Performance of different microalgae-based technologies in nutrient removal and biogas upgrading in response to various GR24 concentrations
Graphical abstract
Introduction
Strigolactone (SL) is a terpene lactone presumed to be a carotenoid isolated from root exudates, which can irritate root parasitic plants sprout (Ma et al., 2017). This group of phytohormones plays a vital role in the regulation of photosynthesis in plants (Tsuchiya and McCourt, 2012). GR24 is one kind of synthesized SL and is involved in the physiological response of algal cells (Tai et al., 2017). As an effective regulator of pressure changes, exogenous GR24 can increase removal of COD and enhance plant cells’ tolerance to the aquatic environment (Kramna et al., 2019). Recently, the research of GR24 has received a great deal of interest. Most researches focus on the regulation of development of plants and germination of parasitical weeds by the induction of GR24. Song et al. (2020) explored the presence of GR24 in the green lineage and tested the concentration of some mosses, liverworts, charophytes, as well as green algae, finding that the early appearance of GR24 in streptophyte lineage was acted as a means of controlling rhizoid elongation. GR24 also play a key role in host recognition of parasitic plants such as genera Striga, Orobanche, and Phelipanche (Wang et al., 2020). GR24 could promote the host detection in convergent evolution of obligate parasitic plants, as well as play an essential role in regulating algae production (Kyslík and Prokop, 2010; Shen et al., 2020). Due to the special ability to produce oils, microalgae are recognized as a significant source of renewable biodiesel (Pal et al., 2019). And the presence of SL could improve the liquid content and liquid productivity of monoraphidium sp. (Song et al., 2019). However, there are few reports on the effects of GR24 on microalgae biomass and photosynthesis, as well as simultaneous purification of anaerobic fermentation liquid and biogas.
Microalgae-based technology such as mixed cultivation of microalgae with fungus or activated sludge was developed in recent years for wastewater treatment and biogas upgrading (Leong et al., 2018; Xu et al., 2017). Although pollutants in wastewater and CO2 could be removed simultaneously by microalgae monoculture, microalgae-based technology shows great advantages, such as greater economic viability, higher removal efficiencies, comparing with wastewater treatment using microalgae (Mohd-Sahib et al., 2017; Rosli et al., 2019; Zhao et al., 2015). There have some reports on the improvement in removing nitrogen and phosphorous from wastewater using algae-bacteria/fungus-algae co-cultivation (Leong et al., 2019; Muradov et al., 2015). Numerous studies have proven that during the co-cultivation process, the interaction between microalgae and fungi or bacteria has a positive impact on the growth of microalgae and fungi or bacteria. In the microalgae-fungi symbiosis system, fungi can effectively degrade large particles of organic pollutants, provide CO2 for photosynthesis of microalgae, store water for the microalgae as well as provide a larger receiving surface for the minerals needed by the microalgae, which are more conducive to remove nutrients in the biogas slurry and capture CO2 by microalgae (Zhao et al., 2019). In the microalgae-bacteria symbiosis system, microalgae and bacteria meet their needs for the substances needed for growth by changing their own metabolism, and promote the rapid growth of each other. In terms of metabolism, the extracellular metabolites released during the growth of microalgae, such as amino acids, carbohydrates, and fats, can be used by bacteria (Ramanan et al., 2016). Meanwhile, bacteria secrete vitamins, quorum sensing signal molecules, and growth hormones such as indole-3-acetic acid, which could promote the rapid growth of microalgae (Liu et al., 2017; Natrah et al., 2014). The content of CO2 in raw biogas is more than 40%, which reduces its calorific value and enhances its energy demand for application (Ran et al., 2020). Therefore, removal of CO2 is necessary for biogas upgrading. Some upgrading methods such as physical or chemical absorption, membrane separation, as well as cryogenic separation have been reported on biogas upgrading (Xu et al., 2020). However, if CO2 from the raw biogas was released into the atmosphere directly, it will lead to greenhouse gas emission (Porté et al., 2019). The key to improve the efficiency of the algae-based biogas purification system is to enhance its CO2 fixation capacity, and this is determined by the activity of carbonic anhydrase in microalgae cells. Some studies have shown that GR24 can effectively increase the activity of alginate carbonic anhydrase and improve the fixation efficiency of CO2 (Kamel et al., 2017; Sytar et al., 2019).
In this study, biogas slurry acted as the nutrient medium was used in three systems of microalgae-based technologies: individual microalgae, a microalgae-fungal association, and microalgae-activated sludge. The effect of GR24 concentration (0, 10−11, 10−9, and 10−7 M) on microalgae biomass, chlorophyll a (CHL-a) content, specific rate of growth as well as carbonic anhydrase was explored. The most suitable GR24 concentration was analyzed to improve the purification efficiency of nutrients and CO2 by microalgae-based technologies.
Section snippets
Mono-culture of Chlorella vulgaris
Microalgae Chlorella vulgaris (FACHB-8) was gotten from Freshwater Algae Culture Collection located at the Institute of Hydrobiology, Chinese Academy of Sciences. The microalgae cultivated in BG11 liquid medium were placed in a photobioreactor with constant LED light of 200 μmol m−2 s−1, temperature of 25 ± 0.5 °C, light/dark cycle of 12 h/12h as well as cultivation time of 7 days.
Cultivation of Ganoderma lucidum
Fungi Ganoderma lucidum (serial number: 5.765) were provided by China General Microbiological Culture Collection
CHL-a content and growth rate
On days 3, 7, and 10, the CHL-a concentration of the three systems mono-culture of Chlorella vulgaris, co-culture of Chlorella vulgaris with Ganoderma lucidum, and co-culture of Chlorella vulgaris with activated sludge, were measured and the results are shown in Fig. 1. The results show that CHL-a contents were the lowest when the concentration of GR24 was 0 M. When the concentration of GR24 was 10−7 M, CHL-a concentration slightly increased, and when the GR24 concentration reduced further, to
Discussion
In the process of microalgae growth, CHL-a is a vital key in the photosynthesis of Chlorella vulgaris. GR24 can be used as inducing factor to make host plants, fungi and bacteria form a symbiotic relationship (Luisa et al.). The optimal concentration of GR24 to obtain high CHL-a is 10−9 M, which may be because that GR24 is a highly active phytohormone, and its effect on algal cells is small, but concentrations that are too high or too low will inhibit the formation of CHL-a (Lu et al., 2019;
Conclusions
The study investigated the effect of GR24 concentration on microalgae-based cultivation system for the purification of nutrients in biogas slurry and the upgrading of biogas. The major outcomes of the study were as follows: (1) The inclusion of GR24 in cultivation systems was beneficial for removal of pollutants in biogas slurry and upgrading of biogas; (2) Co-cultivation of Chlorella vulgaris with activated sludge significantly enhanced the removal efficiency of nitrogen, phosphorus nutrients
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (grant number 31971514 and 31670511), and the Zhejiang Provincial Natural Science Foundation (grant number LY16C030003 and LQ17E080013).
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Prof. Yongjun Zhao will handle Correspondence at all stages of refereeing and publication, also post-publication