Effects of methionine sulfoximine on growth and nitrogen assimilation of the marine microalga Chlorella autotrophica

Chlorella autotrophica Shihira and Krauss (clone 580), a euryhaline microalga from the marine coastal environment is subject to large fluctuations in external salinity and nitrogen supply. The alga exhibits maximum growth at salinities lower than 100% ASW (artificial seawater). Cells divide faster and show higher cell yields when the supply of either NH ₄ ⁺ or NO ₃ ^- is increased above 0.2 mM. Cells growing on NH ₄ ⁺ show high levels of NADPH-glutamate dehydrogenase (GDH) activity, and the levels of glutamine synthetase (GS) are decreased to very low levels under these conditions. Methionine sulfoximine (MSX), an inhibitor of GS, has little effect on cell division and nitrogen assimilation of cells growing on NH ₄ ⁺ . Cells growing on NO ₃ ^- , however, show marked inhibition (65%) in nitrogen assimilation in the presence of 5 mM MSX. This MSX concentration also causes growth retardation and a progressive decrease in cell protein and nitrogen content. GS is almost completely inhibited by 5 mM MSX in both NH ₄ ⁺ and NO ₃ ^- -grown cells. Cells growing on NH ₄ ⁺ maintain high levels of NADPH-GDH activity in the presence of MSX. NADPH-GDH activity in MSX-treated NO ₃ ^- -grown cells increases, and, in the presence of 5 mM MSX, reaches 40% of the level found in NH ₄ ⁺ -grown cells. These results are consistent with NADPH-GDH providing an alternate pathway for NH ₄ ⁺ assimilation by this marine Chlorella species.