be used in eutrophic waters. The samples are then fixed with formalin 

 (0.5 ml per 100 ml of water) and passed through filters impermeable to bac- 

 teria. In the laboratory, the filters are placed for 10 min. upon a filter 

 paper moistened with 1 percent solution of hydrochloric acid, dried and 

 counted. 



The content of hydrocarbonates should always be measured. This analy- 

 sis can be performed by direct titration when the water is clean and trans- 

 parent, and with distillation when it is highly sedimented or polluted. In 

 the former case, 100 ml of water is poured into a conical flask and 3 drops 

 of phenolphtalein solution is added. If the water does not turn pink, 1-2 

 drops of a solution of an alkali are added. The color is then neutralized 

 by addition of 0.1 N solution of HC1. After the color has disappeared (pH 

 8.3), 7 drops of methyl red or methyl orange are added, and titration is 

 repeated with HC1 until a stable pink color appears. The amount of the 

 acid used for titration from the time of addition of the second indicator 

 is multiplied by 12, thus obtaining the quantity in mg C of carbonate in 1 

 1 of the water. 



If the test water is dirty, the carbonates are determined by distilla- 

 tion from acidified solution into an alkali (Kuznetsov and Romanenko, 

 1963). 



The quantity of carbon dioxide assimilated by microorganisms is cal- 

 culated by the formula: 



r a = 



C k 



T 



where: 



ra - heterotrophic assimilation of CO2 (yg C 1/day), 

 r - radioactivity of microorganisms in the whole sample of 



tested water ( imp/mi n), 

 C^ - content of carbonates in water (yg C/l), 

 R - radioactivity of the carbonate solution, added into 



the sample, imp/mi n. 



Heterotrophic Assimilation as an Index of Bacterial Development 



Heterotrophic assimilation of carbon dioxide as an index of bacterial 

 development was first used by the author (Romanenko, 1964). It is based on 

 the proportionality between increase of bacterial biomass and CO? assimila- 

 tion. For this purpose ^ 4 C-labeled organic substances may also be used, 

 but here difficulties arise owing to the fact that organic substances 

 quickly decompose, and it is not always possible to establish the location 

 of the experiment on the non-linear assimilation curve. Further rationale 

 favoring the use of carbonates is the fact that organic substances are not 

 introduced with carbonates. The carbonates are essentially neutral sub- 

 stances and in nutrient media there is almost always an excess of carbonate 

 as a buffer. It is not desirable to create an excess of organic substances 

 in nutrient media. 



68 



