This study describes the effects of five different irradiances on the growth and lipid content of Amphidinium carterae maintained in culture conditions. This work obtained changes in the growth rate of A. carterae by the effect of irradiance, with the highest values from 150 to 200 µmol photon m−2 s−1, and the lowest values at 50 µmol photon m−2 s−1.
When A. carterae (Woods Hole Amphi 1) was maintained at 15 °C under different irradiances (2 to 150 µmol photon m−2 s−1), found that at low irradiances grow well (15 µmol photon m−2 s−1) and has photoinhibition at irradiances of 150 µmol photon m−2 s−1 [34]. When A. carterae (Amphi 1) was grown in “f/2” media at 19 °C, and from 10 to 50 µmol photon m−2 s−1 the growth rate was linearly proportional to irradiance, and the maximum growth was at 80 µmol photon m−2 s−1 in batch cultures [35]. The growth of A. carterae (Amphi 1) was higher under continuous light (24:0), with regards to the values obtained at 12:12 in “f/2” media, shaking at 100 rpm, at 20 °C with 80 µmol photon m−2 s−1 [36].
The growth rate obtained in this study for A. carterae at high irradiances (100 to 250 µmol photon m−2 s−1) was higher (0.59 to 0.67 divisions day−1), with regards to the growth rate values (0.15 to 0.55 divisions day−1) reported to A. carterae isolated from macroalgae and grow in low irradiances (35 to 70 µmol photon m−2 s−1) [37]. Also, the growth rates obtained in this study were higher than the values measured for the cultures of A. cartaerae (CCMP1314) maintained at 20 °C with 100 µmol photon m−2 s−1 supply 14:10 hrs light: dark under P-repleted (0.52 divisions day−1) and P-deprived (0.27 divisions day−1) [38]. For A. carterae cultures maintained with irradiances of 50 to 750 µmol photon m−2 s−1, the higher growth rate was at 300 µmol photon m−2 s−1 during the first 5 days of culture [39]. The growth rate values obtained for A. carterae in this work were different from those mentioned by other authors, possibly for the differences from the origin of the strain and the different culture conditions used.
In this work, it was obtained as a general trend, that growth rate and cell concentration increased proportionally to irradiance level with higher values among 150 to 250 µmol photon m−2 s−1. Over this limit of irradiance level, the rise of light intensity did not increase the growth rates, suggesting that saturation photosynthesis was achieved. This pattern has also been described in Scenedesmus obliquus cultures, when growing at different irradiances 200 to 1000 µmol photon m−2 s−1, and found the higher growth at 150 µmol photon m−2 s−1 [22]. Photoacclimation probably supports the tolerance to high irradiances and tolerates a wide range of light intensities for different phytoplankton species. In the benthic dinoflagellate Gambierdiscus spp. photoacclimation permits that grow at high irradiances [40]. Photoacclimation of A. carterae in batch cultures allowed at high irradiances (300 µmol photon m−2 s−1) the maximum growth rate of 0.65 divisions day−1 [16], which those growth rates are similar to those obtained in this study.
Light is an important energy source and an essential factor in photosynthesis [19]. If the light intensity is too low in phytoplankton cultures, logarithmic growth will not prevail [41]. The results obtained with A. carterae showed that at an irradiance of 50 µmol photon m−2 s−1 grows lower than the different irradiances used; thus, the low irradiance level used was insufficient to promote a high growth rate. A. carterae cultured in 12:12 LED-lighted raceway photobioreactor had high growth at irradiances between 100 to 289 µmol photon m−2 s−1, reaching the stationary growth phase at nine days of culture [16]. The results obtained with A. carterae showed that the logarithmic growth phase was achieved at least for eight days of culture and that irradiances below 100 µmol photon m−2 s−1 are limiting for cell growth.
In cultures of Scedenesdum obliquus, when the cells are stressed, they decrease or stop reproduction and increase cellular weight [22]. This pattern was observed on the cultures of A. carterae maintained at an irradiance of 50 µmol photon m−2 s−1; in this irradiance, the cell decreases their growth, and the cell components used to reproduce are storage and consequently increase cell weight; under unfavorable environmental or stress conditions, many algae increase and accumulate neutral lipids which serve primarily as a storage form of carbon and energy [42]. In this work, the TDW, ODW, and IDW were not modified by irradiance at values between 100 to 250 µmol photon m−2 s−1; this trend was due to the cells maintaining a similar growth under these irradiance values.
The lipid content of A. carterae had significant modifications by the effect of light irradiances and showed the highest values (534 pg cell−1) with the low irradiance. Previous reports mentioned that A. carterae could produce lipids content from 7.2% and 9.2% of dry weight [43]; in this study, lipids represent between 3.6–14% of dry weight (52 to 534 pg cell−1), the lipid content of some marine diatoms can change from 22.7% when grows under normal conditions to 44.6% when was maintained under stress conditions [42]. The lipid content of A. carterae under the five irradiances in this work was higher than the values obtained to other studies (20 to 30 pg cell−1) with the same specie but obtained from Woods Hole (Amphi 1) culture with “f/2” media, 20 °C and 50 µmol photon m−2 s−1 [35]. Microalgae like Scenedesmus obliquus and Dunaliella salina present the highest lipids content in high irradiances [22, 44]. However, Nanochloropsis sp. produce a high lipid content at low irradiances [41]. The effect of the light irradiance on the proximate composition is species-specific, and their modifications are related to each strain characteristic. In this work, A. carterae decreases the growth rate at the lowest irradiance, consequently increasing the storage products as lipids.
The neutral lipids in the mainly form of tryacylglycerols can be converted to fatty acid methyl esters and used as biofuel [45]. Lipids serves in two ways; as energy reserves and structural components of the membranes of the cell [46], tryacylglycerols typically provide a storage form of carbon to convert in energy that enables microalgae to endure adverse environmental conditions [47]. The diatoms Phaeodactylum tricornutum and Isochrysis galbana growing in different irradiance (50, 300, 600 µmol photon m−2 s−1) increase the content of tryacylglycerols when grows in irradiances between 300 and 600 µmol photon m−2 s−1 [48]. However, in the same species diatom P. tricornutum when grows with nitrogen starvation, the highest tryacylglycerols was found in irradiances of 60 µmol photon m−2 s−1 [49]. In this study, A. carterae had the highest lipid content in the lowest irradiance used (50 µmol photon m−2 s−1). Maltsev et al. [20], describes that the optimal light intensity for the highest lipid content and productivity in not the same for different taxa of microalgae.
Other environmental parameters can influence the growth and proximate composition of different microalgae strains, e.g., Porphyridium cruentum maintained with low irradiance (50 µmol photon m−2 s−1) and ammonium as nitrogen source, increase the lipid content [50]. However, Metsoviti et al. [51] mentioned that it is difficult to generalize the specific influence of the environmental factors on growth and biochemical composition in microalgae species due to differences in their metabolism. The results obtained in this work show that the lowest irradiances used on the cultures of A. carterae induce a low growth rate, increasing TDW, ODW, and lipid content by cell.
Proteins content for A. carterae in this work increases proportionally to irradiance level rises. This trend previously described was also observed in Scenedesmus obliquus [22]. Protein synthesis can be stimulated by factors like the nitrogen source, temperature, and light [51]. The protein content of A. carterae obtained in this study under the five irradiances (62 to 87 pg cell−1), was similar to the values measured to the same specie but obtained from Woods Hole (Amphi 1) (100 to 220 pg cell−1) when was maintained with “f/2” media, 20 °C and 50 µmol photon m−2 s−1 [35]. Carbohydrate content obtained in this work had slight variation by the effect of the five irradiances used. The carbohydrate contents of A. carterae under the five irradiances (22 to 30 pg cell−1) were lower than the values obtained to other studies (40 to 90 pg cell−1), with the same specie but obtained from Woods Hole (Amphi 1) and culture with “f/2” media at 20 °C with 50 µmol photon m−2 s−1 [35].
Chlorophyll a content had an inverse trend concerning the irradiance level used to A. carterae cultures in this work; this pattern is typical in all photosynthetic organisms [18]. One general response of microalgae to increase the photon flux densities is reducing cellular pigment content, regulating the light-harvesting antennas size, and modifying the cell size [52]. The chlorophyll a content obtained in this study (3.1 to 4.02 pg cell−1) was higher than the previously reported for A. carterae [15, 38, 53]. We considered that these differences are due to the characteristics of the strain used and distinct culture conditions.
Under high light irradiance, the excess energy absorbed can cause a destructive effect in the photosynthetic apparatus; carotenoids and their antioxidant capacity protect the photosystem from damage [54]. The carotenoid content was not modified by the effect of the irradiance in this work; thus, the light irradiance used is not too high to cause photodamage in the photosynthetic apparatus of A. carterae. The carotenoid content measured in this work (1.5 to 1.84 pg cell−1) was similar to the values mentioned to Amphidinium carterae by other authors (2 to 5 pg cell−1) [15, 53].
For several microalgae groups, Fv/Fm values were described with mean values of 0.65 and indicate that the cells are without stress, when decreasing the values of Fv/Fm indicate stress [55]. The Fv/Fm ratio obtained on this work (0.21 to 0.38) were similar to the values previously mentioned to A. carterae (0.30 to 0.40) [37]; however, the Fv/Fm values were lower than the obtained by Li et al. [38] (0.30 to 0.70) and by Molina-Miras et al. [15] (0.62). The lower Fv/Fm values obtained in the irradiance of 50 µmol photon m−2 s−1 indicates that A. carterae is under stress condition with low photosynthetic efficiency. Moreover, the high Fv/Fm ratio obtained in the different light irradiances used shows that photoinhibition is not produced and explains the photoacclimation of A. carterae to high irradiances. The irradiance of saturation (Ik) observed in this work indicates that A. carterae can tolerate high irradiances.