Epigenetic Regulation in Response to CO 2 Fluctuation in Marine Microalga Nannochloropsis oceanica.

Microalgae often undergo different CO2 experiment in their habitat. To adapt to low CO2 , carbon concentrating mechanism (CCM) could be launched in majority of microalgae and CCM are regulated at RNA level are well known. However, epigenetic modifications and their potential regulation of the transcription of masked genes at the genome level in response to CO2 fluctuation remain unclear. Here epigenetic regulation in response to CO2 fluctuation and epigenome-association with phenotypic plasticity of CCM are firstly uncovered in marine microalga Nannochloropsis oceanica IMET1. The result showed that lysine butyrylation (Kbu) and histone H3K9m2 modifications were present in N. oceanica IMET1. Moreover, Kbu modification positively regulated gene expression. In response to CO2 fluctuation, there were 5,438 and 1,106 genes regulated by Kbu and H3K9m2 in Nannochloropsis, respectively. Gained or lost histone methylations were closely associated with activating or repressing gene expressions. Differential modifications were mainly enriched in carbon fixation, photorespiration, photosynthesis, and lipid metabolism etc. Massive genome-wide epigenetic reprogramming was observed after N. oceanica cells shifted from high CO2 to low CO2 . Particularly, we firstly noted that the transcription of the key low CO2 responsive carbonic anhydrase (CA5), a key component involved in CCM stress signaling, was potentially regulated by bivalent Kbu-H3K9m2 modifications in microalgae. This study provides novel insights into the relationship between gene transcription and epigenetic modification in Nannochloropsis, which will lay foundation on genetic improvement of CCM at epigenetic level.