Research project: SONIC – Shortcut in the Oceanic NItrogen Cycle

The overarching goal of this project is to determine whether a shortcut exists in the remineralisation of organic nitrogen back to nitrate in the ocean’s twilight zone – i.e. direct use of organic nitrogen in the nitrification process – and its potential significance in the oceanic nitrogen budget.

Why nitrogen? - Nitrogen is often a limiting nutrient to biological production, thus its availability in the surface ocean and subsequent export of organic carbon into the deep ocean are major determinants of the Ocean’s ability to sequester atmospheric CO₂: the Ocean’s biological pump.

Why twilight zone? - Once exported from the surface ocean, organic matter is mostly remineralised back to CO₂ and nutrients, with >90% occurring in the twilight zone (mesopelagic ocean) between the sunlit surface and the deep dark ocean.

Why remineralisation? - The depth of remineralisation within this transition zone, and which mechanisms are involved govern the exact amounts and appropriate forms of nutrients that can be returned to surface ocean to support phytoplankton growth.

Hence, accurate understanding of nitrogen remineralisation pathways and their distribution are essential to the projection of how ocean productivity and thus CO₂ sequestration may fare in future ocean scenarios.

We combine state-of-the-art ¹⁵N-labelling experiments, metatranscriptomics, metaproteomics and single-cell techniques to detect and quantify the occurrences of this potential shortcut in the twilight zone, and its significance to oceanic nitrogen cycle relative to the canonical ammonification-nitrification pathway. We make use of two well-established sampling platforms, Bermuda Atlantic Time Series and Atlantic Meridional Transect, which allow us to examine the seasonal and spatial variation of nitrogen remineralisation representative of various productivity regimes in global oceans.

Principle Investigator: Dr. Phyllis Lam

Figure 1. Collecting water samples with a CTD-Niskin rosette package from the mesopelagic ocean at twilight, aboard the R/V Atlantic Explorer.

Figure 2. Ready to deploy in-situ large-volume standalone pumps to collect samples for proteomics analyses, aboard the research vessel Atlantic Explorer at the BATS sampling site.