Effect of oil spill on compound-specific stable carbon isotope composition of amino acid in Nitzschia closterium and Heterosigma akashiwo.

Marine microalgae, the most important primary producers in marine ecosystems, are susceptible to toxicants and usually selected as the promising organisms for chemical risk testing. The stable carbon isotope ratios (δC) of amino acids (AAs) were measured to determine if compound-specific stable isotope analysis could be used to provide insight into the toxic effects of oil spill on the marine microalgae. The water accommodated fractions (WAF) of #180 fuel oil were selected as the toxic targets with different concentrations in acute (96 h) toxic tests. Naphthalene, phenanthrene, anthracene, and fluoranthene were detected as the predominant congeners of polycyclic aromatic hydrocarbons in the WAF, and may be the main toxic components. At the same WAF concentration, the δC values of leucine in Nitzschia closterium increased with the culture time, whereas decreased in Heterosigma akashiwo. However, with the increasing WAF concentrations, the δC values of glutamic acid exhibited the increasing trend in both of the two kinds of microalgae at the same culture time. The similar phenomenon was also observed for histidine in H. akashiwo, while opposite in N. closterium, but both enriching in δC compared to the controlled experiments without the WAF. Under the different culture times and WAF concentrations, the δC values of alanine and methionine showed unexpected trend. These findings indicated that the synthesis pathways of AA in microalgae cells were affected by the WAF, thus leading to the carbon isotopic fractionation in AAs. The present study provide a novel approach to explore the toxicity mechanism of primary producers under oil spill stress, and monitor and evaluate the marine ecological risk of oil spill pollution.