Disclosed is a method for enriching a photosynthetic microorganism (Chlorophyceae or Cyanophyceae) organic selenium, wherein said photosynthetic microorganism is cultured in a medium comprising a selenohydroxy acid compound of general formula (I) as defined within the specification, or a salt or an ester or amide derivative of said compound. Also disclosed is a composition comprising said microorganism and a culture medium comprising selenohydroxy acid compounds of formula (I).