Time series investigations into the dynamics of nitrogen fixing bacteria and rates of nitrogen fixation at Station ALOHA

M. J. Church¹, C. Mahaffey¹, A. A. Fong¹, J. P. Zehr², D. M. Karl¹

¹Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

²Department of Ocean Sciences, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064

Abstract

In the nutrient impoverished regions of the North Pacific Ocean the activities of nitrogen fixing bacteria play important roles in regulating ecosystem productivity and carbon export. Despite increasing data on rates of nitrogen fixation in various marine systems, we still lack understanding of temporal and spatial variability in the distributions and activities of nitrogen fixing bacteria in open ocean ecosystems. In an effort to elucidate time-dependent variability in nitrogen fixation in the subtropical North Pacific Ocean, we coupled time series investigations of the diversity and abundances of nitrogen fixing bacteria with rate measurements of nitrogen fixation based on in situ plankton assimilation of 15N-dinitrogen gas. Temporal variability in the distributions and abundances of several groups nitrogen fixing bacteria including Trichodesmium, unicellular cyanobacteria, and heterocyst-forming cyanobacteria were evaluated based on quantitative PCR (QPCR) analyses of nifH gene abundances. Together, these data reveal striking temporal dynamics in both the abundances of selected diazotroph populations and rates of nitrogen fixation. Abundances of all of the nifH containing cyanobacterial groups examined demonstrated pronounced time and space dependent dynamics. In general, the nifH containing groups were distributed throughout the upper 100 m of the euphotic zone, with nifH gene abundances typically greatest in the upper 45 m of the water where nitrate+nitrite concentrations are generally less than 10 nmoles per liter. Similarly, rates of nitrogen fixation were greatest in the upper 45 m of the water, but demonstrated large month to month variability. By coordinating time-series measurements of rates of nitrogen fixation with quantitative assessments of the abundances and diversity of nitrogen fixing bacteria, our efforts are providing insight into the dynamics of an important determinant of carbon export in oligotrophic ocean ecosystems.