Acidophilic Green Alga Pseudochlorella sp. YKT1 Accumulates High Amount of Lipid Droplets under a Nitrogen-Depleted Condition at a Low-pH

Abstract

Microalgal storage lipids are considered to be a promising source for next-generation biofuel feedstock. However, microalgal biodiesel is not yet economically feasible due to the high cost of production. One of the reasons for this is that the use of a low-cost open pond system is currently limited because of the unavoidable contamination with undesirable organisms. Extremophiles have an advantage in culturing in an open pond system because they grow in extreme environments toxic to other organisms. In this study, we isolated the acidophilic green alga Pseudochlorella sp. YKT1 from sulfuric acid mine drainage in Nagano Prefecture, Japan. The vegetative cells of YKT1 display the morphological characteristics of Trebouxiophyceae and molecular phylogenetic analyses indicated it to be most closely related to Pseudochlorella pringsheimii. The optimal pH and temperature for the growth of YKT1 are pH 3.0–5.0 and a temperature 20–25°C, respectively. Further, YKT1 is able to grow at pH 2.0 and at 32°C, which corresponds to the usual water temperature in the outdoors in summer in many countries. YKT1 accumulates a large amount of storage lipids (∼30% of dry weigh) under a nitrogen-depleted condition at low-pH (pH 3.0). These results show that acidophilic green algae will be useful for industrial applications by acidic open culture systems.

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

“…The spring water media were adjusted to pH 2.0 with KOH and then filter-sterilized, because P. sp. YKT1 does not grow under extremely acidic conditions (pH < 2.0) (Hirooka et al, 2014). …”

“…sp. YKT1 cells form a high amount of lipid droplets under nitrogen-depleted condition and the cellular neutral lipid contents is elevated to ∼30% of the cellular dry weight (Hirooka et al, 2014). Thus, further investigation of acidophilic oleaginous algae and a determination of the appropriate conditions for lipid droplet formation in acidic spring water (or other acidic wastewater) will eventually lead to oil production while maintaining cellular growth at relatively low cost.…”

“…YKT1 (Hirooka et al, 2014) were used in this study. G. sulphuraria was maintained with gyration in M-Allen (MA) medium (Minoda et al, 2004), at pH 2.0 and 30°C under continuous light.…”

“…As representative acidophilic algae, we tested the red alga Galdieria sulphuraria 074G (Gross and Schnarrenberger, 1995) and the green alga Pseudochlorella sp. YKT1 (Hirooka et al, 2014). …”

“…YKT1 (Trebouxiophyceae) was recently isolated from the acidic drainage of an abandoned sulfur mine in Japan (Hirooka et al, 2014). This alga is able to grow at pH 2.0–5.0 and 20–32°C and accumulates a large amount of storage lipids (∼30% of dry weight) under a nitrogen-depleted condition, a characteristic which is likely useful for the production of biofuels.…”