title: Marine humic substances: distribution, cycling pathways, and biogeochemical function
Creators: name: Goddard-Dwyer, Millie
Publisher: name: University of Liverpool Repository
Year: 2024

description: Trace metal availability is an important control on the marine carbon cycle via limitation of primary production. A key factor controlling the availability of trace metals is complexation by components of the dissolved organic matter (DOM) pool called organic ligands. Organic ligands act to stabilise trace metals and are thought to mediate bioavailability to microorganisms. Within the diverse organic ligand pool, trace metal binding electroactive humic substances (eHS) are thought to play an important role due to their abundance, ubiquity, and ability to bind to a range of trace metals.
 
Humics are a heterogenous pool of organic matter, known to be cycled via a suite of processes including primary production, remineralisation, grazing, terrestrial, sedimentary, photo-oxidation, and hydrothermal processes. However, several key knowledge gaps exist in our understanding of humic biogeochemistry. Firstly, the distribution of eHS in the Southern Ocean, a key iron limited area, is largely unknown. Secondly, the processes that control humic cycling are poorly constrained, but could be important interaction factors with trace metal sources. As such, the impact of eHS on trace metals and carbon cycling is unknown. The overarching aim of this thesis was to advance our understanding of humic cycling in these areas and to consider the role of humic substances in trace metal and DOC cycling. 

To reveal eHS distribution in the Southern Ocean and to investigate the processes involved in eHS cycling, eHS was determined in the Southwest Indian sector of the region. The Southwest Indian sector of the Southern Ocean is a predominantly iron limited, low productivity region with islands supplying iron that can stimulate phytoplankton blooms and bioavailable dissolved organic matter (DOM) production. The phytoplankton blooms associated with the islands were linked to elevated levels of eHS, potentially modulated by phytoplankton community composition and grazing. This implies a role of iron in stimulating microbial eHS production, however uncoupling between DFe and eHS also suggested a minor role of eHS in regional DFe transport. Offshore, prokaryote remineralisation was a potential important source of eHS. Incubation experiments were performed to unravel the impact of DOM composition in controlling prokaryote eHS cycling. Higher dissolved organic carbon (DOC) supply and DOM bioavailability increased eHS production suggesting that eHS cycling is constrained by carbon availability. While lower bioavailability DOM and DOC supply promoted the removal of eHS, likely due to prokaryote carbon limitation. These results suggest that DOM bioavailability may interact with prokaryote humic cycling to influence local iron stabilisation and transport, with possible impacts on the magnitude of iron-driven phytoplankton blooms.

Finally, mass balance modelling of eHS inside a hydrothermal plume reveal that hydrothermal activity was a net eHS source due to prokaryote activity and could supply 7 % of the deep (≥2000m) eHS standing stock in the 500 km downstream of the vent site, potentially impacting carbon storage. Similarities between dissolved copper and eHS profiles at plume peripheries suggest that eHS were released by oxidative dissolution of Cu-sulfide particles. Although eHS did not covary with iron, it may complex a significant portion of dissolved iron 10 km from the vent. This emphasises the complex array of interactions 
between eHS and trace metals in hydrothermal systems.