Experimental Dataset: Biomass- and Temperature-Dependent Methylmercury Toxicity in Marine Phytoplankton
Description
Research Description This repository contains datasets from three linked studies examining biomass-dependent methylmercury (MeHg) toxicity, temperature-modulated toxicity, and thermal growth responses in marine phytoplankton. The objective was to determine how biomass and temperature regulate growth and contaminant sensitivity at the base of marine food webs. Research Hypotheses We tested three hypotheses: (1) Apparent EC₅₀ values increase with biomass due to density-dependent buffering and therefore overestimate intrinsic cellular sensitivity. (2) Temperature alters intrinsic MeHg toxicity and shifts community-level protection thresholds. (3) Phytoplankton growth follows canonical exponential (Eppley-type) scaling at suboptimal temperatures but diverges among species in full thermal performance curves. Experimental Design Experiments used axenic cultures of Phaeodactylum tricornutum, Emiliania huxleyi, Synechococcus elongatus, Thalassiosira pseudonana, and Cricosphaera carterae. For toxicity assays, cultures were exposed to graded MeHg concentrations across multiple initial biomass levels measured as optical density (OD₇₅₀). Experiments were performed at 20°C, 25°C, and 30°C under 48 h and 96 h exposures. Growth rates were calculated from OD changes, dose–response curves were fit using Hill models, and EC₅₀ values were estimated. Intrinsic toxicity was derived by regressing log₁₀(EC₅₀) against OD₇₅₀ and extrapolating to OD = 0 to remove biomass-buffering effects. Unified regression models quantified species × temperature × biomass interactions. Species sensitivity distributions (SSDs) were constructed from intrinsic EC₅₀ values to derive HC₅ and Final Acute Value benchmarks. For thermal physiology experiments, three species were incubated across six temperatures (20–35°C). Growth was modeled using quadratic thermal performance curves to estimate optimal temperature (Tₒₚₜ). Log-linear regressions and ANCOVA tested conformity with the canonical Eppley exponent (b = 0.0631). Key Findings and Reuse Biomass strongly inflated apparent EC₅₀ values; once corrected to OD = 0, intrinsic MeHg sensitivity converged across taxa. Temperature significantly modulated toxicity, particularly under longer exposure, with warming generally increasing intrinsic EC₅₀ values. Thermal growth experiments showed unimodal curves with optima near 30–32°C. Although suboptimal scaling aligned with Eppley predictions, species differed in curvature and high-temperature decline. The dataset includes raw OD measurements, growth rates, dose–response fits, EC₅₀ estimates, regression outputs, ANCOVA results, and SSD-derived benchmarks. It supports climate-sensitive ecotoxicological modeling and trait-based analyses of phytoplankton responses in a warming ocean.