Improving biomass and carbohydrate production of microalgae in the rotating cultivation system on natural carriers

Preliminary substrate adhesion

The result of the initial stages of substrate colonization indicated that microalgae were less likely to adhere to the nylon’s smooth surface than to other textured surfaces. Lower attachment of Chlorella sorokiniana, Chlorella sp. and Picochlorum sp. as unicellular strains were observed on all surfaces. Desmonostoc sp. as filamentous strain attached more strongly to jute, cotton and positive charge yarn in batch cultures. In general, the degree of filamentous strain biofilm formation was found to follow the sequence of carrier jute > linen > cotton > positive charge yarn > nylon. Our findings agree with the findings of Lee et al., who found that the architecture of microalgal biofilms is species-dependent, and attachment material selection is influenced by microalgal/ cyanobacteria species (Lee et al. 2014).

Additionally, the positive correlations between initial cell attachment and surface texture and roughness observed in this study are similar to those reported previously (Sekar et al. 2004; Renner and Weibel 2011; Achinas et al. 2019). Our results have been confirmed by the result of the Christenson and Sims study that showed the nylon rope did not achieve any harvestable growth (Christenson and Sims 2012). In this study, rough and textural surfaces were shown to be more effective for biofilm development, which contradicted the findings of Gross et al., who discovered that Chlorella vulgaris (UTEX #265) adhered better to nylon as a smooth surface (Gross & Wen, 2014). Also, Sweat and Johnson found that benthic diatoms attach better to a smoother substrate, such as smooth acrylic panels (Sweat and Johnson 2013).

In brief, we found that the size and shape variation of algae cells is affected by initial attachment on the surface (Achinas et al. 2019). The higher contact between the cell and surface when the cell is larger than surface features (as is the case for filamentous cells Desmonostoc sp.) has a significant impact on the adhesion trend compared to the interaction between the cell and surface when the cell is smaller than the surface features (as in Chlorella sorokiniana, Chlorella sp. and Picochlorum sp. cell cases). These findings were previously reported by Katainen et al. (Katainen et al. 2006).

Compared to the smooth surface of nylon, the texture and roughness of jute aid in capturing filamentous cells, exhibiting the highest amount of cell attachment. A similar phenomenon has been observed in algal-produced river biofilms (Zhang et al. 2019).

More chances for cell colonization on rough surfaces are probably due to having more acceptor sites and a larger surface area for cell attachment (Sousa et al. 2009). Microorganism cells on the rougher surface, on the other hand, are protected from fluid shear forces (Achinas et al. 2019).

Some researchers examined the various types of natural and synthetic carriers for algae biofilm cultivation (Christenson and Sims 2012; Gross 2015). The presence of polar-OH groups in the glucose unit of cotton as natural polysaccharides provides multiple hydrogens for binding to microalgae/ cyanobacteria exopolysaccharides. Recent studies on cotton as natural support have indicated that cotton has the best algal attachment performance (Gross 2015). Similar to this study result, Christenson & Sims confirmed cellulose-based support such as jute and cotton achieved greater cell adherence than polyester and acrylic (Christenson and Sims 2012). Some research suggests that the addition of cotton fabric is an appropriate support candidate for algal attachment, and it’s also more applicable for commercially feasible algal attachment (Johnson and Wen 2010; Gross et al. 2013a). According to findings from the first screening, the filamentous strain Desmonostoc sp. was chosen from the studied strains; jute, cotton, and yarn were selected for the next phases.

Microalgal biofilm growth kinetics

To monitor microalgae growth, the biomass productivity and dry biomass yields of microalgae cultured on three biofilm system materials were evaluated. Our findings are in agreement with the fact that the initial attachment of algal cells to the fresh surface of the materials is crucial and can be time-intensive. Once the initial colonization occurs, the attachment of additional algal cells to the existing algal biofilm layer is relatively easy. This is also highlighted in Ozcan et al. study (Ozkan and Berberoglu 2013a).

The increasing biomass productivity from re-growing (2nd, 3rd and 4th harvesting) based on the hypotenuse of Schnurr et al. could be the result of lag phase elimination and symbiosis cells with each other due to the presence of residual EPS and biomass in previous growth respectively (Schnurr and Allen 2015).

Different surface properties of the substrates, such as roughness and hydrophobicity, have been reported to play roles in algal cell attachment (Fanesi et al. 2019). The influence roles of different surface properties of the substrates, such as roughness and hydrophobicity have been reported to play roles in algal cell attachment. Comparing the three carriers tested in this study, jute and cotton were the rougher materials, respectively, and, as a result, showed a higher adhesion of the cells in the same culture time. However, the roughness of the biofilms changes from rough to smothering over time based on the metabolic and nutritional transport rates of cells. An increase in the adhesion of microalgae can be explained by the use of meshed substratum as an alternative carrier material by increasing the surface roughness (Fanesi et al. 2019). The pore size of the cotton and jute mesh used in this study was smaller than the pore size of the positive charge yarn mesh, once again demonstrating the superior adhesion of the cotton and jute.

The physicochemical features of the carrier such as patterns and texture (surface roughness) due to the enhancement of cell attachment area play a critical role in the algal attachment (Cui et al. 2013; Schnurr and Allen 2015). Different surface topologies, for example, irregular surfaces with brush-like patterns in cotton and texture in jute carrier in this study, not only increase surface area attachment and allow more microalgae cells to anchor to the solid surface but also reduce the detrimental effects of shear stress and cell detachment (Schnurr and Allen 2015). Christenson et al. also observed a more significant attachment to cotton than to polypropylene, nylon, acrylic, and jute carrier materials (Christenson and Sims 2012). Algal cells attaching to the surface of the carrier material help to receive nutrients and light to the cells, whereas light limitation is a significant issue in suspended systems (Johnson and Wen 2010).

In previous studies, a harvesting strategy for minimizing the cost of microalgae/ cyanobacteria harvesting has been suggested (Borowitzka 1997; Griffiths and Harrison 2009; Bruno et al. 2012).

Attached algae culture systems applying the scraping method are easier to harvest and less expensive than suspended microalgae cultures, which typically use centrifugation as their harvesting process (Lee et al. 2014). In this study, the scraping of the total biomass was performed, with a period of approximately 7–10 days between scrapings. Some literature has noted that much longer scraping intervals disrupted the distribution of light and nutrients in cell algae in the lower biofilm layers. The once-weekly frequency of scraping was more proper for the harvesting of adhered biomass in previous biofilm studies (Fanesi et al. 2019).

Evaluation of material surface physico-chemical properties on algal attachmentContact angles and surface properties

The negative surface energy of hydrophobic jute and cotton surfaces coated with algae, as shown in Table 1, indicates that solid-solid contact (material surface and algae) is stronger than solid-water interaction. It agrees well with the hydrophobicity results determined by the water contact angle criterion. As expected, the contact angles of low-energy hydrophobic polymers like cotton and jute are usually larger than those of high-energy, more hydrophilic materials like positively charged yarns.

Contrary to our results, which showed a relationship between microalgae attachment and surface energy and contact angle Gross reported poor correlations between cell attachment and the surface energy and water contact angle (Gross 2015). The reason may be due to the different material types, culture conditions, and algal species used in our study.

In this study, the hydrophobicity of the surface after attaching the filamentous cyanobacterium cells to the surfaces could be attributed to the natural hydrophobicity of the cyanobacterium cells. Our result was in agreement with Hao et al. study, which also identified algae Chlorella vulgaris as planktonic microalgae and Anabaena vasriabilis as a filamentous cyanobacterium, which naturally exhibit hydrophilic and hydrophobic properties, respectively (Hao et al. 2017). In addition, it has been reported that while hydrophobic strains of cyanobacterium Microsystis grow into colonies, hydrophilic strains are unicellular, showing increased cell-to-cell attraction with an increase in the hydrophobicity of the cell surface (Yang et al. 2011). Therefore, the observation that microalgae/ cyanobacteria have a hydrophobic surface suggests that hydrophobicity may be one of the main mechanisms promoting the initial adhesion of algal cells (Ozkan and Berberoglu 2013b).

Characterization by ATR-FTIR spectroscopy

Cyanobacterium and carriers (positive charge yarn, jute, and cotton) display broad and intense absorption bands at 3000–3500 cm− 1corresponding to O-H and N-H stretching vibrations, suggesting that the supports interact with Desmonostoc sp. cells (cell-support) through hydrogen bonding. The water intake was shown to have a considerable influence on the CH stretching bands at 2935 and 2900 cm− 1 (Célino et al. 2014).

The C-O and C-O-C vibrations associated with polysaccharides on crude jute and jute were covered by the characteristic bands related to Desmonostoc sp. at 1036 cm− 1 and 1024 cm− 1, respectively. The C-O-C band attributed to carbohydrates in raw cotton and cotton was covered by vibrations of Desmonostoc sp. at 1174–1134 cm− 1, respectively (Duygu et al. 2012). The interaction between polysaccharides on the carrier surface and cells might influence the cell’s initial adhesion to jute and cotton fibres (Li 2022). The vibrations ascribed to carboxylate ions (1400–1398 cm− 1) were observable in the spectra of almost all the samples. In all samples, the bands at 1423–1357 cm− 1 can be contributed to CH2 bending vibrations of methyl (Cheah and Chan 2022). In addition, the absorption band at 2928 cm− 1 can be assigned to C-H (Desmonostoc sp.) and vibrations at about 2626 cm− 1 (raw jute and raw yarn) can be attributed to C = O of amide I and N-H stretch of amide-II, respectively(Duygu et al. 2012). The bands at 1585 and 1481 cm− 1 related to N-H bending and amide II C-N stretching vibrations of raw jute and jute were covered by cyanobacterium vibrations. The bands at about 1114 cm− 1 attributed to C-O-P and P-O-P in raw cotton and cotton were covered by cyanobacterium vibrations (Cheah and Chan 2022). In general, the results demonstrate that the interaction between jute, yarn, cotton and Desmonostoc sp. cells is independent of the surface’s chemical characteristics, and the interaction between the microalgae cells and the surfaces under investigation appears to be physical. The fact that after 90 days of exposure to cyanobacterium, the absorption spectra of jute, cotton, and the yarn did not change, and that none of the functional groups such as sulfate (-SO4), amino (—NH2), carboxyl (—COOH), hydroxyl (—OH), sulphydryl (—SH), phosphoryl (—PO3O2), etc. in microalgae were involved in the surface binding process, suggests that the biological activities associated with functional groups of biomass biofilm are preserved.

Biomass production and carbohydrate content of attached and suspended systems

In this study compared to the suspended culture system, the attached culture systems (cotton and jute) achieved 3- and 4-times higher biomass productivity. These results are in agreement with Johnson and Wen’s findings that the attached algae culture system produced more biomass than the suspended system (25.65 g DW m− 2 and 1.27 ± 0.12 g DW L− 1 for the attached and suspended culture biomass yield, respectively) (Johnson and Wen 2010). Several features of algal production were evaluated between the attached and suspended systems. The biomass collected from the attached culture system was paste-like and had a similar water content to the cell pellet centrifuged from the suspension culture system (Johnson and Wen 2010). This means that the attached algal growth system has a significant advantage in terms of biomass harvesting. During suspended development, a substantial amount of water must be evacuated from the algal cells. In addition to making harvesting easier, the attached algal growth system produced more biomass than the suspended system when compared to the lowest amount of biomass production yield in the biofilm system obtained in the yarn carrier (Table 2). The simulated model of suspension and biofilm culture employed in this work revealed that open culture consumes more water than biofilm culture (data not shown). Many studies confirm the fact that a key limiting factor for algal development in terms of economic feasibility and ecological sustainability is the fact that large-scale algae production in suspension systems requires a high amount of fresh water. This constraint is overcome by a considerable reduction in the total volume of culture in the biofilm system (Ozkan et al. 2012; Podola et al. 2016).

Among reports on immobilized-based microalgal systems, biomass production in jute support in this study was consistent with the result described by Hodges et al., who found 4.4 g m− 2 day− 1 biomass productivity for filamentous cyanobacterium in the RABR system for petroleum wastewater treatment (Hodges et al. 2017). In another study, Gross et al. used a revolving algal biofilm (RBC) system as immobilized cultivation for Chlorella vulgaris (UTEX #265) growth and obtained biomass productivity of 4.2 g m− 2 day− 1 on nylon and polypropylene, which was similar to the productivity observed for jute in this study (Gross et al. 2016). In addition, cotton was also investigated by Christenson and Sims as a substrate in a bench-scale rotating bioreactor of mixed culture (algal-bacterial) and 2.5 g m− 2 day− 1 productivity was recorded (Christenson and Sims 2012), while in this study, the productivity of 3.6 g m− 2 day− 1on cotton was obtained. Also, Murphy and Berberoglu reported productivities of 2.8 g m− 2 .day− 1 for Anabaena variabilis as filamentous cyanobacterium in porous substrate bioreactor (PSBR) cultivation (Murphy and Berberoglu 2014). The Rhizoclonium hieroglyphicum filamentous green alga was successfully grown on raw and digested dairy effluents in the attachment-based algal turf scrubber (ATS) system, resulting in biomass production of 5.3 g DW m− 2 day− 1 and 4.9 g DW m− 2 day− 1 respectively (Mulbry et al. 2008). For Nitzschia palea and Scenedesmus obliquus, biomass productivity in the algal biofilm culture system was determined by Schnurr et al. at 2.8 and 2.1 g m − 2 d − 1 during the nutrient shortage, respectively (Schnurr et al. 2013).

Microalgae biofilm formed on cotton showed the highest sugar content, followed by jute. In two-way communication, microbial strains, growth phase, external condition, and supporting material are affected by EPS production and their composition. On the other hand, the amount of polysaccharide produced by microbial strains plays a vital role in biofilm formation on a substrate (Sheng et al. 2010; Palma et al. 2017; Jin et al. 2018). As depicted in Table 2, the present study was in line with the previous study’s beliefs that suggested significant relationships between capsular polysaccharides and biomass in microalgae phototrophic biofilm (Bellinger et al. 2005; Pippo et al. 2009).

Two stages are suggested for microorganism adhesion. The first step is reversible attachment, which occurs by macroscopic surface properties, while the second step is irreversible attachment caused by microscopic molecular interaction such as EPS production (Busscher and Weerkamp 1987). It appears that microorganisms with weak initial adhesion to the substrate do not secrete additional EPS. Various amounts of EPS are generated on different surfaces (Becker 1996). In addition to their adhesion and cell-protective roles, EPS have other functions including nutrient trapping, detoxification and microcolony formation (Becker 1996). As a result, cotton and jute proved to be more effective at capturing nutrients, creating an initial microcolony, and developing biofilm with a higher EPS content during the course of this research. It is also important to note that physicochemical properties, microalgae EPS secretion, and the reduction of the free energy in a flowing system, as well as the decrease in interfacial tension between surface and cell attachment, are all influencing biological factors that alter surface characteristics and minimize free energy in a flowing system. It is also important to note that physicochemical properties, microalgae and cyanobacterium EPS secretion, and the reduction of the free energy in a flowing system, as well as the decrease in interfacial tension between surface and cell attachment, are all influencing biological factors (Simões et al. 2008; Barros et al. 2018). Thus, high EPS content increases the hydrophobic interaction and cell adhesion to surfaces (Moghaddam et al. 2018).

Biofilms can be strengthened by rougher surfaces, such as those found in jute and cotton, which increase the production of EPS and bind cells together (Schnurr and Allen 2015). Compared to our previous studies, HPLC data (data not shown) confirmed that there was no difference in the monosaccharide composition of EPS, which was extracted from single-cell and filamentous strains and was composed mainly of structural units of glucose, glucosamine and sucrose (Mousavian et al. 2022).

The widespread application of microalgal biofilms in wastewater treatment, biomass and high-value metabolites production is prevalent today. Treatment of wastewater is a significant application of microalgal biofilm following biodiesel generation (Miranda et al. 2017). It would be ideal to use microalgal biofilm biomass as fertilizer if wastewater did not contain heavy metals (Patwardhan et al. 2022).

In general, the formation of microalgal biofilms seems to be species-dependent and affected by physico-chemical surface features. This study examined a rotating biofilm system for algal culture using different supporting materials. Filamentous strain cyanobacterium and jute and cotton have been identified as appropriate algae strains and effective supporting materials, respectively, for the establishment of biofilm algal culture. In comparison to the suspended growth culture operated under similar conditions, the biofilm system achieved greater biomass production. In this study, the biomass productivity of the rotating cylinders for jute and cotton was 4.76 and 3.61 g m− 2 day − 1, and the biomass production was 66.74 and 44.41 g m− 2, respectively. The production of microalgal and cyanobacterial biomass can be facilitated by biofilm-based microalgal systems. This approach applies to a wide variety of low-cost and long-lasting supporting materials, which has attracted more attention from researchers and industrialists.

留言 (0)

沒有登入
gif