Sentence:  It was possible to maintain aerobic conditions in the pond by regular additions of sodium nitrate until the temperature decreased and the algae population changed from <WSD>blue-green</WSD> to green algae . 
Meaning of the word marked with <WSD> tags:  predominantly photosynthetic prokaryotic organisms containing a blue pigment in addition to chlorophyll; occur singly or in colonies in diverse habitats; important as phytoplankton