Observation and Enumeration of Organisms
The pattern of movement shown by organisms is characteristic of certain groups. For photographing protozoa, a flash unit is most useful in stopping movement including ciliary or flagellar motion. Enumeration of protozoa must be done with fixed samples. The counting procedures used for algae are appropriate for protozoa, although Lugol’s iodine solution cannot be used if organisms are to be identified to species. To preserve samples for enumeration, Lugol’s iodine solution is most suitable. Fixatives such as Lugol’s solution or formalin will kill the protozoa, and organisms respond differently to different fixatives. Other good fixatives include saturated mercuric chloride, 10% formalin, and Bouin’s fixative. Hot saturated mercuric chloride and hot 70 to 80% alcohol are good fixatives, although cold 85% alcohol and 5% formalin are reliable for the field. For gastrotrichs, an excellent fixative is osmic acid. Volatile fixatives should be used under a hood.