Factors controlling carbon turnover and nitrogen loss at a microscale in continental shelf sediments

Approximately 25% of  global marine primary occurs on the continental shelves sustain approximately, making them hot spots of the nutrient cycling (Jahnke et al., 2010). This high primary production, combined with shallow water depths means that the sediments on the shelves receive high amounts of organic matter, which settles from the water column (Otto et al., 1990). However, the sandy sediments which cover 50-70% of the seafloor on the shelves are characterized with low organic content (Huettel et al., 2014). Previous work has shown that part of the reason for this is that remineralization rates are high in sandy sediments due to high advective supply of organic matter and nutrients into the sediment matrix, which support highly active microbial communites which are attached to the sand grains (Gobet et al., 2012).

The transportation of the organic matter from water column to sands is governed by advective supply, the velocity of which varies dependent on bottom water currents, wave action, permability and sediment topography.  Such variations in pore water velocity and subsequently the transport of organics and electron acceptors into the sediments could result in spatial heterogeneities in the oxygen consumption rates (OCR). Ahmerkamp et al., 2020 and Probandt et al., 2017 have showed that the bacterial community attached to sand grains is denser within cracks and depressions on the grains. The selective behavior of the bacterial community towards irregularities on a sand grain could lead to formation of microniches and heterogeneity in the distribution of OCR. However, due to limitation of methods, past studies have been unable to assess whether variations in porewater flow and differences in bacterial colonization result in heterogeneous OCR. In this project we will  study the oxygen dynamics and nitrogen cycling in sediments at a microscale, to see if such heterogeneities exist and their impact on bulk turnover of organic matter and nutrients. The OCR on sediments will be visualized by using a newly developed technique using sensor-particles coated with O2-sensitive particles. In addition to this, combination of in-situ observations using lander deployments with laboratory incubations using stable isotopes will help to better understand the benthic microbial community.

Ahmerkamp, S., Marchant, H.K., Peng, C., Probandt, D., Littmann, S., Kuypers, M.M.M., and Holtappels, M. (2020), ‘The effect of sediment grain properties and porewater flow on microbial abundance and respiration in permeable sediments’, Science Report 10, 3573.

Gobet, A., Böer, S. I., Huse, S. M., van Beusekom, J. E., Quince, C., Sogin, M. L., Boetius, A. and Ramette, A. (2012), ‘Diversity and dynamics of rare and of resident bacterial populations in coastal sands’, The ISME Journal 6, 542–553.

Huettel, M., Berg, P. and Kostka, J. E. (2014), ‘Benthic exchange and biogeochemical cycling in permeable sediments’, Marine Science 6, 23-51.

Jahnke, R. A. (2010), Global synthesis, in ‘Carbon and Nutrient Fluxes in Continental Margins’, Springer, pp. 597–615.

Otto, L., Zimmerman, J., Furnes, G., Mork, M., Saetre, R. and Becker, G. (1990), ‘Review of the physical oceanography of the north sea’, Netherlands Journal of Sea Research 26(2), 161–238.

Probandt, D., Eickhorst, T., Ellrott, A., Amann, R. & Knittel, K. (2017), ‘Microbial life on a sand grain: from bulk sediment to single grains’, The ISME Journal 12, 623-633.