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Growth characteristics of a diatom microalgе Cylindrotheca closterium are investigated. Cylindrotheca closterium cultivated on nutrient mediums which were received by proportional increase all biogenous elements concentration of the environment F. It is shown that culture efficiency increases due to proportional increase of the biogenous elements up to 20 F in a nutrient medium. The maximum efficiency of this microalga is received at cultivation on a nutrient medium 20 F and made 1.74 g/l.

The carbon utilization efficiency is an important characteristic of the cultivated object. Diatom Cylindrotheca closterium (Ehrenberg) Reimann & J. C. Lewin is known to use carbon from aquatic environment quite effectively, as it has many unique carbonic anhydrases and carbon transporters. However, the carbon fixation efficiency for many types of diatoms in culture is still unknown. When calculating the carbon fixation efficiency, researchers use different terminology and methods, and it leads to significant difficulties when comparing the carbon fixation efficiency in the biomass of various types of microalgae. The aims of this study are: 1) to update terms and definitions used in literature on the basis of modern concepts of carbon fixation in microalgae biomass, as well as absorption of inorganic carbon by microalgae culture; 2) to evaluate the carbon fixation efficiency in the biomass of C. closterium diatom under conditions of cumulative cultivation. C. closterium was grown at a temperature of +20 °C on a nutrient medium RS. During the cultivation, the culture was bubbled with air (1.1 L of air per 1 L of culture per minute). The air temperature at the outlet of the suspension was of +19 °C; the maximum productivity of the culture was of 1.254 g·L−1·day−1. According to the results of the CHN analysis, the proportion of carbon in C. closterium dry biomass was of 23 %. Under the conditions of cumulative cultivation in C. closterium, the carbon fixation efficiency in biomass was of 90 %. Compared with other algae species, C. closterium is characterized by a rather high CO2 fixation efficiency. For example, in green microalga Chlorella protothecoides and Ch. vulgaris, the CO2 fixation efficiency was of 20 % and 55.3 %, respectively; in cyanobacteria Spirulina sp. – of 38 %; in red microalgae Porphyridium purpureum – of 69 %. It was observed that to ensure an increase of 1 g of C. closterium dry biomass per day at a temperature of +19 °C, a minimum of 0.46 L of CO2, or 1132 L of air, should be consumed. Possibly, it is high carbon fixation efficiency, as well as low carbon fraction in C. closterium biomass, that explains the high production indices of this species. Under equal conditions of cultivation in terms of light and carbon availability, the productivity of C. closterium can exceed the productivity of other types of microalgae by 5–10 times. So, while Spirulina sp. productivity reaches 0.2 g·L−1·day−1, C. closterium productivity is of 1.254 g·L−1·day−1.