Nitrite cycling in the primary nitrite maxima of the eastern tropical North Pacific

Abstract

Abstract. The primary nitrite maximum (PNM) is a ubiquitous feature of the upper ocean, where nitrite accumulates in a sharp peak at the base of the euphotic zone. This feature is situated where many chemical and hydrographic properties have strong gradients and the activities of several microbial processes overlap. Near the PNM, four major microbial processes are active in nitrite cycling: ammonia oxidation, nitrite oxidation, nitrate reduction and nitrite uptake. The first two processes are mediated by the nitrifying archaeal/bacterial community, while the second two processes are primarily conducted by phytoplankton. The overlapping spatial habitats and substrate requirements for these microbes have made understanding the formation and maintenance of the PNM difficult. In this work, we leverage high-resolution nutrient and hydrographic data and direct rate measurements of the four microbial processes to assess the controls on the PNM in the eastern tropical North Pacific (ETNP). The depths of the nitrite maxima showed strong correlations with several water column features (e.g., top of the nitracline, top of the oxycline, depth of the chlorophyll maximum), whereas the maximum concentration of nitrite correlated weakly with only a few water column features (e.g., nitrate concentration at the nitrite maximum). The balance between microbial production and consumption of nitrite was a poor predictor of the concentration of the nitrite maximum, but rate measurements showed that nitrification was a major source of nitrite in the ETNP, while phytoplankton release occasionally accounted for large nitrite contributions near the coast. The temporal mismatch between rate measurements and nitrite standing stocks suggests that studies of the PNM across multiple timescales are necessary.

Testing the influence of light on nitrite cycling rates in eastern tropical North Pacific

“…Net nitrite production from nitrification (NetNit, ammonia oxidation minus nitrite oxidation) declined with increasing light at each station (Fig. S6), consistent with field observations that NetNit generally decreases from the base of the euphotic zone towards the surface (Travis et al, 2023). The dark treatments had the largest net positive rate of nitrite production from nitrification at all stations.…”

“…However, active nitrification has been observed in the sunlit ocean (Beman et al, 2008;Clark et al, 2008;Santoro et al, 2013;Smith et al, 2014;Francis et al, 2005;Ward, 2005) is variation in apparent photosensitivity across natural communities of ammonia oxidizers (Qin et al, 2014). While decreased ambient nitrification rates have been seen above the nitrite maximum in previous work (Travis et al, 2023;Beman et al, 2012Beman et al, , 2013, those studies were not able to conclude that lower rates were caused by increased light level, because the microbial community and cell abundance were different at each depth measured in a vertical profile. Since the experimental design in the current study manipulated light using a single source water community, we can conclude that the declines in bulk rate measurements observed here were most likely caused directly or indirectly by changes in light intensity.…”

“…However, other work has suggested that nitrite oxidation may be less sensitive to increasing light intensity compared to ammonia oxidation, although recovery after photoinhibition is slower (Horak et al, 2013;Guerrero and Jones, 1996b). Oceanic profiles of nitrite oxidation activity show vertical distributions that are shaped similarly to ammonia oxidation, with maximal rates at the base of the euphotic zone and lower rates in surface waters (Travis et al, 2023;Beman et al, 2012). Overall, our data suggest that light is not the primary reason for generally low rates of nitrite oxidation in the surface ocean, but may still play a role under some conditions, in addition to substrate supply and competition for substrate within the community.…”

“…We targeted source water from PNM depths where ammonia and nitrite oxidizers are typically abundant and active (Santoro et al, 2010(Santoro et al, , 2013Travis et al, 2023). Challenging these communities with higher light levels simulated water moving upward in the water column.…”

“…The relative rates of each process are controlled by the ecophysiological response of each microbial group to environmental conditions, leading to dynamic changes in net accumulation of nitrite (Carlucci et al, 1970). Few direct measurements attempt to separate all of the 80 relevant, overlapping nitrite consumption and production rates in the field (Kiefer et al, 1976;Olson, 1981b;Travis et al, 2023).…”