Hydrothermal sulphur bacteria enhance mercury availability for coastal marine organisms

The hydrothermal compounds massively released into the sea by the geothermal power plant of Bouillante (Guadeloupe, Lesser Antilles) support the growth of sulphur-oxidising bacteria like around black smokers. Opportunistic animals of the bay were previously shown to actively seek and consume the bacterial biofilm. To investigate the role of these bacterial mats in enhancing the transfer of Hg, a highly toxic heavy metal, its concentrations were assessed in sediments, bacteria, and 6 low trophic levels animals from the Bouillante Bay and a Control Site nearby containing only natural sources of Hg. Concentrations of Hg in all samples from Bouillante were greater than those from the Control Site (2 to 627 times higher). A comparison with the Hg concentrations reported in the literature for similar samples types reveals that they are abnormally high in most Bouillante samples. Although bacterial biofilm contained high levels of Hg, the link between bacteria contribution to diet and Hg concentration was more complex than expected, which might be due to interspecific differences in Hg uptake and elimination rates. Species not consuming bacteria (i.e. filter feeders) still presented higher levels of Hg, suggesting that significant amounts of Hg are released along with discharge waters from the Bouillante geothermal plant, and are integrated through diffusion. Differences in Hg concentrations between tissues do not indicate that Hg contained in samples from animals was under the form of MeHg, lowering the biomagnification potential of Hg in the Bouillante Bay trophic food web. Overall, the Bouillante case study emphasises for the first time an important role played by sulphur bacteria mats as a vector of hydrothermal inorganic Hg, and potentially other trace metals emitted in vents area, through dietary pathways.