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. 2015 Aug 10;10(8):e0135401.
doi: 10.1371/journal.pone.0135401. eCollection 2015.

Levels of Daily Light Doses Under Changed Day-Night Cycles Regulate Temporal Segregation of Photosynthesis and N2 Fixation in the Cyanobacterium Trichodesmium Erythraeum IMS101

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Levels of Daily Light Doses Under Changed Day-Night Cycles Regulate Temporal Segregation of Photosynthesis and N2 Fixation in the Cyanobacterium Trichodesmium Erythraeum IMS101

Xiaoni Cai et al. PLoS One. .
Free PMC article

Abstract

While the diazotrophic cyanobacterium Trichodesmium is known to display inverse diurnal performances of photosynthesis and N2 fixation, such a phenomenon has not been well documented under different day-night (L-D) cycles and different levels of light dose exposed to the cells. Here, we show differences in growth, N2 fixation and photosynthetic carbon fixation as well as photochemical performances of Trichodesmium IMS101 grown under 12L:12D, 8L:16D and 16L:8D L-D cycles at 70 μmol photons m-2 s-1 PAR (LL) and 350 μmol photons m-2 s-1 PAR (HL). The specific growth rate was the highest under LL and the lowest under HL under 16L:8D, and it increased under LL and decreased under HL with increased levels of daytime light doses exposed under the different light regimes, respectively. N2 fixation and photosynthetic carbon fixation were affected differentially by changes in the day-night regimes, with the former increasing directly under LL with increased daytime light doses and decreased under HL over growth-saturating light levels. Temporal segregation of N2 fixation from photosynthetic carbon fixation was evidenced under all day-night regimes, showing a time lag between the peak in N2 fixation and dip in carbon fixation. Elongation of light period led to higher N2 fixation rate under LL than under HL, while shortening the light exposure to 8 h delayed the N2 fixation peaking time (at the end of light period) and extended it to night period. Photosynthetic carbon fixation rates and transfer of light photons were always higher under HL than LL, regardless of the day-night cycles. Conclusively, diel performance of N2 fixation possesses functional plasticity, which was regulated by levels of light energy supplies either via changing light levels or length of light exposure.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Specific growth rate of Trichodesmium IMS101.
Growth rates of Trichodesmium IMS101 of LL (70 μmol photons m-2 s-1, dark column) and HL (350 μmol photons m-2 s-1, open column) cultures grown under different Light Dark (LD) regimes. Values represent mean ± SD of triplicate cultures under each regime.
Fig 2
Fig 2. Diel variations of N2 fixation.
Diel variations of N2 fixation of LL (closed circles) and HL (open circles) cultures acclimated to different LD regimes. (A) 12L:12D, (B) 8L:16D and (C) 16L:8D regimes. Shaded areas represent dark periods. Values represent mean ± SD of triplicate cultures under each regime.
Fig 3
Fig 3. Diel variations of carbon fixation.
Diel variations of carbon fixation of LL (closed circles) and HL (open circles) cultures acclimated to different LD regimes. (A) 12L:12D, (B) 8L:16D and (C) 16L:8D regimes. Shaded areas represent dark periods. Values represent mean ± SD of triplicate cultures under each regime.
Fig 4
Fig 4. Diel variations of ΦPSII.
Diel variations of quantum yield of PSII (ΦPSII) of low light (LL, closed circles) and high light (HL, open circles) cultures acclimated to different LD regimes, (A) 12L:12D, (B) 8L:16D and (C) 16L:8D. Shaded areas represent dark periods. Values represent mean ± SD of triplicate cultures under each regime.
Fig 5
Fig 5. Diel variations of rETR.
Diel variations of relative ETR of low light (LL, closed circles) and high light (HL, open circles) cultures acclimated to different LD regimes, (A) 12L:12D, (B) 8L:16D and (C) 16L:8D. The values of rETR was determined from the instant ΦPSII multiplied by growth light irradiances. Shaded areas represent dark periods. Values represent mean ± SD of triplicate cultures under each regime.
Fig 6
Fig 6. Relationship with daily light doses.
Correlations of light doses with (A) daily amount of carbon fixed, (B) daily amount of N2 fixed and (C) growth rate in the cells grown under of low (LL, closed circles) and high (HL, open circles) light levels.

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Grant support

The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. This study was supported by National Natural Science Foundation (NSFC, No. 41430967; 41120164007), Joint project of NSFC and Shandong province (Grant No. U1406403), Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA11020302), State Oceanic Administration (SOA,GASI-03-01-02-04).
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