Cultivation of Acidophilic Algae Galdieria sulphuraria and Pseudochlorella sp. YKT1 in Media Derived from Acidic Hot Springs

Abstract

Microalgae possess a high potential for producing pigments, antioxidants, and lipophilic compounds for industrial applications. However, the cultivation of microalgae comes at a high cost. To reduce the cost, changes from a closed bioreactor to open pond system and from a synthetic medium to environmental or wastewater-based medium are being sought. However, the use of open pond systems is currently limited because of contamination by undesirable organisms. To overcome this issue, one strategy is to combine acidophilic algae and acidic drainage in which other organisms are unable to thrive. Here, we tested waters from sulfuric acidic hot springs (Tamagawa, pH 1.15 and Tsukahara, pH 1.14) in Japan for the cultivation of the red alga Galdieria sulphuraria 074G and the green alga Pseudochlorella sp. YKT1. Both of these spring waters are rich in phosphate (0.043 and 0.145 mM, respectively) compared to other environmental freshwater sources. Neither alga grew in the spring water but they grew very well when the waters were supplemented with an inorganic nitrogen source. The algal yields were ∼2.73 g dry weight/L for G. sulphuraria and ∼2.49 g dry weight/L for P. sp. YKT1, which were comparable to those in an autotrophic synthetic medium. P. sp. YKT1 grew in the spring waters supplemented either of NH4+, NO3- or urea, while G. sulphuraria grew only when NH4+ was supplemented. For P. sp. YKT1, the spring water was adjusted to pH 2.0, while for G. sulphuraria, no pH adjustment was required. In both cases, no additional pH-buffering compound was required. The phycocyanin of the thermophilic G. sulphuraria is known to be more thermostable than that from the Spirulina platensis currently used in phycocyanin production for commercial use. The phycocyanin content in G. sulphuraria in the Tsukahara water supplemented with NH4+ was 107.42 ± 1.81 μg/mg dry weight, which is comparable to the level in S. platensis (148.3 μg/mg dry weight). P. sp. YKT1 cells in the Tamagawa water supplemented with a nitrogen source formed a large amount of lipid droplets while maintaining cellular growth. These results indicate the potential of sulfuric hot spring waters for large-scale algal cultivation at a low cost.

Unexpected conservation of the RNA splicing apparatus in the highly streamlined genome of Galdieria sulphuraria

“…This is true in G. sulphuraria , as demonstrated by previous analysis of transcriptome data derived from Sanger sequences and 454 long-reads that revealed alternatively spliced isoforms for about 500 genes [ 8 ]. To test if AS in G. sulphuraria responds to environmental changes, we generated and analyzed extensive RNA-seq data from this alga under two arbitrary different temperature conditions: ‘heat’ (42 °C and 46 °C; non-stressed, because this alga normally lives at temperatures between 35 and 56 °C [ 24 ]) and ‘cold’ (28 °C; stressed) (see Methods). A total of 1766 introns were identified as being alternatively spliced (mostly via intron retention) under one or both temperature conditions (Additional file 14 : Table S6).…”

Comparison of Galdieria growth and photosynthetic activity in different culture systems

“…For the latter, exposition at this light intensity leads to an increase of phycobiliprotein production: Carbone et al (2020) e.g. showed, in an experiment with a Twin Layer-S using different light intensities that 100 μmol photons m −2 s −1 was the optimal light intensity for production of phycobiliproteins, also Hirooka and Miyagishima (2016) obtained good production of phycocyanin at this light intensity in a suspended cultivation system using hot spring water supplemented with NH 4 + as culture medium.…”

Extremophile Microalgae: the potential for biotechnological application

“…, Škaloud et al. , Hirooka and Miyagishima ). In addition to adaptations for acidic environments, acidophiles are also typically found in environments with high temperatures, high salinity, or heavy metal concentrations since these conditions often co‐occur, for example, in areas of acid drainage (Novis and Harding , Raddadi et al.…”

“…YKT1 isolated from an acidic mine drainage in Japan is able to accumulate a large amount of storage lipids (~30% of dry weight) under nitrogen‐depleted condition and low pH (Hirooka et al. , Hirooka and Miyagishima ).…”