Date of Award

Fall 2023

Document Type


Degree Name

Master of Science (MS)


Ocean & Earth Sciences


Ocean and Earth Sciences

Committee Director

Alexander B. Bochdansky

Committee Member

P. Dreux Chappell

Committee Member

H. Rodger Harvey


Substantial remineralization of organic carbon occurs in the mesopelagic zone (i.e., the biological pump), the efficiency of which is responsible for the oceans’ capacity to store carbon originally derived from the atmosphere. To better understand how a substrate’s composition influences its degradation by mesopelagic microbial communities, we added treatments made of live algal cells, dead particulate organic carbon and dissolved organic carbon from 14C-labeled algal cultures to mesopelagic water collected in-situ. Each incubation took place in the laboratory over a period of months, during which PO14C, DO14C, ATP, prokaryote abundances and biochemical fractions were measured. Three algal species were used: Emiliania huxleyi, Thalassiosira weissflogii and Tetraselmis sp. Prokaryote biomass measured via cell counts and adenosine triphosphate responded quickly to the addition of organic material; peaks in both occurred within four to 10 days in most incubations. Thereafter, prokaryote abundances declined because of organic resource depletion and protist grazing. There were significant differences in decay rates between the live, POC, and DOC fractions, in POC between each species of algae, and in DOC between some algal species. DOC treatments exhibited the most rapid decay, followed by POC and lastly live treatments. E. huxleyi POC was initially degraded slower than any other dead substrate (POC or DOC). Our data suggest that carbon decay generally occurs in two significantly different phases: a rapid decay over the first three weeks (or less), followed by a slower, more gradual decline. On a more time-resolved level, the decay rate constants for the dead treatments rapidly increased and peaked after one to two days. Our results show that the use of a frequently applied, simple exponential function (i.e., first-order kinetics) hardly ever applies and is thus unsuitable for describing the trajectory of organic carbon decay. Lipids decayed more rapidly than proteins, polysaccharides-nucleic acids and the low molecular weight fractions during the first phase of decay. During the second phase, the lipid decay rates were similar to those of the other fractions. Overall, the carbon fraction (POC or DOC) and its status (live or dead) exerted more control over carbon decay rates than the type of algal species.


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