The potential of green microalgae cultures for agriculture application is discussed in the article. Microalgae are cultured for high value products such as food additives, biologically active substances, pigments, unicellular protein, renewable energy, methane, biodiesel, ethanol or hydrogen, wastewater treatment, CO₂ fixation. Microalgae’s natural metabolic process makes them feasible to use while along with water purification and providing a renewable feedstock supply. For implementation of high efficient production systems for biomass generation or algolization, the development of high productive cultivation in conditions of natural illumination is required. The use and understanding of controlled growth and nutrients supply for algocenosis in natural water bodies or raceway ponds it is particularly important from an operational stand point as far as it gives the sufficient information for scaling up the systems that can provide effective algolization in natural water body or pond culturing.

Growth systems are based mainly on the principle of concentration increase in the fixed  volume. The developed model provides a different principle, i.e. volume increase under fixed concentration with close to maximal growth rate which allows to achieve constant concentration, controlled culturing and biomass stable yield. Since such calculations are based on the use of the established cell quotes, it is necessary to determine them for the type or species (Levich, 2000). The need for a cell, or "quota", is actually the content of a specific element in a biomass unit or in a single cell. This value is also called the intracellular concentration of the substance, or the amount of substrate required to increase the unit cell biomass. There are methods for experimental identifying quotas suggested by a number of researchers, but they are quite complicated to apply (Levich, 2000). Some researchers introduce the concept of a specific absorption rate for each element since the intensity of absorption of cell nutrients depends on several factors, primarily on photosynthesis intensity, there is also a number of discrepancies regarding the term of cellular quota (Oglesby, 1977).

The suggested model is aimed to define the conditions for each growth period and keeping the culture in this boundaries. The conducted tests series aimed to prove our concept that in most systems algae growth can be supported by stable input of nutrients equal to “cell quota” while light input is stable. The total conversion is higher if nutrient provision for each of basic nutrients is equal to the “quota”. The model can be applied for both quasi-continuous and storage culture mode at different culture depths and, in addition to incident sunlight and water temperature data, it requires the following experimentally determined strain-specific input parameters: growth rate as a function of light intensity and temperature, biomass loss rate in the dark as a function of temperature and light intensity during the preceding light period, and the scatter-corrected biomass light absorption coefficient. The model is also applicable to photobioreactor cultures. Solar energy is usually used in cultivation systems and thus concentration increase results in conversion level decrease as light energy amount is stable and the amount of culture consuming light is growing. Proceeding from the foregoing, a simplified model of calculation, values of the gross demand of cells depending on their initial biomass is suggested to calculate the total cell needs depending on the start biomass at a fixed interval of time, which makes it possible to maintain growth factors at a level that meets the needs of a phytoplankton group or monoculture, and hence maintaining "predictable" productivity.

Total cell requirement in nitrogen input can be expressed as following: qNX(t) = 2 × qNX0 × eµt , where qNX(t) – total cell quota on nitrogen, qNX0 – initial nitrogen content in biomass, µ – growth rate coefficient, t – time required for biomass add. Similar equation can be proposed for the phosphorous: qPX(t) = 2 × qPX0 × eµt , where qPX(t) – bulk quota on phosphorous for the biomass, qPX0 – initial phosphorous concentration in the biomass, µ – growth rate coefficient, t – time required for biomass growth.

The developed growth method allows to cultivate on constant parameters (concentration) with the volume increase which is very important when we use sunlight. For daytime conditions, it is important to determine the specific growth rate (μ) in each of the n culture volume layers using experimentally determined strain-specific growth rate data. Since cells in well mixed dense cultures exposed to high average light intensities at or near the surface of the pond, integral growth rate was assumed as integral constant meaning and experimentally determined for the case of high average light intensity during the late exponential growth phase. Obtaining these parameters for each strain is rather laborious, though labour costs can be minimized or the process can be automatized in the industrial producrion. This will significantly reduce the requirement for outdoor pond cultivation. Finally, the biomass growth model, allows for the generation of strain-specific biomass productivity for natural water bodies, which is a topic of currently ongoing research.

Key words: autotrophs culture, cell quota, time culturing model, chlorella, algolization.