The nature of the culture media is the main determining factor in the growth of microalgae, their productivity and ultimately their biomass as long as the pH, light intensity, and temperature needs have been fulfilled (da Silva Ferreira and Sant'Anna, 2017; Gani et al. 2019; Chowdury et al. 2020). The light provided to the culture was by white light-emitting diode tubes (3200K, 9W). Csavina et al. (2011) illustrated that Oocystis sp. requires the optimum light intensity of 150 µmol m− 2 s− 1. Latala et al. (1991) found that the growth of Oocystis sp. was completely inhibited at high light intensities ranging between 270–380 µE/m2s.
pH influences the quantity of free carbon in the culture media and the balance between carbonate and bicarbonate. In this study, the pH was adjusted to range between 6.0 to 8.0, as reported by RAo (1963), to suit the growth of Oocystis sp. optimally. Mayo (1997) found similar results while culturing Chlorella sp. with the maximum growth rate at pH ranging from 6.31 to 6.84. Changes in the media pH were attributed to the photosynthetic and decomposition process of the cultured Oocystis sp. ( Tucker and D'Abramo 2008; Rahardini et al. 2018).
During the study, the water temperature range was in optimal range as reported by Nalley et al. (2018), where they found the best temperature for optimum growth of Oocystis sp. ranged between 26.600C to 27.780C. Chowdury et al. (2020) state that temperature is critical in micro-algae culture as it affects the algal growth rate, cell size, biochemical composition, and nutrient requirements.
The temporal chlorophyll-a concentration varied amongst the treatments, mainly due to the different constituents in the culture treatment media, with the differences being significant with time in all the treatments. Sutkowy et al. (2019) state that the composition of culture media influences the growth of micro-algae in vitro. Overall, all treatments followed a similar trend where there was a decrease in the chlorophyll-a concentration once it was at its highest biomass. The pattern achieved is similar to that obtained by Rahardini et al. (2018) while assessing the growth of Chlorella sp., which is also found in the Chlorophyceae class of Green algae.
At the begging of the experiment, there was a steady increase (exponential phase) in the chlorophyll-a concentration due to sufficient nutrient concentration in all the treatments. There was no lag phase observed during the study, which can be attributed to the conditions of the inoculum (Spencer 1954). Talling (1966) notes that an inoculum obtained from a healthy exponentially growing population is unlikely to have a lag phase when transferred to a freshly prepared media under similar growth conditions (light, temperature, and pH); consequently, there is no need for physiological adaptations for growth. The lack of a lag phase is crucial as it reduces the time required for upscaling the culture, allowing the harvesting of cultured cells sooner. The exponential phase is characterised by the production of materials that are also capable of growth (Fogg and Thake 1987), with its length depending on the size of the inoculum, the growth rate, and the capacity of the medium and culturing conditions to support algal growth.
The exponential phase was followed by the stationary phase, characterised by a decline in population growth, resulting from reduced nutrient concentration in the culture media, causing the number of cells to remain constant. The stationary phase is characterised by zero net growth, as the rate of the growth of the cells is equal to the rate of cell death ( Nyström 2004; Maier and Pepper 2015). Finally, the death phase follows, as nutrients in the culture media run out, resulting in the death of cultured algal cells; therefore, a decline in the chlorophyll-a concentration was witnessed. The death rate of cells in the media became higher than the rate of cell growth (Maier and Pepper 2015).
The study demonstrated that the inorganic fertilizers' specific compositions and comparison between the culture media did not have any significant impact on the growth rate, divisions per day, and the doubling time of cultured Oocystis sp.. Similar results were also obtained by Rahardini et al. (2018) who also found no significant differences in the growth rate and doubling time of Chlorella sp. cultured in media with different compositions. The divisions per day obtained was higher than those from Csavina et al. (2011), which can be attributed to the continuous agitation of the algal cells during culture, consequently not allowing the clamping of cells together, resulting in deaths and ultimately decline in the growth rate and divisions per day. Research carried out by Sobczuk et al. (2006) showed an increase in the growth rate of microalgae with an increase with agitation; it should also be noted that excessive agitation can result in cell death.