Epitranscriptomic reprogramming in response to low CO2 stress and m6A engineering to enhance biomass production in Nannochloropsis oceanica.

N6-adenine methylation (m6A) as an epitranscriptomic mark is the most abundant modification in eukaryotic RNA and plays a dynamically regulated role. However, m6A dynamics, deposition and engineering in microalgae remain largely unknown. Here, in Nannochloropsis oceanica, the dynamic alterations and reprogramming in m6A RNA modifications after the shift from high to low CO2 conditions were first investigated using methylated RNA immunoprecipitation sequencing. The m6A peaks in N. oceanica were mainly enriched in 3'UTR. A positive association between m6A abundance and mRNA transcription of CO2-responsive genes was observed; moreover, N. oceanica cells adopted versatile strategies in a dynamic reprogramming of m6A in response to low CO2 stress. Secondly, knockout of two putative m6A methylases including NoMTA (NO04G02990) and NoMTB (NO07G02450) by genome editing induced methylation reprogramming, which was associated with expression changes of low-CO2 responsive genes such as carbon/nitrogen metabolism, and photorespiration genes that underlie reductions in growth and biomass. Lastly, m6A modification reprogramming was first engineered to increase low-CO2 stress tolerance and biomass productivity by the CRISPR/dCas13 system combined with MTA and NoMTB under low CO2 in N. oceanica. Therefore, these strides would pave the way for microalgal epigenetics and future industrial applications.