Light and depth dependency of nitrogen fixation by the non-photosynthetic, symbiotic cyanobacterium UCYN-A

Abstract

The symbiotic cyanobacterium UCYN-A is one of the most globally abundant marine dinitrogen (N₂ )-fixers, but cultures have not been available and its biology and ecology are poorly understood. We used cultivation-independent approaches to investigate how UCYN-A single-cell N₂ fixation rates (NFRs) and nifH gene expression vary as a function of depth and photoperiod. Twelve-hour day/night incubations showed that UCYN-A only fixed N₂ during the day. Experiments conducted using in situ arrays showed a light-dependence of NFRs by the UCYN-A symbiosis, with the highest rates in surface waters (5-45 m) and lower rates at depth (≥ 75 m). Analysis of NFRs versus in situ light intensity yielded a light saturation parameter (I_k ) for UCYN-A of 44 μmol quanta m^-2 s^-1 . This is low compared with other marine diazotrophs, suggesting an ecological advantage for the UCYN-A symbiosis under low-light conditions. In contrast to cell-specific NFRs, nifH gene-specific expression levels did not vary with depth, indicating that light regulates N₂ fixation by UCYN-A through processes other than transcription, likely including host-symbiont interactions. These results offer new insights into the physiology of the UCYN-A symbiosis in the subtropical North Pacific Ocean and provide clues to the environmental drivers of its global distributions.

Fig. 1

Sampling locations superimposed onto a corrected satellite Sea Level Anomaly (SLA_corr) field for April 3, 2018. Station A: anticyclone array, Day 2, and Night 2 experiments. Station C1: cyclone array 1 and diel nucleic acid sampling experiments. Station C2: cyclone array 2, Day 1, and Night 1 experiments. Exact locations of all experiments are provided in Table S1. The open circle shows the location of Station ALOHA. Sea level anomaly data were retrieved from the Copernicus Marine and Environment Monitoring Service (http://www.marine.copernicus.eu) and corrected for an interannual linear trend and the seasonal cycle, following Barone et al. (2019). [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 2

UCYN‐A1 cell‐specific NFRs measured using 12‐h deck‐board incubations during the day and night periods. Each circle represents the rate measured from an individual UCYN‐A1 association. Boxplots represent summary statistics; black points represent outliers. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 3

UCYN‐A1 cell‐specific NFRs measured over 24‐h using free‐floating in situ arrays deployed in anticyclonic (red) and cyclonic (blue) eddies. Each circle represents the rate measured from an individual UCYN‐A1 association. Boxplots represent summary statistics; black points represent outliers. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 4

UCYN‐A1 cell‐specific NFR as a function of average daily in situ light intensity. Symbols and error bars represent mean and standard deviation (n = 8–12 symbioses analysed per sample). The dashed line represents the fitted light response curve, using the model of Webb et al. (1974). Parameters for the light curve are presented in Table 1. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 5

UCYN‐A1 nifH gene expression pattern measured at three depths over a 24‐h time course. The mean nifH transcripts gene copy⁻¹ at each time‐point were normalized to the median transcripts gene copy⁻¹ at each depth from the full time‐series, as in Thompson et al. (2014). Previous measurements by Church et al. (2005b) at Station ALOHA (25 m depth) using the same normalization method are plotted for comparison. Shaded areas represent times of darkness. [Color figure can be viewed at wileyonlinelibrary.com]