Surface Ammonia-Oxidizer Abundance During the Late Summer in the West Antarctic Coastal System

Abstract

Marine ammonia oxidizers that oxidize ammonium to nitrite are abundant in polar waters, especially during the winter in the deeper mixed-layer of West Antarctic Peninsula (WAP) waters. However, the activity and abundance of ammonia-oxidizers during the summer in surface coastal Antarctic waters remain unclear. In this study, the ammonia-oxidation rates, abundance and identity of ammonia-oxidizing bacteria (AOB) and archaea (AOA) were evaluated in the marine surface layer (to 30 m depth) in Chile Bay (Greenwich Island, WAP) over three consecutive late-summer periods (2017, 2018, and 2019). Ammonia-oxidation rates of 68.31 nmol N L−1 day−1 (2018) and 37.28 nmol N L−1 day−1 (2019) were detected from illuminated 2 m seawater incubations. However, high ammonia-oxidation rates between 267.75 and 109.38 nmol N L−1 day−1 were obtained under the dark condition at 30 m in 2018 and 2019, respectively. During the late-summer sampling periods both stratifying and mixing events occurring in the water column over short timescales (February–March). Metagenomic analysis of seven nitrogen cycle modules revealed the presence of ammonia-oxidizers, such as the Archaea Nitrosopumilus and the Bacteria Nitrosomonas and Nitrosospira, with AOA often being more abundant than AOB. However, quantification of specific amoA gene transcripts showed number of AOB being two orders of magnitude higher than AOA, with Nitrosomonas representing the most transcriptionally active AOB in the surface waters. Additionally, Candidatus Nitrosopelagicus and Nitrosopumilus, phylogenetically related to surface members of the NP-ε and NP-γ clades respectively, were the predominant AOA. Our findings expand the known distribution of ammonium-oxidizers to the marine surface layer, exposing their potential ecological role in supporting the marine Antarctic system during the productive summer periods.

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

“…On average the seawater temperatures were 0.8 • C and 1.2 • C at 2 and 30 m, respectively. Average salinity values reached 33.9 in the surface samples and 34.14 at 30 m, denoting a shallow haline stratification as reported recently [53]. Variability in Chl-a concentrations was detected both during and between summers for the monitored days.…”

“…Our observations show that bacterioplankton were less diverse and dominant during low productivity periods compared to the highly diverse bacterioplankton community associated with a eukaryotic photoautotrophic bloom [70]. In addition, previous work has described the activity of the microbial community and its contribution to relevant ecological functions, mainly associated with biogeochemical C cycling (diatoms) and N assimilation (bacterioplankton) during bloom events [39,53], and the effect of environmental variables, such as changes in salinity due to glacial melt, and the shallow haline stratification on these (active) microbial communities has been investigated [53,69].…”

“…In that sense, our study in Chile Bay (Western Antarctic coastal seawater) contributes new knowledge to diazotrophy in the WAP. In this bay, we previously characterized the water column and the identity and dynamics of phytoplankton, bacterioplankton and virioplankton under different productivity conditions during the summertime [39,53,70,71]. Our observations show that bacterioplankton were less diverse and dominant during low productivity periods compared to the highly diverse bacterioplankton community associated with a eukaryotic photoautotrophic bloom [70].…”

“…Furthermore, the interior of carbon-rich particles has been described as a suitable microenvironment for chemosynthetic and heterotrophic N 2 fixation [72,73,84,85]. This scenario could also explain the presence of nif H sequences closely affiliated with soil and marine sediments in the shallow waters of Chile Bay (maximum depth between 80-230 m), where water mixing conditions have been reported [53], which could enhance the suspension of particulate matter from bottom sediment. Another possible explanation for the presence of these sequences is that they originated from a nearby glacial source in the bay as a result of runoff due to ice melt [69].…”

“…For samples taken at 2 m, uncovered transparent polycarbonate bottles were incubated in triplicate, while another triplicate set of covered bottles (using a black bag and aluminum foil) was used to obtain information on N 2 fixation in the absence of light by the non-photosynthetic community. For all 30 m samples, covered bottles (using a black bag and aluminum foil) were incubated in triplicate to simulate the dark conditions that exist during the day time at this depth (1% of the PAR only reaches <20 m in the bay [53]). Due to the complicated logistics, it was not possible to perform these incubations directly offshore at station P3; thus, all incubations were performed at 2 m depth in near-shore bay water (average seawater temperature 1-1.5 • C) for a period of 24 h to obtain a daily rate.…”