The effects of temperature, salinity, and sediment organic carbon on MeHg uptake in phytoplankton

Phytoplankton uptake of MeHg represents a key, but understudied step in the transfer of MeHg in marine food webs. Additionally, as phytoplankton represent the first bioconcentration (~10⁵ increase from water) of MeHg in food webs, understanding which factors influence MeHg uptake will assist in understanding MeHg bioaccumulation in marine food webs. Further, how climate change may impact MeHg uptake in phytoplankton or how this could alter marine food webs is unknown. Here, we examine how changes in salinity and carbon due to predicted increase in precipitation and changes in temperature would impact MeHg uptake in phytoplankton. We conducted a laboratory experiment, in which we used modified sediment cores with internally generated mixing to maintain cells in the water column. To these, we added ~5 cm of sediment from contaminated sites that are high in Hg (Mill Creek, NJ or Penobscot, ME) to cores. Then we added phytoplankton (Tetraselmis impellucida) and individually manipulated salinity (10 ppt, 20 ppt, or 30 ppt), organic carbon (Mill Creek: 5.3% LOI or 15.5% LOI; Penobscot: 5.2% LOI or 12% LOI), and temperature (15°C or 25°C). Phytoplankton were counted daily and harvested at stationary phase by filtering on GFFs in a trace metal clean room. In general, we found that at warmer temperatures, phytoplankton grew faster and MeHg uptake per cell was lower. Additionally, we found that MeHg uptake was higher when grown over sediment with lower organic carbon, but that no differences in uptake between sediments was observed. Lastly, we found that lower salinity resulted in higher uptake of MeHg into phytoplankton. Thus, sediment and water chemistry influences MeHg uptake from contaminated sediments into phytoplankton.