1.
Introduction
Microalgae like Chlorella vulgaris are not only edible, but can also be used for diverse applications such as pharmaceutics, cosmetic, dyes and even biofuel, making them interesting for research and industry since decades [1]–[6]. This is why large-scale commercial cultivation of these and other microalgae began already in the 1960s and is still going on [7]–[10].
C. vulgaris is a unicellular microalga growing in fresh water with a high protein content of more than 55% of its dry weight, anti-cancer and immune-modulating properties as well as protection against age-related diseases [3],[11]–[15]. Typically, these microalgae are cultivated under photoautotrophic conditions in open ponds or closed bioreactors [16]. Growth in photobioreactors, however, is economically not efficient since only a part of the applied light is used, and production costs are high. Another important factor is the harvesting and dewatering process if algae are grown in relatively strongly diluted suspensions, which is thus investigated by many research groups.
The most straightforward way to make harvesting easier is letting the microalgae grow adhering to a substrate, i.e. as a biofilm [17]. Some studies reported on the general adhesion properties of C. vulgaris on rigid substrates [18]–[20]. Gao et al. investigated a membrane photobioreactor with solid carriers for C. vulgaris growth in adhesion, with a relatively complicated harvesting process [21]. Melo et al. built a rotating flat plate photobioreactor and found good adhesion to different polymer flat planes [22]. Shen et al. investigated a porous substrate to enable attached C. vulgaris culture in a photobioreactor [23]. Here, the substrate was a not nearer defined canvas, i.e. a woven textile fabric of unclear material, areal weight, thickness etc. In an earlier study, the group used also a cotton rope and Spandex, finding the canvas to be the optimum substrate [24]. In that study, rotating drum and plate biofilm reactors were used of which only the bottom was submerged in the medium reservoir, thus making the fluid retaining capacity of these substrates highly important.
Previous studies of this group found, on the one hand, good adhesion and growth of the green microalga Chlamydomonas reinhardtii on nanofiber mats and macroscopic nonwovens [25], and on the other hand, a positive influence of knitted fabrics as possible substrates for growth of higher plants [26],[27]. No investigation was found in the literature about the growth of C. vulgaris on different textile substrates in common flat-bed reactors where only textile material and structure can influence the microalgae growth. Here for the first time an experimental investigation of different textile materials is shown, finding that especially an open-pore jute woven fabric as well as similarly hairy knitted fabrics resulted in significantly higher adhesion than flatter, denser fabrics, thus suggesting new experiments with more textile fabrics beyond the canvas used in [23] and [24].
The paper is structured as follows: After describing the diverse textile substrates and the algae cultivation, results of algae concentration in suspension and on substrates are reported. At the end, an estimation is given which textile fabrics should be preferred.
2.
Materials and method
2.1. Textile fabrics
The textile fabrics under examination are presented in Table 1, images of the surfaces are shown in Figure 1. The fabrics were chosen to give a broad overview of possible fabric constructions (woven/knitted), man-made and natural fibers, thicknesses, areal weights, open (Jute) and dense fabrics.
2.2. Microalgae cultivation
Microalgae C. vulgaris (Interaquaristi.de Shop, Biedenkopf-Breidenstein, Germany) were cultivated on the textile fabrics, cut in samples of 5 cm × 5 cm (n = 3) and cultured in petri dishes with TAP (tris-acetate-phosphate) medium [28], in which they were found to grow well in pre-experiments. Colonization was started with 40 ml medium including approx. 10,000 algae cells/ml, i.e. with approx. 400,000 algae cells per petri dish.
The petri dishes were illuminated with warm-white LED lights (CRI930 linear flex band, purchased from ISOLED, Schwoich/Austria) with an average intensity of 14–15 W/m² during a photoperiod of 10 h/d. An impression of some petri dishes with textile fabrics after 6 d is given in Figure 2.
2.3. Investigation
After cultivation for 7 d, the textile fabrics were carefully taken out of the petri dishes, drained above the respective dish and afterwards put into a Greiner tube where 20 ml of distilled water were added. The adhered algae were separated from the textile fabrics using an ultrasonic device (UP200Ht, hielscher, Teltow, Germany). This method was chosen due to its high reliability, enabling completely detaching the microalgae from the textiles, which is necessary for the recent basic investigation, while less important in large-scale cultivation.
Algae suspensions were counted in an Improved Neubauer ruled hemocytometer (Paul Marienfeld GmbH & Co. KG, Lauda Königshofen, Germany) (undiluted, i.e. with a concentration of approx. 10,000 algae cells/ml, and in dilutions of 1:1, 1:2, 1:3, 1:5 and 1:10). The same algae concentrations were investigated in a UV/Vis spectrophotometer Genesys 10S (Thermo Fisher Scientific, Waltham, MA, USA), using a wavelength of 750 nm [29],[30], to evaluate the absorbance. The correlation of both values was used to develop a straight calibration line. To investigate the concentration of algae in the suspension and in the supernatant after ultrasonic treatment, both methods were applied, thus improving the accuracy of the results.
3.
Results
The concentration of algae measured in the supernatant is shown in Figure 3. Generally, comparing these values with the seeding value of 400,000 algae cells per petri dish shows a strong increase of the concentration of algae per petri dish, approximately by a factor of 55–130, depending on the textile fabric in the respective petri dish. This shows clearly that none of the textile substrates inhibited algae proliferation. Since here only parts of the overall algae in the petri dishes are measured, a more quantitative evaluation will be given later.
Comparing the impact of the different textile materials, Jute shows the highest concentration of algae in suspension; however, most differences are not significant, as visible by the error bars.
Next, Figure 4 depicts the algae adhered on the fabric. Opposite to the concentration of algae in suspension, here a clear difference is visible between Jute and most other fabrics. While the Jute fabric already showed a slightly advantageous growth of the algae in the suspension around the fabric, here up to one order of magnitude more algae can be found on the Jute fabric than on most others, followed by Spacer 1 and CW 2. This finding clearly shows that a hairy, open-pore textile like the jute woven fabric or the knitted fabrics Spacer 1 and CW 2 are not only supportive for growth of macroalgae, as investigated in a previous study [31], but also for the microalgae under examination here.
Finally, Figure 5 depicts the overall concentration of algae after 7 d. Here, the concentration of microalgae grown in the petri dishes with Jute fabrics is approx. twice as large as the concentration of microalgae found in most other samples, clearly underlining the possibilities of using simple and inexpensive jute woven fabrics to support microalgae growth and harvesting.
Quantitatively, the overall concentration of algae per petri dish increased by a factor between (67 ± 19) for cotton and (179 ± 18) for the jute woven fabric during one week of cultivation. In comparison with typical literature values of C. vulgaris growth in suspension, showing an increase by approx. a factor of 2–6 for 7 d of cultivation [32]–[34], these values are significantly higher. Growing C. vulgaris as a biofilm on rigid substrates was reported to result in a biomass increase by approx. a factor of 30 [35], which is still lower than the values found here. This quantitative comparison suggests further investigations of jute and other hairy fabrics as possible substrates for better growth and easier harvesting of C. vulgaris and other green microalgae.
4.
Conclusion and outlook
For the first time, microalgae C. vulgaris were cultivated on different textile fabrics. For a woven fabric from jute, approximately twice the concentration of algae was found at the end of the cultivation than in the other samples, with approximately one order of magnitude more algae adhered on the jute fabric than on most others.
These first results clearly show the great potential of jute and other hairy, open-pore fabric for microalgae growth and harvesting, thus offering a solution for the problematic cultivation and harvesting of green microalgae. Nevertheless, more research is necessary to investigate how the adhesion on the fabric can further be increased, to support the harvesting process, e.g. by using more fabric per fluid volume. The time-dependent daily growth rate is also an important factor which will be examined in the near future, concentrating on the most promising substrates. Alternatively, it has to be investigated whether steady microalgae harvesting from a continuous culture, harvested daily or in similar short time intervals, is enabled by the recent percentage of adhered algae.
DownLoad:
