Heterologous mannitol-1-phosphate dehydrogenase gene over-expression in Parachlorella kessleri for enhanced microalgal biomass productivity

Abstract

Background: Microalgae have tremendous potential in CO₂ sequestration, bioenergy, biofuels, wastewater treatment, and high-value metabolites production. However, large-scale production of microalgae is hampered due to photo-inhibition in outdoor cultivation. Mannitol, as an osmolyte, is known to relieve the stress produced under different abiotic stress conditions during the growth of a photosynthetic organism.

Results: In the present study, Mannitol-1-phosphate 5-dehydrogenase (Mt1D) was over-expressed to study the effect of mannitol over-production in Parachlorella kessleri under high-light induced stress. Over-expression of Mt1D led to 65% increased mannitol content in the transformed P. kessleri compared to that of wild type. Mannitol transformant demonstrated > 20-fold reduction in reactive oxygen species generation and 15% higher biomass productivity when grown in outdoor cultivation with high-light irradiance of 1200 μmol photons m^-2 s^-1.

Conclusions: The current study establishes that a higher mannitol concentration provides stress shielding and leads to better acclimatization of transgenic microalgae against high-light generated stress. It also led to reduced ROS generation and improved growth of microalga under study. Thus, overexpression of the Mt1D gene in microalgae can be a suitable strategy to combat high-light stress.

Keywords: Abiotic stress, High-light, ROS; Mannitol; Microalgae.

Fig. 1

PCR analysis for Mt1D gene with gene-specific primers in wild-type and transformed P. kessleri. Lane 1: 100 bp ladder (from bottom to top 1000 bp, 1100 bp, 1200 bp, and 1300 bp), lane 2: PCR amplicon in positive control, lane 3: amplicon of wild-type P. kessleri, and lane 4: PCR amplicon of Mt1D transformant

Fig. 2

SDS PAGE analysis of wild-type and transformed P. kessleri. Lane 1: protein marker, Lanes 2, 3, and 4: total protein of WT and lane 5: total protein of transformant

Fig. 3

Growth study and nitrate uptake profile of transformed and wild-type P. kessleri under natural light of 0–1200 μmol photons m⁻² s⁻¹. Values are mean ± standard deviation (n = 3). Student’s t test was significant at P < 0.05 except for nitrate content which was significant at P < 0.09

Fig. 4

Mannitol content of wild-type and transformed P. kessleri in the light regime up to 1200 μmol photons m⁻² s⁻¹. Values are mean ± standard deviation (n = 3). Student’s t test was significant at P < 0.05