Molecular characterization of organically bound copper in the marine environment

Abstract

Marine microbes require copper (Cu) for a variety of key enzymes and can therefore experience limitation when concentrations are low. However, when Cu concentrations are too high, it becomes toxic causing decreased cell growth and even cell death. Laboratory culture experiments have shown that a diverse array of microbes produce organic ligands that complex Cu (CuL) and buffer the free ion concentration, which is the most bioavailable fraction. In this way, the microbes impose a control on the speciation of Cu, decreasing the toxic effects of Cu and making seawater conditions favorable for growth. Studies have shown that CuL complexes produced in laboratory cultures have similar complexation strengths to those found in seawater samples, which suggests a biological source of CuLs in seawater where dissolved Cu is almost entirely bound by organic ligands. However, information about individual CuL complexes is lacking which limits our understanding of the sources, sinks, and cycling of dissolved Cu. In order to fill this gap in knowledge, molecular level information about CuL complexes produced in culture and found in seawater must be obtained. To investigate this, liquid chromatography (LC) was coupled to two mass spectrometers (MS), an inductively coupled plasma (ICP) MS and an electrospray ionization (ESI) MS. By using data supplied by both techniques, the molecular charateristics of CuLs were determined laboratory cultures of the marine diatom Phaeodactylum tricornutum and the cyanobacterium Synechococcus, as well as investigating the distribution of CuLs in natural seawater samples along a line from 56°N to 20°S, along 152°W through the north and central Pacific Ocean. The CuLs identified in laboratory cultures had molecular formulae and fragmentation patterns characteristic of linear tetrapyrroles, a group of organic compounds commonly found in biological systems. This identification was further supported by absorbance and nuclear magnetic resonance spectroscopy. The distribution of CuLs in the Pacific Ocean showed a highly dynamic and complex mixture of ligands, closely tied to biological cycles.