Complex sentence: Under oxic conditions, MoO 4 2--is very unreactive, but in modern euxinic environments such as the Black Sea, molybdenum precipitates out as oxythiomolybdate (MoO 4-x S x 2 --).
Split into simpler sentences: Under oxic conditions, MoO 4 2--is very unreactive but in modern euxinic environments such as the Black Sea, molybdenum falls out of solution. Under euxinic condtions, Molybdenum most likely leaves as a form of oxythiomolybdate (MoO 4-x S x 2 --).