The effect of bacterial contamination on the heterotrophic cultivation of Chlorella pyrenoidosa in wastewater from the production of soybean products
Graphical abstract
Biomass changes in the pure and contaminated algal cultures.
Highlights
► Soybean processing wastewater can promote C. pyrenoidosa growth and algal lipids accumulation. ► Cultivation of C. pyrenoidosa in SPW could yield cleaner water and useful biomass. ► Bacteria coexisting with algae improved the degradation of TN, TP, glucose and COD. ► Bacterial contamination reduced total lipid content and algal lipid production rate. ► Apparent interaction between bacterial and algae varied based on organics in cultures.
Introduction
In recent years, the use of the microalgae cultivation and production in food manufacture, health care, pharmaceutical industry, agricultural industry, livestock, biofuels, and other areas has attracted significant attention (Chisti, 2007; Perez-Garcia et al., 2011). However, the production cost of employing microalgae as an alternative fuel source is still high due to the required nutrients and freshwater resources (Bhatnagar et al., 2010), and the economic feasibility of its technique and operating process are still challenging. Therefore, searching low production cost method is therefore timely.
In order to promote the growth of microalgae and enhance its lipid content at a lower cost, a number of investigation have been carried out on the screening of microalgal strain (Chisti, 2007; Sydney et al., 2011), optimization of culture condition (Miao and Wu, 2006), design of bioreactor for microalgal culture, culture style, etc. (Huntley and Redalje, 2007; Xiong et al., 2010). Similarly, some studies on coupled algal growth and wastewater treatment have been also conducted with the goal of reducing costs (Li et al., 2008; Bhatnagar et al., 2010; Perez-Garcia et al., 2011; Su et al., 2011). They utilized pollutants in wastewater to cultivate microalgae instead of nutritional raw materials, especially for heterotrophic microalgae (e.g., chlorella, etc.) which can use organic pollutants in the wastewater directly to obtain rapid growth. Therefore, it is a very promising research field and will be helpful to develop a new green process both for the production of microalgae and wastewater treatment.
However, bacterial infection, resulted from introducing organic matters in microalgae culture system under the aerobic condition, is inevitable to influence the removal of organic substances and the growth of microalgae. On the one hand, the competition for nutrients between the algae and bacteria may change the wastewater composition (Perez-Garcia et al., 2011), which probably represses the growth of algae and simultaneously affects the algal cell density, lipid content, and protein content. Several previous studies reported that some bacteria could cause the microalgae death by releasing soluble cellulose enzyme (Kim et al., 2007), and/or extracellular substances (Fergola et al., 2007). Meanwhile, the bacteria can also attach the inner-wall of the photo-bioreactor to form bacteria film which will hinder the transmission of light and inhibit the microalgae photosynthetic growth. On the other hand, bacteria can help degrade some recalcitrant compounds, allowing them to be easily assimilated by algae. For example, in algae cultivation reactors, the degradation by bacteria can result in the production of carbon dioxide, water, ammonium, nitrate, and phosphate, which can be used as carbon, nitrogen, and phosphorous sources of algae. Therefore, bacteria and algae are often expressed as the reciprocal relationship in the algae pond system (Glöckner et al., 2000). Some other studies also showed that the presence of certain bacteria can promote the growth of microalgae in the microalgae light autotrophic process (de-Bashan et al., 2002a, de-Bashan et al., 2004). Thus, there are some very complicated relationships between bacteria and algae, which probably include competition, syntrophy, mutualism and antagonism. The type of interactions between bacteria and algae is influenced by environmental conditions. We mainly focus on how bacterial contamination impacts microalgae growth and lipid content in a heterotrophic culture of microalgae with high concentrations of glucose or organic wastewater, and whether it is possible to enhance the lipid yields by changing the culture medium.
Basal (BG) medium containing glucose is commonly used in heterotrophic cultivation of Chlorella. In this work, we used soybean-processing wastewater (SW) and glucose to cultivate Chlorella and studied the effects of bacteria on algal growth, lipid production, and wastewater treatment. We also evaluated the interaction between bacteria and Chlorella. The results obtained from this study can serve as guidelines for the efficient regulation of bacterial contamination with good wastewater treatment efficiency. This study also lays a foundation for determining the ways in which the algal biomass and lipid yields can be increased.
Section snippets
Composition and pre-treatment of SW
SW was obtained from a local soybean-processing plant, and the soybean wastewater composition was: COD 8087 mg L−1, TN 189.9 mg L−1, 169.8 mg L−1, TP 45.6 mg L−1, Na+ 1387 mg L−1, Cu2+ 0.55 mg L−1, Zn2+ 6.91 mg L−1, Mg2+ 173.5 mg L−1, Mn2+ 0.21 mg L−1, Fe2+ 5.16 mg L−1, Ca2+ 51.47 mg L−1. The basal medium composition was: KNO3 1250 mg L−1, KH2PO4 1250 mg L−1, MgSO4·7H2O 1000 mg L−1, H3BO3 114.2 mg L−1, CaCl2·2H2O 111 mg L−1, FeSO4·7H2O 49.8 mg L−1, ZnSO4·7H2O 88.2 mg L−1, MnCl2·4H2O
Effects of bacterial contamination on the algal biomass
To investigate the influence of bacteria on algal growth and lipid content, we examined a pure algal culture in SWG medium (SWG + C), a contaminated algal culture in SWG medium (SWG + C/B), a pure algal culture in BG medium (BG + C), and a contaminated algal culture in BG medium (BG + C/B). Algae growth during the course of cultivation was expressed in terms of the biomass (Fig. 1).
As shown in the Fig. 1, the growth patterns of algae with and without contamination are similar in each medium.
Effect of contaminating bacteria and mediums on lipid content of C. pyrenoidosa
The major differences between the compositions of the SWG and BG mediums are as follows: ① the TP content in the BG medium is 7.5 times that in the SWG medium; ② the TN content in the SWG medium is slightly larger than that in the BG medium, and the form of the nitrogen source in the two mediums is different (ammonium is the main form in the SWG medium and nitrate is the main form in the BG medium); ③ the trace element contents of the two mediums are different; and ④ in addition to glucose,
Conclusion
Applying SW can enhance the accumulation of lipids in algal cells and increase the algal lipid yield. The co-existing bacteria in the culture enhanced the degradations of TN, TP, COD, and glucose. TN, TP, and COD were mainly reduced through algal cell synthesis under heterotrophic cultivation. However, the bacteria did not affect the algal biomass. The total lipid content and lipid yield were lowered in the presence of bacteria, although this effect may be indirect. It is likely that the
Acknowledgement
This study was supported by China National Science Fund (No. 20976139, 51138009), the National Key Technology R&D Program (2009BAC62B02) and Shanghai Youth Science and Technology “Phosphor” (Tracking) Project (11QH1402600).
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