Comparison of the Photoautotrophic Growth Regimens of Chlorella sorokiniana in a Photobioreactor for Enhanced Biomass Productivity

Abstract

Microalgae have a wide industrial potential because of their high metabolic diversity and plasticity. Selection of optimal cultivation methods is important to optimize multi-purpose microalgal biotechnologies. In this research, Chlorella sorokiniana AM-02 that was isolated from a freshwater lake was cultured under various high photosynthetic photon flux density (PPFD) conditions and CO2 gas levels in standard Bold’s basal medium (BBM). Furthermore, a wide range of nitrate levels (180–1440 mg L−1) was tested on the growth of C. sorokiniana. Microalgae growth, pigment concentration, medium pH, exit gas composition, as well as nitrate, phosphate, and sulfate levels were measured during an experimental period. The preferred high PPFD and optimal CO2 levels were found to be 1000–1400 μmol photons m−2 s−1 and 0.5–2.0% (v/v), respectively. The addition of nitrate ions (up to 1440 mg L−1) to the standard growth medium increased final optical density (OD750), cell count, pigment concentration, and total biomass yield but decreased the initial growth rate at high nitrate levels. Our findings can serve as the basis for a robust photoautotrophic cultivation system to maximize the productivity of large-scale microalgal cultures.

Assessment of Chlorella sorokiniana Growth in Anaerobic Digester Effluent

“…The growth of microalgae and biomass productivity when grown in wastewater or anaerobic digester effluent depend on different factors (such as the features of the culture, physicochemical properties of the wastes, type of a photobioreactor, and technological parameters of the process) [ 28 ]. The microalgal strain Chlorella sorokiniana AM-02 was tested in our previous study [ 27 ], in which we identified the optimal growth conditions in a standard Bold’s Basal Medium (BBM). In this research, a synthetic medium for supplying algae with all the necessary growth compounds was replaced with a diluted anaerobic digester effluent (ADE).…”

“…Centrifugation of the digestate contributed to removing particles as well, which gave it a dark color. photons m -2 s -1 [27]. Therefore, we chose 1200 μmol of photons m -2 s -1 in the present study.…”

“…Moreover, we continuously sparged cultures with air supplemented with 2% CO2, since it was found that nutrients are consumed faster under these conditions [27]. Previous work results showed that the preferred high levels of photosynthetic photon flux density for C. sorokiniana strain AM-02 in standard BBM are 1000-1400 µmol of photons m −2 •s −1 [27]. Therefore, we chose 1200 µmol of photons m −2 •s −1 in the present study.…”

“…Under such conditions, microalgae should receive enough light, since the addition of digestate increases the medium's turbidity. Moreover, we continuously sparged cultures with air supplemented with 2% CO 2 , since it was found that nutrients are consumed faster under these conditions [27]. Figure 3 shows the changes in pH under different cultivation conditions.…”

“…Thus, vast new data on the screening, characterization, and efficient cultivation of green microalgae using affordable and nutrient-rich substrates could significantly increase microalgae productivity for the food, pharmaceutical, and biofuel industries. In our recent work [ 27 ], we optimized the nutrients levels and light intensity for the high growth rate of alga C. sorokiniana AM-02 in photoautotrophic growth regimens. Thus, a wide range of nitrate levels (180–1440 mg·L −1 ) and different photosynthetic photon flux density conditions (1000–1400 μmol·m −2 ·s −1 ) were tested on the growth efficiency of C. sorokiniana AM-02.…”