The influence of light on nitrogen cycling and the primary nitrite maximum in a seasonally stratified sea |
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Authors: | Katherine RM Mackey Laura Bristow David R Parks Mark A Altabet Anton F Post Adina Paytan |
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Institution: | aDepartment of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305, United States;bInstitute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA 95064, United States;cSchool for Marine Science and Technology, University of Massachusetts Dartmouth, New Bedford, MA 02744, United States;dStanford Shared FACS Facility, Stanford University School of Medicine, Stanford, CA 94305, United States;eThe Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA 02543, United States |
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Abstract: | In the seasonally stratified Gulf of Aqaba Red Sea, both release by phytoplankton and oxidation by nitrifying microbes contributed to the formation of a primary nitrite maximum (PNM) over different seasons and depths in the water column. In the winter and during the days immediately following spring stratification, formation was strongly correlated (R2 = 0.99) with decreasing irradiance and chlorophyll, suggesting that incomplete reduction by light limited phytoplankton was a major source of . However, as stratification progressed, continued to be generated below the euphotic depth by microbial oxidation, likely due to differential photoinhibition of and oxidizing populations. Natural abundance stable nitrogen isotope analyses revealed a decoupling of the δ15N and δ18O in the combined and pool, suggesting that assimilation and nitrification were co-occurring in surface waters. As stratification progressed, the δ15N of particulate N below the euphotic depth increased from −5‰ to up to +20‰.N uptake rates were also influenced by light; based on 15N tracer experiments, assimilation of , , and urea was more rapid in the light (434 ± 24, 94 ± 17, and 1194 ± 48 nmol N L−1 day−1 respectively) than in the dark (58 ± 14, 29 ± 14, and 476 ± 31 nmol N L−1 day−1 respectively). Dark assimilation was 314 ± 31 nmol N L−1 day−1, while light assimilation was much faster, resulting in complete consumption of the 15N spike in less than 7 h from spike addition. The overall rate of coupled urea mineralization and oxidation (14.1 ± 7.6 nmol N L−1 day−1) was similar to that of oxidation alone (16.4 ± 8.1 nmol N L−1 day−1), suggesting that mineralization of labile dissolved organic N compounds like urea was not a rate limiting step for nitrification. Our results suggest that assimilation and nitrification compete for and that N transformation rates throughout the water column are influenced by light over diel and seasonal cycles, allowing phytoplankton and nitrifying microbes to contribute jointly to PNM formation. We identify important factors that influence the N cycle throughout the year, including light intensity, substrate availability, and microbial community structure. These processes could be relevant to other regions worldwide where seasonal variability in mixing depth and stratification influence the contributions of phytoplankton and non-photosynthetic microbes to the N cycle. |
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