Chemical speciation of copper in a salt marsh estuary near Sapelo Island, Georgia

The concentrations of dissolved copper (Cud), copper-binding ligands, thiourea-type thiols and humic substances (HSCu) were measured in estuarine waters adjacent to Sapelo Island, Georgia, USA, on a monthly basis from April to December 2014. Here we present the seasonal cycle of copper speciation within the estuary and compare it to the development of an annually occurring bloom of ammonia oxidising archaea (AOA) Thaumarchaeota, which require copper for many enzymes. Two types of complexing ligands (L1 and L2) were found to dominate with mean complex stabilities (log K'CuL) of 14.5 and 12.8. Strong complexation resulted in lowering of the free cupric ion (Cu2+) concentration to femtomolar (fM) levels throughout the study and to sub-fM levels during the summer months. A Thaumarchaeota bloom during this period suggests that this organism manages to grow at very low Cu2+ concentrations. Correlation of the concentration of the L1 ligand class with a thiourea-type thiol and the L2 ligand class with HSCu provide convincing evidence for the identity of the ligands. Due to the stronger complex stability, 82 - 99 % of the copper was bound to L1. Thiourea-type thiols form strong Cu(I) species, suggesting that ~90% copper is present as Cu(I) in this region, upsetting the paradigm of its predominance as Cu(II). In view of the very low concentration of free copper (pCu >15 at the onset and during the bloom) and a reputedly high requirement for copper, it is likely that the Thaumarchaeota are able to access the thiol-bound copper directly.