Trace metal adsorption in three serial flow-through systems: implications for foraminiferal culturing experiments

Abstract. Benthic foraminifera have been utilized extensively in laboratory culturing experiments to study the effects of elevated and/or depleted seawater concentrations of trace metals (e.g., on morphological shell abnormalities). In addition, past oceanic concentrations of various elements have been reconstructed through the analyses of benthic foraminiferal calcite. Paleoproxies of several trace metals have been used to track geologically ancient surficial processes (e.g., weathering and redox) by using black shales; however, analyses of foraminiferal calcite remain underconstrained for many transition metals. Growth experiments with trace elements, such as molybdenum (Mo) in this study, could be used to explore their potential as proxies to expand the sedimentary archives beyond those of black shales. Additional calcite proxies would provide more robust long-term paleo-redox reconstructions, especially for periods of Earth's history with limited archives. Concentrations of trace elements in seawater are often extremely low and measurements can be altered by adsorption, contamination, and/or stratification. To assess possible effects in calcifying organisms from trace metal concentration fluctuations, closed-system experiments using serial flow-through culturing devices can help ensure reliable results. Minor concentration changes in a trace element, particularly near or lower than seawater concentrations, can alter the equilibration time required for the modified source seawater to normalize with the seawater that has passed through the experimental setup. In this study, the stabilization was investigated after Mo-enriched seawater passed through three serial flow-through culturing systems. System A was an apparatus using a typical 24-well plastic cell culture plate, System B was based on System A but included modifications such as a customized block of polytetrafluoroethylene (PTFE) in place of the plastic cell culture plate, and System C was a serial system of synthetic perfluoroalkoxy alkane (PFA) vials connected using polytetrafluoroethylene tubing. The three systems were tested for the duration required to reach steady state to examine Mo adsorption by each system (A, B, and C) using 5-times and 2-times natural seawater concentrations of Mo ([Mo]sw). Though the three serial flow-through culturing systems reached equilibrium early in the process when [Mo]sw was 5-times seawater concentration, the experiments demonstrated that, when the Mo concentration was closer to modern seawater levels, systems reached equilibrium at different times. Specifically, systems that incorporated more adsorption-resistant materials and minimized seawater contact with “common” plastic (e.g., polypropylene and polystyrene) components reached equilibrium more quickly, where System C was the fastest. At low concentrations, equilibration takes longer due to the abundance of the element that can be adsorbed by components over time being lower, while the amount of time may become insignificant at far higher concentrations. Since equilibration time at lower concentrations with this trace metal differed by system, other foraminiferal trace element experiments should also consider the effect of adsorption. These setups were further tested in two experiments that compared the incorporation of Mo at various [Mo]sw into foraminiferal calcite, and their very low measurements, near current detection limits, demonstrate the importance of minimizing external effects such as adsorption. Importantly, with the best experimental setup tested, there was a small but measurable difference in the foraminiferal incorporation. Therefore, greater utilization of adsorption-resistant materials in experimental design is advised when studying very low oceanic elemental concentrations and their incorporation into foraminiferal calcite.