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The University of Southampton
Ocean and Earth Science, National Oceanography Centre Southampton

Vertical export of materials into the ocean's interior and processes in the 'twilight zone'


The Ocean’s twilight zone, or mesopelagic ocean, is a critical buffer zone between the sunlit surface and the dark deep ocean. It intercepts all material transfers from the surface into the deep, including the gravitationally sinking particles that drive the biological carbon pump (BCP) – the delivery of organic carbon ‘fixed’ from atmospheric CO2 by phytoplankton into the deep ocean, where it is locked away for roughly a millennium – a principal means of CO2 sequestration by global oceans. The twilight zone holds the key to the success of this BCP, as processes occurring herein can enhance or hamper its efficiency; while it is also crucial to the biogeochemical cycling of many other elements, including the regeneration of the essential nutrients that resupply surface primary production.

Figure 1.

Key Questions:

1. Is remineralisation of organic matter the same for all nutrients in the mesopelagic ocean? – Are there differential remineralisation rates and depths of iron, silica and nitrogen relative to carbon?

2. Are suspended particles remineralised the same way as sinking particles? – Do they contribute the same to the carbon budget of the ocean interior?

3. Are the microbial communities the same between those associated with sinking particles and those with suspended particles? – What are their origins, and do they perform the same biogeochemical functions?

4. How exactly is nitrogen regenerated from organic matter in the twilight ocean? – What is the role of organic nitrogen in nitrification (i.e. oxidation of reduced nitrogen to nitrate)?


Marine Snow Catcher
Figure 2.

How do we do it?

We combine in situ observations and bioassay manipulation experiments, laboratory studies and at sea experimentation, to determine differential remineralisation rates and length scales of micro- and macro- nutrients relative to carbon, and to develop mechanistic understanding of the physical, chemical and biological controls of these processes.

We deploy a suite of sampling technologies to meet our varied needs of sample types, including the PELAGRA and Marine Snow Catcher (Fig. 1 and 2) to collect our particle samples, and standalone pumps (Fig. 3) to filter large volumes of seawater in situ for e.g. protein analysis.

We use state-of-the-art analytical techniques, including radioactive and stable isotope tracers and microelectrodes, at sea and in onshore facilities, to determine microbial carbon uptake and respiration, as well as nitrogen respiration, regeneration and nitrification processes. 

We employ a range of bioimaging (Fig. 4) and omics technologies, including the use of our Environmental Sequencing Facility and the University’s Centre for Proteomics Research, to identify the key responsible microorganisms of these processes, and to examine the underpinning molecular mechanisms that drive these communities, the up- and down-regulation of expressed genes and proteins along environmental gradients and in response to environmental stimuli, and the interactions among members within and between communities.

We use this mechanistic understanding, further complemented by data from the wider related projects and partnered long-term time-series programmes (e.g. BATS, AMT), to predict how system activity and biogeochemical cycles will alter under a range of different scenarios at ecosystem scales.

South Atlantic deployment
Figure 3.
Epifluorescence micrograph
Figure 4.

Figure 1. Deployment of the free-floating sediment trap PELAGRA.

Figure 2. Subsampling water from the Marine Snow Catcher on a cold yet sunny day in the Southern Ocean.


Figure 3. Deployment of the large-volume standalone pump at daybreak in the South Atlantic.

Figure 4. Epifluorescence micrograph of a marine snow particle teaming with microbes - blue indicates any microbial cells, while green denotes cells targeted with specific stains.

Who in the Marine Biogeochemistry Group is involved?

Dr Phyllis Lam; Prof Mark Moore; Prof Tom Bibby; Prof Nicholas Bates.

Links to other Research Themes

Chemical, biological and physical controls on primary production in the surface ocean

Biogeochemical cycles and their response to climate change

Dynamics of marine planktonic and microbial communities – from single cells to large scale processes and ecosystem functions

Development of photosynthetic microbes for algal biofuel and other biotechnologies

Institute for Life Sciences

Associated Projects

SONIC - Shortcut in the Oceanic Nitrogen Cycle

COMICS - Controls over Ocean Mesopelagic Interior Carbon Storage

CUSTARD - Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth

BATS - Bermuda Atlantic Time Series

AMT - Atlantic Meridional Transect

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