Award: OCE-1851368

The mesopelagic layer of the ocean, defined as the depth range between 200 and 1000 m, is an important site for the reprocessing of organic carbon produced in the upper sunlit surface layer. Here, particulate organic carbon sinking from the surface is either degraded into dissolved organic carbon or respired by microbial communities, creating either more refractory organic carbon or carbon dioxide. Alternatively, it is consumed and repackaged by zooplankton into fast-sinking fecal pellets.

In this project, we addressed the reprocessing of organic material by ocean microbes using a combination of in situ and laboratory techniques. Decay experiments were conducted in situ employing a newly designed deep-sea incubator, and the resulting rates were compared with incubations performed in parallel under controlled laboratory conditions. 13C-labeled organic material was introduced into the field incubations, while either 13C- or 14C was used in laboratory incubations. Organic material was added either as live cells or as dead particulate or dissolved material derived from three different algal species.

The response of microbial biomass to carbon additions was monitored through cell counts and measurements of adenosine triphosphate. Fluorescence in situ hybridization and single-cell interrogation by Raman microspectroscopy confirmed that most of the fresh organic material was taken up and processed by fast-growing copiotrophs belonging to the phylum Gammaproteobacteria. In situ measurements of decay were very similar to those conducted in the laboratory, as long as experiments were carried out at the same temperatures and under fully oxygenated conditions. This agreement provides confidence that controlled laboratory time-series experiments yield realistic results.

A major finding of this investigation was that the commonly used single first-order decay rate models do not apply. Instead, we observed two distinct phases of first-order decay. The type of source material (i.e., the algal species from which the organic material was derived) significantly altered the trajectory of decay rates, but only to a limited extent, and organic carbon was decayed to a similar degree across treatments after three months, with approximately 13% remaining unremineralized. More important than the origin of the organic material was the fraction in which it was introduced (either dissolved organic carbon (DOC) or particulate organic carbon (POC)). DOC decayed much faster than POC during phase 1, but decay rates were very similar between the two fractions during phase 2.

When live algae were introduced into incubations with mesopelagic water, remineralization was lower than in DOC- and POC-amended incubations because organic carbon contained within live cells was not accessible to prokaryotic decay. Remineralization in live treatments was largely due to respiration by the algae themselves. However, decay in incubations containing live algae accelerated once the phytoplankton cells died due to the absence of light, at which point they provided an easily accessible carbon source for mesopelagic microbes.

Approximate biochemical fractionation of the organic substrates into proteins, lipids, and polysaccharides revealed that lipids were the most labile and most easily digestible fraction during phase 1. During phase 2 decay, all fractions were degraded at similar rates.

The results of this project will inform biogeochemical and ecosystem models of organic carbon decay in the ocean and will help better constrain models of the biological pump, a process by which carbon dioxide is removed from the atmosphere and sequestered in the deep ocean via biological processes. The project also contributed substantially to the development of scientific infrastructure through the design, construction, and testing of two types of deep-sea incubators. Training and participation of both undergraduate and graduate students were essential components of this project. Oceanographic equipment developed during this project was exhibited to the general public during university-organized outreach activities in Norfolk, Virginia.

 

Last Modified: 12/17/2025
Modified by: Alexander B Bochdansky