The addition of 2.5 mᴍ glufosinate ammonium (BASTA), a well known plant killer, to Haematococcus pluvialis culture efficiently inhibits cell growth, blocks the activity of glutamine synthetase (GS) and induces astaxanthin accumulation. Conversely, methionine-S-sulfoximine (MSX), a well known GS inhibitor, had no effect on neither these parameters. When GS activity was tested in vitro, MSX inhibited the activity at high concentrations (mᴍ), while glufosinate was effective in the μm range. We have found that in the presence of glufosinate, ammonia is excreted from the cells. Therefore, we suggest that this process enables Haematococcus cells to escape the potentially harmful effect of glufosinate. As a consequence of the inability to assimilate nitrogen, astaxanthin is accumulated. This situation resembles the response of Haematococcus cells to nitrogen starvation
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