Shortcuts in the Oceanic Nitrogen Cycle: Fluxes and Microbial Pathways of Nitrogen Remineralization in the Ocean's Twilight Zone

The Ocean's twilight (mesopelagic, 100-1000m) zone lies beneath the sunlit surface ocean, with too little light for photosynthesis but above the pitch-black deep ocean, where large animals can no longer see their prey. Of all organic matter that sinks out from the surface, >90% is degraded in the mesopelagic with only a small fraction escaping into the deep ocean (>1000 m). The mesopelagic thus represents an important barrier: most material falling into it is prevented from sinking further by remineralisation - the degradation process that breaks down organic matter and releases CO2 and inorganic nutrients to solution. Eventually, physical mixing or ocean circulation deliver the nutrients back to the surface to fuel phytoplankton growth. Hence, remineralisation in the mesopelagic is critical to controlling the oceanic biological pump, and can affect the ocean's ability to sequester atmospheric CO2. Nitrogen is often the limiting nutrient for biological production in global oceans, its remineralisation would thus be key to biological pump efficiency. However, the mechanisms of N-remineralisation are poorly characterized, and there are no rate measurements of this process in the mesopelagic.

The remineralisation of nitrogen (N) in the oceans encompasses ammonification: the degradation of organic N to ammonium (NH4+), and subsequently nitrification: the oxidation of NH4+ to nitrite and then nitrate. However, a recent study suggests that some nitrifiers (microorganisms conducting nitrification) can utilise organic N directly, thus presenting a possible shortcut in the N-cycle. Because the respective organisms have different feeding styles regarding carbon (CO2- fixing or producing), the relative abundance and activities of these functional groups of organisms will have different direct impacts on the CO2 balance, and the existence of the potential shortcut will likely cause a shift towards autotrophy (CO2 fixation).

This project aims to determine exactly how and how much nitrogen is remineralised in the twilight ocean, using a combination of state-of-the-art geochemical rate measurements and molecular biological analyses. In particular, we will determine whether the above-mentioned shortcut exists in the remineralisation of organic nitrogen (N) to nitrate (NO3-), and quantitatively assess its potential significance to the oceanic N-cycle relative to the conventional ammonification-nitrification pathway. Together, these planned analyses will give the most complete dataset of directly measured N-remineralisation fluxes ever attempted in the oceans.
State-of-the-art 15N-stable-isotope-labeling experiments will be conducted to measure rates of concurrent N-conversions for a more accurate assessment of upper ocean N-budget: ammonification, nitrification, assimilation (incorporation of N into biomass) and release of dissolved organic N. We will do this by tracing 15N (the heavy stable isotope of N that is rare in nature, as opposed to the common 14N) from various amended organic substrates into different N-pools at the same time, to determine whether organic-N is directly channeled to nitrification or via ammonification. In parallel, major remineralisation pathways will be identified by elucidating the expression of biomarker enzymes key to these N-conversions at both gene transcript and protein levels, as quantifiable activity indicators for the respective processes.

Sampling is planned along the Atlantic Meridional Transect (AMT) from the north (UK) to south Atlantic (Falklands/Chile) to examine N-remineralisation in diverse nutrient regimes, while temporal variability is explored via seasonal sampling at the Bermuda Atlantic Time Series (BATS) site. Such spatiotemporal coverage and complementary, interdisciplinary dataset would yield a highly representative depiction of mesopelagic N-remineralisation in the oceans, and the most comprehensive assessment to date on the significance of the twilight zone in oceanic N-cycle.

Increasing human activities and pressures on our planet have resulted in, in addition to the greater emissions of greenhouse gases, higher inputs of anthropogenic nitrogen into oceans. In particular, out of ~67 Tg N/y of atmospheric nitrogen deposition into the ocean, ~80% is anthropogenic, an increase from 29% in pre-industrial times (Duce et al., 2008, Science 320:893-897). This enhanced nutrient level to the surface ocean, whether from the atmosphere, rivers or land, has significantly stimulated surface production and so CO2 drawdown. How long this enhanced export production may last, however, depends on a combination of feedbacks and factors, including the efficiency of the biological pump mediated by the remineralisation of organic matter in both of the euphotic zone and the mesopelagic.

Since this proposed study plans to address the efficiency of remineralisation in both the lower euphotic and the mesopelagic, while comparing different nutrient regimes and seasons, data from this study will provide invaluable information on how such enhanced N inputs may impact the export of organic matter into the deep, the degree of remineralisation within the mesopelagic and how the increase in nitrogen may alter such balance, and if there exist other possible feedbacks via the little known resident microbial communities. These data can be further coupled to e.g. global circulation models for more comprehensive assessment, and to yield better understanding on how increased surface production may impact subsurface processes and their feedbacks to surface production. These model outputs may provide the necessary basis and guidance for governments and policy makers, such as DEFRA and the Environmental Agency in the UK and other countries, to the design of appropriate policies to control the amounts and forms of anthropogenic nitrogen released into the environment from different channels. Therefore, this proposed study would have direct and indirect impacts on government policy makers, and would also be beneficial to the well being of wider general public.

The mesopelagic zone harbours microbial communities distinct from the surface ocean. As a result, there is a good chance of encountering new organisms, new proteins and new enzymes from the metaproteomics analyses. Of particular interests are the detection of enzymes and proteins involved in the degradation of organic matter, which may find applications in bioprocessing and bioremediation and thus interest industrial sectors like biotechnology or wastewater treatment.