NITROGEN CYCLING IN EUXINIC SAKINAW LAKE

The Proterozoic and Phanerozic Eons were punctuated by intervals in which euxinic conditions developed in the oceans. These euxinic intervals caused large-scale shifts in the biosphere including mass extinctions and the re-organization of biogeochemical cycles. The N-cycle in particular would have been strongly affected with the oceans experiencing massive N-loss through denitrification.

Modern euxinic basins serve as analogues to the sulfidic oceans of the past. Biogeochemical and microbiological studies of these modern euxinic basins can inform models for the evolution of marine chemistry and life through these turbulent intervals in Earth history. Meromictic Sakinaw Lake is a euxinic basin located on the Sunshine Coast of British Columbia, Canada. To determine rates and pathways of N-transformations under euxinic conditions, we have conducted a suite of geochemical and molecular microbiological analyses spanning water column redox gradients. Incubations with ¹⁵N-labeled N compounds revealed potential denitrification rates up to 50 nM N hr^-1 within a narrow interval below the depth of oxygen penetration, whereas anammox was not detected in our measurements. Denitrification exhibited a strong dependency on NO₃^- concentrations and this NO₃^- dependency could be modeled using Michaelis-Menten kinetics with K_m and V_max values of 28 µM and 150 µM hr^-1, respectively. Furthermore, denitrification was strongly inhibited at NO₃^- concentrations of 25 µM. Corrected to in situ NO₃^- concentrations using our Michaelis-Menten model, natural denitrification rates were up to 1.6 nM N hr^-1. These rates are similar to those found in other euxinic lakes and oceanic oxygen minimum zones. Ongoing work is determining the contribution of different electron donors (sulfide vs. organic molecules) to the denitrification process, while pathway analyses are being conducted using metagenomic data from several depth intervals.