## 1 Introduction  
Microalgae are among the most widespread organisms. According to [PERSON] [1], it is estimated that there are up to 1 million species of algae on Earth. We meet them in the entire biosphere, but above all in fresh, salt, and saline waters. A characteristic feature of algae is the presence of chlorophyll [2, 3], which enables photosynthesis to be carried out in each individual cell [4]. The efficiency of this process and the related biomass production is much higher compared to land plants [5].  
Microalgae biomass has been widely used in various industries: pharmaceutical, food, animal feed, water purification, and biofuel production [6, 7, 8]. Under suitable conditions, microalgae convert solar energy into chemical energy of compounds accumulated in their cells, including not only in carbohydrates and proteins but also in lipids, which are used in the production of biodiesel [9, 10]. The biomass yield and the biochemical composition of microalgae depend on environmental and physiological factors such as culture temperature, pH, lighting intensity and type, availability of nutrients, and carbon dioxide [11].  
Lighting directly influences the growth of microalgae through photosynthesis. Algae cells, like most organisms, require not only access to light but also the dark phase, where photochemical processes occur and both ATP and NADPH are formed in algae cells [12]. Research on determining the optimal photoperiod in microalgae cultivation indicates that the efficiency of algal biomass production increases with the lengthening of the light to dark ratio [12; 13]. When selecting the optimal light, the quality of the spectrum and the light intensity must be taken into account. The quality of the spectrum is defined by the absorption spectrum of chlorophyll and other components present in algae cells, which include phycobilins and carotenoids. The energy consumption of algae cells depends on the chemical conditions of the constitutive pigments [14]. The assimilation pigments have two main bands of blue-green light absorption with a wavelength of 450-475 nm and red light, 630-675 nm [15]. Lighting should be evenly distributed throughout the reactor allowing the photons to reach the cells. Excessive light intensity can lead to photooxidation and photoinhibition, while low light levels will reduce the growth of microalgae [16].  
The pH of a culture medium not only affects the composition and lifetime of microalgae cells but also determines the availability of nutrients, the proper course of the photosynthesis process, and the uptake of carbon dioxide by cells [17]. The acidic conditions adversely affect the absorption of nutrients and disrupt the functioning of microalgae cells, while alkaline conditions reduce the ability to assimilate carbon dioxide. Research indicates that for most microalgae, the optimal pH range is between 6 and 10 [18]. Optimization of cultivation conditions is crucial for industrial productivity of microalgae. The objective aim of this work was to analyze some of the parameters important for algal biomass production and methods suitable for estimating _Chlorella vulgaris_ growth. In this study, we hypothesized that optimization of basic culture conditions leads to higher biomass concentration in the photobioreactors.