Copper sensitivity of Gonyaulax tamarensis1

The copper sensitivity of the dinoflagellate Gonyuulax tamarensis was examined in artificial seawater medium. Two short term responses of the organism to copper toxicity are rapid loss of motility and reduced photosynthetic carbon fixation. The chelators tris(hydroxymethylamino)methane (Tris) and cthylenedinitrilotetraacetic acid (EDTA) were used to dcmonstrate that copper toxicity is a unique function of cupric ion activity. Copper additions to medium containing EDTA equilibrated with the chelator relatively slowly, resulting in misleading short term data. This kinetic effect was not seen when the major copper chelator was Tris or when the copper was added in a chelated form with EDTA. Variations in manganese concentrations over two orders of magnitude did not alter the results. Cells of G. tamarensis are 100% nonmotile at a calculated cupric ion activity of lO-“.7 M with 50% of the cells nonmotile at lo‘n.4 M. Nonmotile cells do not divide or grow larger. Gonyaulux tamarensis growth is totally inhibited at cupric ion activities that only partially inhibit the growth of four other species that have been studied extensively. Furthermore, this toxicity occurs at the calculated copper activity of natural waters, assuming only inorganic copper complexation. Thus organic chelation may be necessary before G. tamarensis can successfully compete with other algal spccics in coastal waters. Concentrations of copper as low as 1 ppb can be toxic to various phytoplankton species (e.g. Steemann Nielsen et al. 1969; Stecmann Nielsen and Kamp-Nielsen 1970; Mandelli 1969; Erickson 1972; Martin and Olander 1971). Complexing agents are thus important for reducing copper toxicity in culture media (Manahan and Smith 1973; Fitzgerald and Faust 1963). Steemann Nielsen and Wium-Andersen (1970) suggested that the effect of organic chelators in stimulating the growth of algae in seawater, as observed by Johnston (1963, 1964) and Barber and Ryther (1969), was due to their ability to decrease the activity of the free cupric ion. This interpretation has been supported by the computations of Jackson and Morgan (1978) using the results of Davey et al. (1973), Barber (1973), and Huntsman (unpublished), and by the laboratory results of Sunda and Guillard (1976) who found that copper uptake and toxicity were uniquely determined by the cupric ion activity for two species of marine algae. ’ Research supported in part by the Office of Sea Grant, NOAA, U.S. Department of Commerce, through grant 04-6-158-44007, in part by National Science Foundation grant DES7515023, and in part by International Copper Research Association project 252. We have found the response of the dinoflagellate Gonyaulax tamarensis to copper toxicity is a rapid loss of motility-a reaction to stress that provides a unique opportunity to study the organism’s short term sensitivity to copper, relatively free from the potential complications of cellular exudates and other changes in the trace metal composition of the culture medium with time. Other types of chemical and physical stresses also cause this organism to become nonmotile. Temperature has been cited as a major factor (Prakash 1967; Yentsch et al. 1975), although Prakash also noted that nonoptimal conditions of nutrients, salinity, and light can induce a limited loss of motility. Nonmotile cells do not divide or increase in size if the stress is maintained, yet in almost all cases they can be “revived” if the stress has not been too severe and normal culture conditions are restored. This has led many researchers to refer to stress-induced nonmotile cells as cysts, but recent comparisons between laboratory cultures and cells of G. tamarensis taken from marinc sediments indicate that this terminology may be incorrect (Anderson and Wall 1978). WC will not call the stationary cell observed in laboratory cultures after exposure to toxic concentrations of copper a cyst, but LJMNOLOGY AND OCEANOGRAPHY 283 MARCH 1978, V. 23(2) 284 Anderson and Morel Table 1. Modified Aquil seawater medium. Analyt. concn Computed -log (free ion) Computed major species