hydrothermal conditions. Using combined Ge/Si and estimated Ge isotope signatures of Ge sinks and sources in seawater, we propose a preliminary oceanic budget of Ge which reveals that an important sink, referred as the “missing Ge sink”, may correspond to Ge sequestration into authigenic Fe-oxyhydroxides in marine sediments. This study shows that combining Ge/Si and δ74/70Ge systematics provides a useful tool to trace hydrothermal Ge and Si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits.