Degree
Doctor of Philosophy (PhD)
Department
Environmental Science
Document Type
Dissertation
Abstract
Methylmercury (MeHg) is a potent neurotoxin that enters marine food webs through phytoplankton, yet the factors controlling its toxicity remain insufficiently understood. This dissertation examined how phytoplankton biomass, temperature, species identity, and exposure duration influence MeHg toxicity across marine taxa. Four hypotheses were tested: MeHg EC₅₀ increases with increasing phytoplankton biomass; EC₅₀ decreases with increasing temperature; EC₅₀ differs among species; and 96-hour EC₅₀ values are lower than 48-hour values, reflecting greater chronic toxicity.
Chapter 2 evaluated biomass effects using five marine phytoplankton species: Phaeodactylum tricornutum, Emiliania huxleyi, Synechococcus elongatus, Cricosphaera carterae, and Thalassiosira pseudonana. Acute 48-hour dose-response assays across gradients of initial optical density showed a consistent positive relationship between biomass and EC₅₀. Regression-based extrapolation to OD = 0 produced intrinsic, biomass-normalized EC₅₀ estimates that converged more closely across taxa than raw values, indicating that much apparent interspecific variation reflects biomass-dependent buffering rather than physiological sensitivity.
Chapter 3 tested temperature, species identity, and exposure duration using T. pseudonana, S. elongatus, and C. carterae across 20-30 °C and 48- and 96-hour exposures. Contrary to expectations, EC₅₀ values did not consistently decline with increasing temperature; in several cases, apparent MeHg tolerance increased at warmer temperatures. Species identity did not significantly affect intrinsic EC₅₀ after biomass normalization, and longer exposure did not consistently reduce EC₅₀ relative to acute assays. Temperature exerted the strongest influence on toxicity responses, suggesting that thermal conditions can reshape MeHg sensitivity and complicate comparisons.
Chapter 4 characterized thermal performance by modeling temperature-growth relationships across the same representative taxa. Suboptimal growth generally followed conserved exponential scaling consistent with the Eppley relationship, whereas quadratic models revealed species-specific differences in thermal breadth and warm-edge sensitivity. Two taxa exhibited unimodal performance curves with optima near 30-31 °C, while one continued increasing across the experimental range.
Collectively, these findings show that MeHg toxicity in marine phytoplankton is context-dependent. Biomass systematically buffers MeHg effects, while temperature alters apparent sensitivity and growth performance. By integrating biomass-normalized toxicity thresholds with thermal performance analysis, this dissertation improves predictions of phytoplankton vulnerability to MeHg in warming, oligotrophic marine systems.
Date
6-8-2026
Recommended Citation
Ihunwo, Owhonda C., "Temperature- and Biomass-Dependent Toxicity of Methylmercury in Marine Phytoplankton" (2026). LSU Doctoral Dissertations. 7104.
https://repository.lsu.edu/gradschool_dissertations/7104
Committee Chair
Laws, Edward A
LSU Acknowledgement
1
LSU Accessibility Acknowledgment
1