Dark Diazotrophy during the Late Summer in Surface Waters of Chile Bay, West Antarctic Peninsula

Although crucial for the addition of new nitrogen in marine ecosystems, dinitrogen (N-2) fixation remains an understudied process, especially under dark conditions and in polar coastal areas, such as the West Antarctic Peninsula (WAP). New measurements of light and dark N-2 fixation rates in parallel with carbon (C) fixation rates, as well as analysis of the genetic marker nifH for diazotrophic organisms, were conducted during the late summer in the coastal waters of Chile Bay, South Shetland Islands, WAP. During six late summers (February 2013 to 2019), Chile Bay was characterized by high NO3- concentrations (similar to 20 mu M) and an NH4+ content that remained stable near 0.5 mu M. The N:P ratio was approximately 14.1, thus close to that of the Redfield ratio (16:1). The presence of Cluster I and Cluster III nifH gene sequences closely related to Alpha-, Delta- and, to a lesser extent, Gammaproteobacteria, suggests that chemosynthetic and heterotrophic bacteria are primarily responsible for N-2 fixation in the bay. Photosynthetic carbon assimilation ranged from 51.18 to 1471 nmol C L-1 d(-1), while dark chemosynthesis ranged from 9.24 to 805 nmol C L-1 d(-1). N2 fixation rates were higher under dark conditions (up to 45.40 nmol N L-1 d(-1)) than under light conditions (up to 7.70 nmol N L-1 d(-1)), possibly contributing more than 37% to new nitrogen-based production (>2.5 g N m(-2) y(-1)). Of all the environmental factors measured, only PO43--exhibited a significant correlation with C and N-2 rates, being negatively correlated (p < 0.05) with dark chemosynthesis and N-2 fixation under the light condition, revealing the importance of the N:P ratio for these processes in Chile Bay. This significant contribution of N-2 fixation expands the ubiquity and biological potential of these marine chemosynthetic diazotrophs. As such, this process should be considered along with the entire N cycle when further reviewing highly productive Antarctic coastal waters and the diazotrophic potential of the global marine ecosystem.

Automated summary

Nitrogen fixation is an important process for marine ecosystems, and this study found that it occurs at higher rates under dark conditions in Chile Bay. Chemosynthetic and heterotrophic bacteria play a significant role in nitrogen fixation in this area. The N:P ratio is an important factor affecting the nitrogen fixation process.