Enhanced lipid productivity and photosynthesis efficiency in a Desmodesmus sp. mutant induced by heavy carbon ions

Abstract

The unicellular green microalga Desmodesmus sp. S1 can produce more than 50% total lipid of cell dry weight under high light and nitrogen-limitation conditions. After irradiation by heavy (12)C(6+) ion beam of 10, 30, 60, 90 or 120 Gy, followed by screening of resulting mutants on 24-well microplates, more than 500 mutants were obtained. One of those, named D90G-19, exhibited lipid productivity of 0.298 g L(-1)⋅d(-1), 20.6% higher than wild type, likely owing to an improved maximum quantum efficiency (Fv/Fm) of photosynthesis under stress. This work demonstrated that heavy-ion irradiation combined with high-throughput screening is an effective means for trait improvement. The resulting mutant D90G-19 may be used for enhanced lipid production.

Figure 1. Effects of irradiation on the cells viability of Desmodesmus sp. S1.

Figure 2. Distribution histogram of Desmodesmus sp. mutant phenotypes.

(A) Distribution histogram of photosynthesis efficiency mutants (PEMs), (B) distribution histogram of lipid-over-production mutants (LOMs), and (C) the relationship between lipid contents and high light-adapted photosystem II efficiency under stress conditions in the PEMs and LOMs of Desmodesmus sp. S1. The distribution histograms of LOMs and PEMs were obtained using SPSS 10.0.

Figure 3. The consumption of NaNO₃ and growth kinetics of Desmodesmus sp.

(A) The consumption of NaNO₃ in the nitrogen-limited (4.25 mM) BG-11 medium (closed symbols) and growth kinetics (open symbols) of Desmodesmus sp. S1 under low light (closed square, open triangle) and high light (closed circle, open diamond), and (B) biomass concentration of Desmodesmus sp. S1 wild type (WT, open square) and mutant D90G-19 (closed square) cultivated in BG-11 medium under LL+N conditions (50 µmol photons m⁻²⋅s⁻¹, BG-11 medium with 17 mM NaNO₃).

Figure 4. Total lipid contents of Desmodesmus sp.

WT and D90G-19 were grown under HL-N conditions (300–400 µmol photons m⁻²⋅s⁻¹, nitrogen-limited BG-11 medium with 4.25 mM NaNO₃). The total lipid of the mutant D90G-19 was significantly higher than WT at day 4 (P = 0.0051), day 6 (P = 0.006) and day 8 (P = 0.0039). D90G-19: closed square; WT: open square.

Figure 5. Starch contents of Desmodesmus sp. WT and D90G-19.

The initial starch content was measured in algae cells grown under LL+N for 4 days. Afterwards, the cells were grown under HL-N (300–400 µmol photons m⁻²⋅s⁻¹, nitrogen-limited BG-11 medium with 4.25 mM NaNO₃). D90G-19: closed square; WT: open square.

Figure 6. Photosynthesis efficiency of Desmodesmus sp. (A) Potential maximum quantum efficiency (Fv/Fm) and (B) oxygen evolution rate.

WT and D90G-19 were grown under HL-N (300–400 µmol photons m⁻²⋅s⁻¹, nitrogen-limited BG-11 medium with 4.25 mM NaNO₃). D90G-19: closed circle; WT: open square.