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.
Highlights
Trichodesmium. spp are the most abundant dizaotrophic cyanobacteria [1] in oligotrophic tropical and subtropical oceans [2]
When the light period was extended to 16 h, the growth rate significantly increased in the LL-grown cells by 68% (P = 0.006), but was significantly reduced by 57% in the HL-grown ones (P = 0.001), compared to that in the 12L:12D regime (Fig 1)
Elongation of the light period led to 3.8 times higher maximal N2 fixation rates in the LL grown cells than that in the HL grown ones (Fig 2C), the amount of N2 fixed in 24 h of LL grown cells were 5 times higher than HL cells (Table 1)
Summary
Culture condition and experimental set-upTrichodesmium erythraeum IMS101, originally isolated from the North Atlantic Ocean, were grown in 1 L glass flasks (500–800 ml cultures) in YBC-II medium without combined nitrogen source [18]. The light was supplied by white fluorescent tubes (Philips) within the chamber These two different light levels were achieved by using neutral density screen, and were measured using a LI-COR 2π PAR sensor (PMA2100, Solar light, USA). Three independent cultures under each light level and each light regime were run and used to measure the growth rate and all subsequent physiological parameters. The cultures were initially run with a 12L:12D (Light: Dark) cycle under the two light levels for 60 generations (more than 180 days) before being shifted to 8L:16D or 16L:8D regime. The cultures were run in triplicates and semi-continuously diluted every 4–5 days to achieve steady-state exponential growth for 50–55 days (10–40 generations) prior to the measurements of growth, N2 fixation and photosynthetic parameters. The measurements of N2 and carbon fixation were carried out in 2 h intervals during the light period and at the onset of the dark period
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