Development of mutant microalgae that accumulate lipids under nitrate-replete conditions

Evaluation of cultivation conditions in hydrogel systems to enhance Chlorella vulgaris growth

Microalgae have gained significant attention as sustainable alternatives to traditional agriculture and energy resources. The cultivation efficiency of microalgae within hydrogel systems presents a promising avenue for spatially efficient bio-production. However, the optimum cultivation conditions of hydrogel cultivation systems have not been elucidated. This study focused on evaluating the hydrogel-based cultivation of Chlorella vulgaris in symbiosis with Bacillus Strain Salmah Ismail (SI) 139SI. It investigated the impact of hydrogel concentration, pH, light exposure, and system thickness on the growth and chlorophyll production of the algae. Our findings highlighted that, in coculture, a 7% (w/v) hydrogel concentration, pH of 7.4, a 12-h light/dark cycle at a hydrogel concentration of 7% but continuous light exposure under 5% hydrogel concentration, and a system thickness of 10 mm have provided the most favorable environmental conditions for the proliferation and chlorophyll production of C. vulgaris. These conditions significantly enhanced the biomass yields, suggesting that tailored hydrogel environments can substantially improve microalgae productivity.

Strategies to improve the efficiency and quality of mutant breeding using heavy-ion beam irradiation

Heavy-ion beam irradiation (HIBI) is useful for generating new germplasm in plants and microorganisms due to its ability to induce high mutagenesis rate, broad mutagenesis spectrum, and excellent stability of mutants. However, due to the random mutagenesis and associated mutant breeding modalities, it is imperative to improve HIBI-based mutant breeding efficiency and quality. This review discusses and summarizes the findings of existing theoretical and technical studies and presents a set of tandem strategies to enable efficient and high-quality HIBI-based mutant breeding practices. These strategies: adjust the mutation-inducing techniques, regulate cellular response states, formulate high-throughput screening schemes, and apply the generated superior genetic elements to genetic engineering approaches, thereby, improving the implications and expanding the scope of HIBI-based mutant breeding. These strategies aim to improve the mutagenesis rate, screening efficiency, and utilization of positive mutations. Here, we propose a model based on the integration of these strategies that would leverage the advantages of HIBI while compensating for its present shortcomings. Owing to the unique advantages of HIBI in creating high-quality genetic resources, we believe this review will contribute toward improving HIBI-based breeding.

Lethal and mutagenic effects of different LET radiations on Bacillus subtilis spores

New, useful microorganism resources have been generated by ionizing radiation breeding technology. However, the mutagenic effects of ionizing radiation on microorganisms have not been systematically clarified. For a deeper understanding and characterization of ionizing radiation-induced mutations in microorganisms, we investigated the lethal effects of seven different linear energy transfer (LET) radiations based on the survival fraction (SF) and whole-genome sequencing analysis of the mutagenic effects of a dose resulting in an SF of around 1% in Bacillus subtilis spores. Consequently, the lower LET radiations (gamma [surface LET: 0.2 keV/µm] and 4He2+ [24 keV/µm]) showed low lethality and high mutation frequency (MF), resulting in the major induction of single-base substitutions. Whereas higher LET radiations (12C5+ [156 keV/µm] and 12C6+ [179 keV/µm]) showed high lethality and low MF, resulting in the preferential induction of deletion mutations. In addition, 12C6+ (111) ion beams likely possess characteristics of both low- and high-LET radiations simultaneously. A decrease in the relative biological effectiveness and an evaluation of the inactivation cross section indicated that 20Ne8+ (468 keV/µm) and 40Ar13+ (2214 keV/µm) ion beams had overkill effects. In conclusion, in the mutation breeding of microorganisms, it should be possible to regulate the proportions, types, and frequencies of induced mutations by selecting an ionizing radiation of an appropriate LET in accordance with the intended purpose.

Development of a stable semi-continuous lipid production system of an oleaginous Chlamydomonas sp. mutant using multi-omics profiling

Background

Microalgal lipid production has attracted global attention in next-generation biofuel research. Nitrogen starvation, which drastically suppresses cell growth, is a common and strong trigger for lipid accumulation in microalgae. We previously developed a mutant Chlamydomonas sp. KAC1801, which can accumulate lipids irrespective of the presence or absence of nitrates. This study aimed to develop a feasible strategy for stable and continuous lipid production through semi-continuous culture of KAC1801.

Results

KAC1801 continuously accumulated > 20% lipid throughout the subculture (five generations) when inoculated with a dry cell weight of 0.8–0.9 g L⁻¹ and cultured in a medium containing 18.7 mM nitrate, whereas the parent strain KOR1 accumulated only 9% lipid. Under these conditions, KAC1801 continuously produced biomass and consumed nitrates. Lipid productivity of 116.9 mg L⁻¹ day⁻¹ was achieved by semi-continuous cultivation of KAC1801, which was 2.3-fold higher than that of KOR1 (50.5 mg L⁻¹ day⁻¹). Metabolome and transcriptome analyses revealed a depression in photosynthesis and activation of nitrogen assimilation in KAC1801, which are the typical phenotypes of microalgae under nitrogen starvation.

Conclusions

By optimizing nitrate supply and cell density, a one-step cultivation system for Chlamydomonas sp. KAC1801 under nitrate-replete conditions was successfully developed. KAC1801 achieved a lipid productivity comparable to previously reported levels under nitrogen-limiting conditions. In the culture system of this study, metabolome and transcriptome analyses revealed a nitrogen starvation-like response in KAC1801.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02196-w.