Dioxygenase that catalyzes DNA modification by mediating the conversion of the modified genomic base 5-methylcytosine (5mC) into 5-glyceryl-methylcytosine (5gmC) (PubMed:31043749, PubMed:33531488). Catalyzes the conjugation of a glyceryl moiety from L-ascorbate (vitamin C) to the methyl group of 5mC through a carbon-carbon bond (PubMed:31043749, PubMed:33531488). 5gmC DNA modification may be required during photosynthesis as an epigenetic mark that couteracts DNA methylation (PubMed:31043749). a 5-methyl-2'-deoxycytidine in DNA + L-ascorbate + O2 = a (8S,9S)-5-glyceryl-2'-deoxycytidine in DNA + CO2 + glyoxylate a 5-methyl-2'-deoxycytidine in DNA + L-ascorbate + O2 = a (8S,9R)-5-glyceryl-2'-deoxycytidine in DNA + CO2 + glyoxylate Binds 1 Fe(2+) ion per subunit. Decreased 5-glyceryl-methylcytosines (5gmC) DNA modification in the genome (PubMed:31043749). Reduced fitness of cells during exposure to high light levels (PubMed:31043749). Defects may be caused by hypermethylation and down-regulation of LHCSR3, a protein-coding gene required for the protection of C.reinhardtii cells from photo-oxidative damage under high light conditions, causing a reduced capacity for photoprotective non-photochemical quenching (PubMed:31043749). Belongs to the TET family.