# Malaysian Virgin Soil Extracts as Natural Growth Enhancer for Targeted Green Microalgae Species
## 3 Results and Discussion
### Effects of Soil Extracts on the Growth of Microalgae
Although the autoclave treatment method at 105 \({}^{\circ}\)C twice exhibited slightly lower growth of _Chlorella sorbiciniana_, during the cultivation period, the optical density on day 9 reached 0.373 \(\pm\) 0.014, resembling the results obtained from autoclave treatment methods at 121 \({}^{\circ}\)C once and at 121 \({}^{\circ}\)C twice, respectively, with no significant difference (\(p\) > 0.05). The use of soil extracts can be sustainable and cheaper support for mass cultivation of microalgae in addition to commercial medium [31]. Since the soil extracts contain the presence of important nutrients for microalgae growth such as carbon, nitrogen, phosphorus, and trace elements due to the oligotrophic nature [32], they assist the formation of protein which is vital for microalgae cultivation. The occurrence of trace elements such as phosphorus and calcium on the other hand stimulates an accelerated rate of cell division and specific growth rate of microalgae resulting in increased cell density. Furthermore, soil contains many free enzymes that play an important role in catalyzing reactions that lead to the decomposition of organic matter that stimulates the growth of microalgae [33]. A study carried out by [PERSON] [34] reported enhanced production of _Chlorella vulgaris_ due to the addition of soil extracts and vitamins to the culture medium as evidenced by the increase in the optical density. Similarly, _Porphyridium purpneum_ cultivated in Rock medium in addition to soil extracts promoted the growth of the microalgae as revealed by the research established by [PERSON] [35]. Figure 4: Optical density at 680 nm of _Chlorella sorbiciniana_. Error bars represent the standard deviation between three replicates (\(n\) = 3). Although the soil extracts obtained from the autoclave treatment method at 121 \({}^{\circ}\)C twice yielded the highest ratio of carbon to nitrogen to phosphorus of 240.4:9:5:0.1, the utilization of different autoclave methods aided the growth of the tested microalgae, namely _Nannochloropsis oculata_, _Nannochloropsis ocanica_, and _Chlorella sorokiniana_, significantly. These findings indicated that the existence of a relatively high level of nutrients on top of distinctive extraction methods may reduce and precipitate the necessary vitamins and minerals present in the soil which enabled increased growth of microalgae [36]. In addition, the Redfeld ratio (C:N:P = 106:16:1 atomic ratio) has been set as the benchmark nitrogen to phosphorus ratio to optimally grow phytoplankton. However, different phytoplankton has different needs for nutrients to support their growth process [35]. In the present study, the ratio of nitrogen to phosphorus was quite similar between the soil extracts and the presence of a carbon ratio within the range of 162 to 164 supported the growth of _Nannochloropsis_ spp. and _Chlorella_ sp. effectively. Evaluation of Specific Growth Rate (SGR, \(\mu\)) and Division Rate (k) for Selected microalgae in Modified Soil Extract Medium  
The specific growth rate (\(\mu\), d\({}^{-1}\)) showed a significant difference (\(p\) < 0.05) between the modified soil extract medium when tested using three different species of microalgae, namely _Nannochloropsis oculata_, _Nannochloropsis ocanica_, and _Chlorella sorokiniana_ (Table 4). _Nannochloropsis oculata_ recorded the highest specific growth rate of 0.114 \({}^{\circ}\)\(\pm\) 0.007 d\({}^{-1}\) when a medium supplemented with soil extracts obtained from natural extraction for 24 h was used. The addition of soil extracts obtained from autoclave 121 \({}^{\circ}\)C once treatment method to the conventional medium was found competent to generate the highest specific growth rate of 0.069 \({}^{\circ}\)\(\pm\) 0.003 d\({}^{-1}\) and 0.080 \({}^{\circ}\)\(\pm\) 0.001 d\({}^{-1}\) for _Nannochloropsis ocanica_ and _Chlorella sorokiniana_, respectively. The relatively higher specific growth rate indicates that the addition of extraction parameters and nutrients in the modified soil extracts medium is essential to the exponential rise.