Knorr (Ship : 1970-) Cruise KN207-3
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The remineralization of marine organic matter by diverse biological and abiotic processes
Knorr (Ship : 1970-) Cruise KN207-3
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Thesis
While aerobic respiration is typically invoked as the dominant mass-balance sink for organic
matter in the upper ocean, many other biological and abiotic processes can degrade particulate
and dissolved substrates on globally significant scales. The relative strengths of these
other remineralization processes — including mechanical mechanisms such as dissolution
and disaggregation of sinking particles, and abiotic processes such as photooxidation — remain
poorly constrained. In this thesis, I examine the biogeochemical significance of various
alternative pathways of organic matter remineralization using a combination of field experiments,
modeling approaches, geochemical analyses, and a new, high-throughput lipidomics
method for identification of lipid biomarkers. I first assess the relative importance of particleattached
microbial respiration compared to other processes that can degrade sinking marine
particles. A hybrid methodological approach — comparison of substrate-specific respiration
rates from across the North Atlantic basin with Monte Carlo-style sensitivity analyses of a
simple mechanistic model — suggested sinking particle material was transferred to the water
column by various biological and mechanical processes nearly 3.5 times as fast as it was
directly respired, questioning the conventional assumption that direct respiration dominates
remineralization. I next present and demonstrate a new lipidomics method and open-source
software package for discovery and identification of molecular biomarkers for organic matter
degradation in large, high-mass-accuracy HPLC-ESI-MS datasets. I use the software
to unambiguously identify more than 1,100 unique lipids, oxidized lipids, and oxylipins in
data from cultures of the marine diatom Phaeodactylum tricornutum that were subjected to
oxidative stress. Finally, I present the results of photooxidation experiments conducted with
liposomes — nonliving aggregations of lipids — in natural waters of the Southern Ocean.
A broadband polychromatic apparent quantum yield (AQY) is applied to estimate rates
of lipid photooxidation in surface waters of the West Antarctic Peninsula, which receive
seasonally elevated doses of ultraviolet radiation as a consequence of anthropogenic ozone
depletion in the stratosphere. The mean daily rate of lipid photooxidation (50 ± 11 pmol
IP-DAG L−1 d−1, equivalent to 31 ± 7 𝜇g C m−3 d−1) represented between 2 and 8 % of
the total bacterial production observed in surface waters immediately following the retreat
of the sea ice.