FORAMINIFERA IN EXTREME ENVIRONMENTS: HOW BENTHIC POPULATIONS RESPOND TO EXPOSURE TO HIGH-[AS], LOW-PH HYDROTHERMAL VENT FLUIDS IN PAPUA NEW GUINEA

Several shallow-water hydrothermal vents discharge into Tutum Bay, Ambitle Island, northeastern Papua New Guinea. Vent waters feature some of the world’s highest naturally-occurring arsenic concentrations ([As]) – up to 11.8µmol/L, or almost 800X higher than normal seawater. Discharge by the vents has been estimated at >1500g As/day into an area approximately 5000m² with an average depth of 6m. This As is mitigated by mixing with seawater and removal of As by incorporation into As-containing iron-oxyhydroxide precipitates, which form as crusts when hydrothermal fluids mix with seawater; crusts contain up to 76,000ppm As. Consequently, [As] are considerably higher near vent sites, in porewaters, and near the sediment/water interface, than in overlying seawater. Despite the high [As], the biota of the bay does not show an obvious severely adverse response; healthy-appearing hard and soft coral and fishes abound. The effects on benthic foraminiferal populations, however, are more complicated. Benthic foraminifera live in close proximity to pore waters and in intimate association with As-rich sediments. The relatively small sizes of the foraminifers, their high surface-to-volume ratios, which promotes diffusive processes, and the extension of their reticulopodia into high As microenvironments, all may amplify their exposure. In addition, As ions may be incorporated into calcite tests and As-laden sediment grains may be used by agglutinated taxa. Foraminiferal abundance and diversity increase with distance from the vents, as [As] decrease and porewater pH increases. A sharp transition zone between very low abundance and higher, more normal abundance occurs 140-150m from the vents, defining zones of higher and lower hydrothermal influence. Laboratory experiments show that foraminifers are adversely affected by [As⁵⁺] of 1000ppb and [As³⁺] of 200ppb, and that growth rate and reproduction are also affected by As exposure. The hydrothermal system at Tutum Bay provides an ideal opportunity to examine the effects of arsenic, a toxin with implications for human and environmental health, in an environment that is minimally impacted by human activities. Furthermore, by examining these effects on Foraminifera, we are observing organisms with general utility as bioindicators of environmental health.