Temporal Patterns and Intra- and Inter-Cellular Variability in Carbon and Nitrogen Assimilation by the Unicellular Cyanobacterium Cyanothece sp. ATCC 51142.

Lùbos Polerecký ,Jan Červený,Mario Giordano,Marie Vancová,Ana Teresa Lombardi,Kimberly H Halsey,Martin Lukeš,Takako Masuda,Sophie Rabouille,Susanne Stephan,Pascal Claquin,Michiel V M Kienhuis,Eva Kotabová,Meri Eichner,Jacco C Kromkamp ,Douglas A Campbell ,David J Suggett ,Tomáš Zavřel ,Ondřej Prášil ,José Bonomi-Barufi ,Gabor Bernát

doi:10.3389/fmicb.2021.620915

Abstract

Unicellular nitrogen fixing cyanobacteria (UCYN) are abundant members of phytoplankton communities in a wide range of marine environments, including those with rapidly changing nitrogen (N) concentrations. We hypothesized that differences in N availability (N2 vs. combined N) would cause UCYN to shift strategies of intracellular N and C allocation. We used transmission electron microscopy and nanoscale secondary ion mass spectrometry imaging to track assimilation and intracellular allocation of 13C-labeled CO2 and 15N-labeled N2 or NO3 at different periods across a diel cycle in Cyanothece sp. ATCC 51142. We present new ideas on interpreting these imaging data, including the influences of pre-incubation cellular C and N contents and turnover rates of inclusion bodies. Within cultures growing diazotrophically, distinct subpopulations were detected that fixed N2 at night or in the morning. Additional significant within-population heterogeneity was likely caused by differences in the relative amounts of N assimilated into cyanophycin from sources external and internal to the cells. Whether growing on N2 or NO3, cells prioritized cyanophycin synthesis when N assimilation rates were highest. N assimilation in cells growing on NO3 switched from cyanophycin synthesis to protein synthesis, suggesting that once a cyanophycin quota is met, it is bypassed in favor of protein synthesis. Growth on NO3 also revealed that at night, there is a very low level of CO2 assimilation into polysaccharides simultaneous with their catabolism for protein synthesis. This study revealed multiple, detailed mechanisms underlying C and N management in Cyanothece that facilitate its success in dynamic aquatic environments.

Highlights

Nitrogen fixing microorganisms are critical suppliers of bioavailable forms of nitrogen (N, e.g., ammonium) in natural ecosystems
N fixed during the night is initially stored as cyanophycin until it is used for processes such as protein and nucleic acid synthesis, whereas C fixed during the day is stored as polysaccharides until it is respired the following night to supply reducing equivalents (NADPH) and ATP needed to support N2 fixation (Schneegurt et al, 1994; Li et al, 2001; Großkopf and LaRoche, 2012; Inomura et al, 2019)
Prominent intracellular inclusion bodies identified in transmission electron microscopy (TEM) images of Cyanothece 51142 included carboxysomes, cyanophycin inclusions, polyphosphate bodies, polysaccharide granules, and thylakoid membranes (Figure 1)

Summary

Introduction

Nitrogen fixing microorganisms (diazotrophs) are critical suppliers of bioavailable forms of nitrogen (N, e.g., ammonium) in natural ecosystems. Cyanothece 51142 appear to restrict N2 fixation to the night time to protect the N2 fixing enzyme complex, nitrogenase, from inactivation by molecular oxygen produced by photosynthesis during the day time (Reddy et al, 1993; Welsh et al, 2008). C respiration during the late afternoon and night facilitates nitrogenase activity by depleting molecular oxygen that diffuses into the cell from the environment (Dron et al, 2012; Großkopf and LaRoche, 2012; Cervený et al, 2013; Inomura et al, 2019)

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Conclusion