Development of a photobioreactor for the cultivation of the freshwater microalga Haematococcus pluvialis

Abstract

Haernatococcus pluvialis is a flagellated green alga that accumulates large quantities of the carotenoid astaxanthin. Astaxanthin is commercially important as a aquaculture pigment, and as a human health supplement. Currently the majority of astaxanthin used in aquaculture is synthetic. However recent advances in algal mass culture technology and growing consumer awareness about the origins of foodstuffs mean that the use of H. pluvialis for the production of astaxanthin may become a viable alternative to synthetically produced astaxanthin. This study focused on the development of a scaleable photobioreactor and a methodology that could be used for the commercial production of H. pluvialis. Four successive air-lift driven novel tubular photobioreactors (termed TBRI, TBR2, AAPS™I and AAPS™2) were developed for the photosynthetic cultivation of H. pluvialis. Physical assessment revealed that the four systems differed in terms of their mass transfer, gas hold-up, flow rate and photostage turbulence (Reynolds number). The photo stage of each photobioreactor acted as the main area for light absorbance, (analysis of the spectral absorbance of the tubing revealed that it transmitted light of 320 - 820nm). The AAPS™I and AAPS™2 had a manifold photo stage, comprising of three windings. They also had a riser and downcomer of increased length when compared to TBRI and TBR2. This increased the bulk density difference between these stages and resulted in an increased fluid flow rate and consequently improved turbulence within the photostage. As well as being longer, the riser of the AAPSTM systems also had an increased diameter. This, combined with the improved gas hold-up of the systems, increased the mass transfer of the AAPSTM 1 and AAPS™2. The manifold assembly also allowed the construction of a larger volume photo stage while frequently returning the cells to the riser/header tank where gas exchange occurred. This prevented the detrimental build-up of photosynthetic 0₂.