Uptake and efflux of 64Cu by the marine cyanobacterium Synechococcus (WH7803)

The uptake and efflux of 64Cu was studied in the marine cyanobacterium Synechoccous strain WH7803 (DC2). Uptake followed classical Michaelis-Menten type kinetics in metal-buffered seawater. The maximum uptake rate, Vmax, was 0.236 6 0.016 3 10218 mol Cu cell21 h21, with the half-saturation constant, KS, of 10210.8160.11 mol L21. An efflux mechanism was also observed in WH7803, whose growth was inhibited by high internal Cu concentrations. Efflux of Cu enabled WH7803 to maintain homeostasis for Cu at typical seawater ambient free copper concentrations ([Cu]f). The sensitivity of WH7803 growth to Cu was related to a simple inability to regulate internal Cu concentrations when external concentrations were .10211 mol L21. Copper is known to be extremely toxic to many marine phytoplankton species (Brand et al. 1986). In particular, the growth of many dinoflagellate and cyanobacteria species is reduced at free Cu concentrations ([Cu]f) of $10211 mol L21 (pCu 5 11, where pCu 5 2log [Cu]). Intracellular trace metal uptake by phytoplankton typically follows Michaelis– Menten type kinetics (Sunda 1994); however, there are currently only sparse kinetic data available for the uptake of Cu. Studies on vascular plants have shown that cells can adapt a variety of mechanisms to reduce the toxicity of Cu. Such mechanisms include (1) internalization of Cu in the cell by intracellular binding with metallothioneins (e.g., phytochelatin) and (2) efflux from the cell of strong Cu-binding organic ligands (Macnair 1993). Currently, there is very little information linking the Cu sensitivity of phytoplankton to uptake rates and the mechanisms by which cells control their internal Cu concentrations. Organic Cu complexes, which researchers in the field classify according to the determined conditional stability constants, dominate copper complexation in seawater (Coale and Bruland 1988; Moffett et al. 1990; Moffett 1995). Class 1 ligands, denoted L1 with log K . 13, are strong Cu-binding ligands that are typically found at concentrations slightly above the ambient dissolved Cu concentration. Weaker class 2 ligands, denoted L2 with log K , 12, are found at con1 To whom correspondence should be addressed. Present address: Institut für Meeereskunde, Kiel, Germany ([email protected]).