High production costs have often obstructed the use of microalgae cultivation units in many hatcheries (Hemaiswarya et al. 2011) as these must be optimally designed to use light and nutrients efficiently in order to obtain high biomass densities (Ranglová et al. 2022). Furthermore, in every cultivation system, effective gas exchange (e.g. O2 degassing) must take place (Masojídek and Torzillo 2014). After meeting these requirements, the microalgae biomass will become more useful and profitable for hatcheries. Such requirements have been guaranteed by the AC-PBR with a short light path of only 46 mm, where high LI of up to 1,600 µmol photons m− 2 −1 can be used for cultivation of dense microalgae cultures (see detailed technical description of the AC-PBR in Ranglová et al. (2022)). When growing Vischeria stellata in the ACPBR, a maximum biomass density of 4.4 ± 0.05 g DW L− 1 and the specific growth rate of µ = 0.16 d− 1 was achieved (Ranglová et al. 2022).
In this work, we have achieved the same growth rate when cultivating Trachydiscus. Moreover, a significantly higher growth rate was determined for Monodopsis. This can be contributed to better photosynthetic performance in the late exponential phase of growth compared to the Trachydiscus culture. The highest growth rate of Monodopsis among the other strains used in this work was also observed during the cultivation in 400 mL glass bubble columns characterized by a short optical light path of 35 mm (Ranglová et al., unpublished data). A higher growth rate of Monoraphidium could be achieved by decreasing the growth temperature, as this strain is cold-tolerant and can grow at a wide temperature range of 1–20°C (Nedbalová et al. 2017). On the other hand, keeping a low temperature is complicated and uneconomical from a biotechnological point of view.
The volumetric productivity values measured in this work were even higher compared to the number achieved previously (Ranglová et al. 2022). Compared to the present trials, much lower daily volumetric productivity of Monodopsis of only 2.7 mg DW L− 1 d− 1 was found by Arora and Mishra (2021); the cause of which could be the very low light intensity which they used when culturing in Erlenmeyer flasks, as light is the most important factor for microalgae growth (Masojídek and Torzillo 2014). The light availability affects not only growth (biomass production) but also the biochemical composition such as the production and composition of FAs (Lukavský 2012; Van Wagenen et al. 2012; Hu et al. 2013; Ranglová 2020). In the process of photoacclimation or photoadaptation, microalgae can change cell composition. The content of PUFAs, including EPA, is inversely related to light intensity during culturing. Low light usually increases the production of PUFAs (Hu et al. 2013; Conceição et al. 2020), whereas high light results in an increase of saturated fatty acids (SFAs), this can be seen in the samples with the highest concentration of PUFAs determined at the end of the cultivation of Chlorella and Monoraphidium in this work, as they were the cultures with the lowest light available in proportion to the cultures of the highest density.
Other authors, Petkov and Garcia (2007) also studied the composition of FAs of Chlorella sp. in laboratory experiments using high density biomass up to 4.5 g DW L− 1. When compared to our data, the cells reflected the same photo-limitation showing the same concentration of PUFAs in the biomass at the end of the trial.
During the study of FA production in the biomass of Trachydiscus minutus, the highest concentration of EPA 42 ± 6% of wet weight (WW) was measured at the end of laboratory cultivation under 83 µmol photons m− 2 s− 1, while only 27 ± 4% of WW was quntified at the higher LI of 161 µmol photons m− 2 s− 1 (Cepák et al. 2014). The further increase in LI to 1,200 µmol photons m− 2 s − 1 available under outdoor cultivation caused a decrease in EPA of about 3 ± 1% (Iliev et al. 2010; Cepák et al. 2014). Even though it is the same strain, we cannot compare our results (2.2 ± 0.3% of DW of EPA) with these as they determined FA content in WW. The effect of various nitrogen sources in the cultivation medium on the FA composition of Trachydiscus biomass was studied by Cepák et al. (2014). Using KNO3, they reached MUFA and PUFA concentrations of 15.5 ± 1.7 and 40.4 ± 4.2% of TFA, respectively, which is very similar to our results (14.0 ± 0.3 of TFA for MUFAs and 48.6 ± 1.2% of TFA for PUFAs). Neither the use of alternative nitrogen sources such as urea, ammonium nitrate and ammonium carbonate, nor the increase in salinity of the cultivation medium have resulted in a higher concentration of PUFAs (e.g. EPA) in the harvested biomass compared to our data confirming the suitability of the AC-PBR to produce microalgae biomass with high (FA) quality.
By decreasing the cultivation temperature below the optimum level, the degree of unsaturation of lipids in membrane systems can be increased as the cells start to protect the photosynthetic apparatus from photo-inhibition at low temperatures (Hu et al. 2013; Ranglová 2020). This was obvious when Monoraphidium sp. was cultivated at 3°C. The content of PUFAs increased to 69.7% of TFA (Nedbalová et al. 2017) from 58.1% of TFA when cultivating at optimum temperature in this work. Here, again the problem is the maintenance of low-temperature cultivation.
Of the four microalgae strains used in this work, only two of them were the producers of EPA - Trachydiscus and Monodopsis as previously discussed. The EPA concentration in Monodopsis was almost similar and, in addition, the yield can be enhanced not only by the temperature and irradiance change, but also by the addition of auxins and cytokinins into the cultivation medium (Arora and Mishra 2019).
The use of Chlorella as an aquaculture feed has been discussed previously in several studies (Safi et al. 2013; Yanes-Roca et al. 2020; Carneiro et al. 2020; Chen et al. 2021). Only scarce information is available about the use of Monoraphidium (Fujii et al. 2010), and no data has been published on the use of Monodopsis and Trachydiscus in aquaculture. This work has shown the possibility of application of these two eustigmatophyceans, Monodopsis and Trachydiscus, as a promising aquaculture feed being rich sources of PUFA (namely EPA), which the growth rate is either comparable or even higher compared to that of Chlorella.