Toxic effects of Heterosigma akashiwo do not appear to be mediated by hydrogen peroxide

The ichthyotoxic red tide organism Heterosigma akashiwo (Raphidophyceae) ha s been associated with fish kill,ani events within the aquaculture industry for many years. The precise toxicological mechanism involved in these fish kills is unclear, however, much research attention has focused on the prod uction of reactive oxygen species (ROS) by these toxic algae. In this study , we investigated the production of hydrogen peroxide (H2O2) by isolates of H. akashiwo and the nontoxic chlorophyte Tetroselmis apiculata. Subsequent ly, we tested those concentrations of H2O2 on vertebrate cell lines and the invertebrate Artemia salina (brine shrimp) to investigate mortality. Net p roduction rates for the H. akashiwo isolates ranged from 0.46 to 7.89 pmol H2O2 min(-1) (10(4) cells)(-1) while obtaining maximum concentrations betwe en 0.14 and 0.91 muM H2O2. Conversely, T. apiculata produced only 0.03 pmol H2O2 min(-1) (10(4) cells)(-1) with a maximum level on 0.04 muM. However, toxic effects on UNIR-106 and HEK-293 cells were only induced by acute and protracted exposure to concentrations of H2O2 greater than or equal to 0.1 mM. Additionally, significant mortality of A. salina in the presence or abs ence of ferric and ferrous iron was induced by H2O2 levels greater than or equal to 1 mM. Iron is a redox metal that reduces H2O2 to hydroxy radicals. These data collectively indicate that production of H2O2 by multiple isola tes of H. akashiwo is orders of magnitude less than that required for morta lity of either the Vertebrate cell lines or the invertebrate A. salina. Oth er nonichthyotoxic roles for extracellular ROS are proposed.