- Arachidonic acid (AA) is a precursor of biologically activeprostaglandines and leukotrienes. The commercial source for AA at present is afungus, but the recently discovered coccoid green alga,Parietochlorisincisa comb. nov., in which over 90% of total AA is deposited intriacylglycerols, makes this species a potential candidate for commercialproduction of AA. We investigated the effect of the light-regime on cell-AAcontent and on culture productivity, by manipulating the intensity of the lightsource, the length of the light-path (LPL), and the population density ofcultures grown in flat plate glass reactors under both controlled laboratoryconditions (continuously illuminated) as well as outdoors. The effect ofnitrogen deprivation on culture content of AA and its productivity was alsostudied.In all experiments, the longer light-path reactors with the highest arealvolumes (L m−2) yielded the highest culture-AA or the highestamount of AA harvested per illuminated reactor surface. Highest culture contentof AA was obtained in cultures exposed to strong light andnitrogen-deprivation.In contrast, highest cell-AA content was obtained in cultures receiving thelowest light-dose. Maximum culture content of AA obtained in the laboratory was2667 mg L−1, reached after a 38-day growth period(of which the final 17 days took place in nitrogen-free medium), undercontinuous exposure to 2000 μmol photon m−2s−1. Maximal culture content of AA outdoors wassignificantly lower compared with the maximum obtained in the laboratory.