The extremophilic nature and metabolic flexibility of Galdieria spp. have major potential in several biotechnological applications1. However, limiting research into successful large scale continuous cultivation of Galdieria sulphuraria has restricted industrial application of the species1. This research focused on investigating the feasibility of light dependant and independent continuous cultivation of Galdieria sp. RTK37.1 in various trophic and light conditions. Stable growth of Galdieria sp. RTK37.1 was achieved during photoautotrophic, mixotrophic and heterotrophic conditions, highlighting the ability to achieve submaximal specific growth while continuous collecting biomass for product retrieval. These results have potential industrial ramifications as they demonstrate that the growth of Galdieria sp. RTK37.1 can be manipulated to obtain desired biomass and product yields over long cultivation periods.

Continuous cultivation of Galdieria sp. RTK37.1 under various light intensities highlighted that operating chemostats under non-optimal light conditions not only affect the growth of Galdieria sp. RTK37.1 but also the ability to control and achieve desired stable growth. Light intensities greater than ~ 101 mol m-2 s-1 increased wall adhesion and decreased overall photosynthesis rates in photoautotrophically grown Galdieria sp. RTK37.1. Although biomass production increased during cultivation under warm white LED lighting at 191.3 mol m-2 s-1, the presence of immobilised cells is not desired during chemostat cultivation as can lead to non-optimal operation and the inability to obtain steady state growth.

Mixotrophic growth is defined as the ability for a cell to combine the mechanisms of photoautotrophic and heterotrophic growth, utilising both inorganic carbons through photosynthesis and organic carbon through respiration simultaneously6. Mixotrophic cultivation of Galdieria sp. RTK37.1 was shown to be an ideal growth condition for obtaining greater biomass production compared to photoautotrophically and heterotrophically grown Galdieria sp. RTK37.1. During steady state mixotrophic growth, Galdieria sp. RTK37.1 achieved a biomass concentration of 1.43 ( 0.12) g L-1, significantly greater than biomass concentrations during photoautotrophic and heterotrophic growth, which were only 0.93 ( 0.01) g L-1 and 0.63 ( 0.02) g L-1 respectively (t-test: p-value < 0.0001). The increased biomass production, along with the decreased net specific oxygen evolution rates and crude protein percentage (which indicated a decrease in photosynthesis compared with photoautotrophy) lead to the conclusion that Galdieria sp. RTK37.1 is a true mixotroph. The results demonstrated a synergistic effect between photosynthesis and aerobic respiration during mixotrophy, indicating both metabolism methods being utilised simultaneously in mixotrophic Galdieria sp. RTK37.1.

This research demonstrated that Galdieria sp. RTK37.1 is a promising strain for larger scale production, with evidence towards its ability to be used for large scale continuous cultivation in a range of applications such as phycocyanin production. Biomass production and growth of Galdieria sp. RTK37.1 can be controlled during continuous cultivation by limiting nutrients such as carbon or ammonia, or through various light intensity and wavelength conditions. Additionally, this research highlighted the ability for Galdieria sp. RTK37.1 to adapt to a wide range of conditions, achieving stable growth even under multiply limiting factors.