Nannochloropsis oceanica promises to be an industrial-level producer of polyunsaturated fatty acids.In this study,the fastest and slowest growing N.oceanica mutants were selected through N-methyl-N’-nitro-N-nitrosoguanidine mutation,and two mutant strains and the wild type(WT) subjected to transcriptome profiling.It was found that the OD_(680) reads at stationary growth phase of both WT and its mutants were proportional to their cell density,thus indicating their division rate and growth speed during culture.This chemical mutation was effective for improving growth performance,and the fast strain divided faster by upregulating the expression of genes functioning in the cell cycle and downregulating genes involved in synthesis of amino acids,fatty acids,and sugars as well as the construction of ribosome and photosynthetic machinery.However,the relationship among the effected genes responsible for cell cycle,metabolism of fatty and amino acids,and construction of ribosome and photosynthetic machinery remained unclear.Further genetic studies are required for clarifying the genetic/metabolic networks underpinning the growth performance of N.oceanica.These findings demonstrated that this mutation strategy was effective for improving the growth performance of this species and explored a means of microalgal genetic improvement,particularly in species possessing a monoploid nucleus and asexual reproduction. Nannochloropsis oceanica promises to be an industrial-level producer of polyunsaturated fatty acids.In this study, the fastest and slowest growing N.oceanica mutants were selected through N-methyl-N’-nitro-N-nitrosoguanidine mutation, and two mutant strains and the wild type (WT) subjected to transcriptome profiling. It was found that the OD 680 (680) reads at stationary growth phase of both WT and its mutants were proportional to their cell density, thus indicating their division rate and growth speed during culture.This chemical mutation was effective for improving growth performance, and the fast strain divided faster than upregulating the expression of genes functioning in the cell cycle and downregulating genes involved in synthesis of amino acids, fatty acids, and sugars as well as the construction of ribosome and photosynthetic machinery.However, the relationship among the affected genes responsible for cell cycle, metabolism of fatty and amino acids, and construction of ribosome and photosynthe fic machinery remained unclear.Further genetic studies are required for clarifying the genetic / metabolic networks underpinning the growth performance of N.oceanica.These findings demonstrating that this mutation strategy was effective for improving the growth performance of this species and explored a means of microalgal genetic improvement, particularly in species possessing a monoploid nucleus and asexual reproduction.