Molecular Characterization of Organically Bound Copper in the Marine Environment

Lydia Babcock-Adams, Ph.D., 2022
Daniel Repeta, Advisor

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.