Engineered Solutions for Optimizing Growth and Bioproduction of the Microalgae Arthrospira Platensis and Haematococcus Pluvialis in the Scalable Air Accordion Photobioreactor

Abstract

The microalga Haematococcus pluvialis and the cyanobacterium Arthrospira platensis are photosynthetic microorganisms that have diverse applications in aquaculture, livestock and the health food industry as essential nutritional feed or food supplement. Astaxanthin is a natural bright-red secondary-metabolite pigment. It is one of the most valuable microalgae-produced carotenoids, synthesized by H. pluvialis, with a wide range of applications. A. platensis contains essential amino acids and fatty acids and the blue pigment phycocyanin. The large-scale production of H. pluvialis and A. platensis remains limited and cannot meet market demand, however, owing to the lack of effective cultivation systems while the market demand increases year by year. The use of scalable photobioreactors for the commercial cultivation of H. pluvialis and A. platensis for production of high-value secondary-metabolite products is desired to achieve high productivity and production consistency. The regular bubble column (RBC) photobioreactor commonly used by the industry to culture microalgae or cyanobacteria biomass in large scale for subsequent extraction of various bioactive compounds has shown its limitations, which include a requirement for high investment costs. As an alternative, a cost-effective Air Accordion photobioreactor (AAPBR) has been designed and developed at The University of Arizona. The first part of the current study yielded the following results: (1) the cell density of 1.116 ± 0.051 g L-1 on day 13 for H. pluvialis in the AAPBR significantly exceeded by 25.07% that in the RBC of 0.836 ± 0.008 g L-1. Similarly, the cell density of 1.015 ± 0.024 g L-1 on day 14 for A. platensis in the AAPBR significantly exceeded by 60.65% that in the RBC of 1.631 ± 0.027 g L-1; (2) the biomass productivity of 0.037 ± 0.000 g L-1 day-1 over 13 days for H. pluvialis in the AAPBR significantly exceeded by 36.67% that in the RBC of 0.058 ± 0.005 g L-1 day-1. Similarly, the biomass productivity of 0.054 ± 0.001 g L-1 day-1 on day 14 for A. platensis in the AAPBR significantly exceeded by 81.90% that in the RBC of 0.088 ± 0.002 g L-1 day-1; (3) the average specific growth rate of 0.066 ± 0.002 day-1 over 13 days for H. pluvialis in the AAPBR significantly exceeded by 25.58% that in the RBC of 0.087 ± 0.002 day-1. Similarly, the average specific growth rate of 0.098 ± 0.003 day-1 over 14 days for A. platensis in the AAPBR significantly exceeded by 35.83% that in the RBC of 0.133 ± 0.001 day-1; and (4) iInitiation of phase transition from Green to Red for H. pluvialis became noticeably observable in the AAPBR on day 13, but not in the RBC. In the second study, electrical elicitation (EE) was employed with the aim of improving the astaxanthin and biomass production of H. pluvialis. The results showed that: (1) the astaxanthin concentration of 5.937 ± 0.060 mg g-1 on day 12 for the 10 mins EE plus N treatment for H. pluvialis at stationary phase significantly exceeded by 12.92% that for the nitrogen starvation astaxanthin-producing positive control of 5.257 ± 0.149 mg g-1. This result also significantly exceeded by 28.43% that of the original basal level on day 0 of 4.623 ± 0.078 mg g-1; (2) electrical elicitation had a negative effect on the chlorophyll production of H. pluvialis cells at exponential phase and was not suitable to predict astaxanthin levels under the different treatments of H. pluvialis at stationary phase; and (3) enhancement of biomass could only be achieved in the 60 mins EE treatment for H. pluvialis at exponential phase such that the final biomass production of 0.438 ± 0.006 g L-1 on day 12 for the 60 mins EE treatment for H. pluvialis at exponential phase significantly exceeded by 68.78% that for the wild-type non elicited control of 0.260 ± 0.001 g L-1. Similarly, this result significantly exceeded by 20.62% that of its original basal level on day 0 of 0.363 ± 0.011 g L-1.