New insights into raceway cultivation of Euglena gracilis under long-term semi-continuous nitrogen starvation

This study aimed to investigate the physiological responses of Euglena gracilis (E. gracilis) when subjected to semicontinuous N-starvation (N-) for an extended period in open ponds. The results indicated that the growth rates of E. gracilis under the N- condition (11 +/- 3.3 g m(-2) d(-1)) were higher by 23% compared to the N-sufficient (N+, 8.9 +/- 2.8 g m(-2) d(-1)) condition. Furthermore, the paramylon content of E.gracilis was above 40% (w/w) of dry biomass in N- condition compared to N+ (7%) condition. Interestingly, E. gracilis exhibited similar cell numbers regardless of nitrogen concentrations after a certain time point. Additionally, it demonstrated relatively smaller cell size over time, and unaffected photosynthetic apparatus under N- condition. These findings suggest that there is a tradeoff between cell growth and photosynthesis in E. gracilis, as it adapts to semi-continuous N- conditions without a decrease in its growth rate and paramylon productivity. Notably, to the author's knowledge, this is the only study reporting high biomass and product accumulation by a wild-type E. gracilis strain under N- conditions. This newly identified long-term adaptation ability of E. gracilis may offer a promising direction for the algal industry to achieve high productivity without relying on genetically modified organisms.

Automated summary

This study investigated the physiological responses of Euglena gracilis (E. gracilis) under semicontinuous N-starvation (N-) conditions in open ponds. The results showed that E. gracilis had higher growth rates and a higher paramylon content under N- conditions compared to N+ conditions. The study also revealed that E. gracilis exhibited similar cell numbers regardless of nitrogen concentrations, with relatively smaller cell size and unaffected photosynthetic apparatus under N- conditions. This adaptation ability of E. gracilis may have promising implications for the algal industry to achieve high productivity without relying on genetically modified organisms.