Not quite sure if this is what you want to do -- if you are working with carbon biomass (also in your "plankton concentration" -- mg C m^3 ?), then just divide by the molar mass (12 g/mol) to arrive at mmol C m^3.

Helena Hauss thanks for your reply. My query is regarding this data (https://data.marine.copernicus.eu/product/OCEANCOLOUR_GLO_BGC_L4_MY_009_104/description), where different phytoplankton species are expressed in mg Chla/m3. I was hoping to convert each of them into mol C/m3.

@Kunal Madkaiker I see, that information was missing from your question. To do that, you could start with this one: Behrenfeld, M.J.; Boss, E.; Siegel, D.A.; Shea, D.M. Carbon-based ocean productivity and phytoplankton physiology from space. Glob. Biogeochem. Cycles 2005, 19.

Helena Hauss thanks!