Limnol. Oceanogr., 45(3), 2000, 619–627

Organic chelators that are thought to be of recent biological origin control copper speciation in seawater. Previous studies have often resolved these copper binding ligands into two classes, class 1 ligands (conditional stability constants 1012–1014 M21) and class 2 ligands (conditional stability constants 109–1012 M21). At present the sources and structures of these chelators are unknown. Previous work has shown that in response to copper toxicity, cultures of the marine cyanobacteria Synechococcus spp. produce a strong ligand, similar in binding strength to class 1 ligands. We extended this work by surveying a suite of copper-stressed marine eucaryotic phytoplankton for the production of copper binding ligands. Most eucaryotic species we studied did not produce ligands with the binding strength of class 1 ligands, but many produced class 2 ligands. A marine dinoflagellate Amphidinium carterae was found to produce very strong ligands; however, the low abundance of dinoflagellates in the open ocean may mean that this source of class 1 ligands is more important in coastal waters. This survey shows that there are many biological sources of copper binding ligands, but that procaryotes such as cyanobacteria are more plausible sources of class 1 ligands in the open ocean than eucaryotes. There is now considerable evidence to suggest that the distribution and speciation of trace metals in the upper water column play an important role in the species composition and physiology of phytoplankton assemblages (Sunda 1994). The chemical speciation of trace metals is important because only certain forms of a given metal are biologically available. In the upper water column, speciation of many biologically active trace metals is controlled by complexation with strong organic ligands (Bruland et al. 1991 and references therein). Complexation of the metal by an organic ligand generally lowers biological availability because the free metal ions are the most biologically available forms (Sunda 1994; Campbell 1995). In addition, complexation may play an important role in the geochemical cycling of these elements in the upper ocean. For many elements, concentrations of these ligands are highest in the euphotic zone and decline to non-detectable levels in the deep waters (Bruland et al. 1991; Rue and Bruland 1995). This distribution suggests that the ligands are of recent biological origin and are not refractory compounds. However, their biological function, if any, is unknown. Complexation is particularly important for Cu, an essential micronutrient that is also toxic. Field studies in coastal 1 Present address: Department of Marine Chemistry and Geology, Netherlands Institute for Sea Research (NIOZ), Postbus 59, 1790 AB Den Burg—Texel, Netherlands. 2 Corresponding author.