Effects of light intensity and quality of three kinds of LED monochromatic lights (blue, green, and red) on the growth of Skeletonema costatum are investigated in batch culture conditions. Seven light intensities (20, 30, 40, 45, 50, 60 and 80 μmol·m-2·s-1) are used to evaluate the specific growth rate, spectrum absorption coefficient and saturated light intensity of LED monochromatic light. Results show that the growth rates of Skeletonema costatum increase with the enhanced light intensity; however, the light level beyond the saturation light intensity inhibited the growth of Skeletonema costatum. Compared with red and green light, the growth rate of Skeletonema costatum under blue light is higher within saturated light intensity, and saturated light intensity of LED monochromatic light is lower under blue light and higher under green light. It is concluded that under different monochromatic light, the saturated light intensity decreases and the growth rate increases with the increasing of spectrum absorption coefficient.
Under certain ecological conditions, some marine microalgae may grow rapidly in a short time and lead to discoloration of waterbody or water blooms, which refer to harmful algal blooms. Such over-proliferated blooms are harmful to other organisms and other organisms can be affected directly or indirectly [
Many studies on the effects of light intensity, temperature and salinity have been reported [10-14]. However, in terms of light intensity, most studies focused on the effects of fluorescent or the sun-light on the growth of red tide species [10,11]. Studies of the influence of light on the Skeletonema costatum have been reported by Baiye Sun with filed culture experiments and model calculations [
In this work, we use the monochromatic light-emitting diode (LED) as the light source and study the effects of three monochromatic lights on the growth of Skeletonema costatum with the same photosynthetic available quantum flux density. Based on the study on the effects of different wavelength of monochromatic light and different light intensities on the growth of Skeletonema costatum, we obtain the saturated light intensities of different monochromatic lights, and further provide the basis of analysis of the mechanism of the red tides occurrence.
The strain of Skeletonema costatum is obtained from the Institute of Fisheries College, Ocean University of China. Seawater used in this work is sterilized artificial seawater that is filtered through a 0.45 μm filter before used for culture medium preparation. Algal cells in the exponenttial growth phase are used, which are inoculated into 300 mL Erlenmeyer flasks containing 100 mL fresh f/2 enriched seawater. Stock cultures are maintained in sterilized Erlenmeyer flasks and cultivated in a plant growth chamber under the common background conditions of 21˚C with a 12:12 light:dark photoperiod. The lights used for the culture are provided by monochromatic LED with the center wavelength of 456 nm, 512 nm and 656 nm, respectively. The light intensity can be adjusted from 20 μmol·m–2·s–1 to 80 μmol·m–2·s–1 by changing the distance between the light source and vessel. The spectra of light sources are measured by PR-650 spectrophotometer/spectrocolorimeter, as shown in
For researching the growth condition of Skeletonema costatum, a 50 μL algal sample is fetched from each flask everyday, and then supported by a glass slide. The numbers of algal cell are counted under optical microscope, and cell number countings are repeated for at least three times. The specific growth rate μ was calculated during the exponential growth period by [18,19]
where N1 and N2 are cell numbers on days t1 and t2, respectively.
Firstly, we study the effect of blue light on the growth rate of Skeletonema costatum. Skeletonema costatum strain culture is maintained in conditions as mentioned above until the end of the exponential phase. The typical light intensity used are 20, 30, 40, 45, 50, 60 and 80 μmol·m–2·s–1. For all light intensities, the Skeletonema costatum has an exponential growth phase in seven days, as is shown in
growth of Skeletonema costatum is significantly different, especially after two days. Based on the cell number, we can calculate the growth rates under different light intensities using Equation (1). As is shown in
growth rates increase with the increasing light intensity from 20 to 40 μmol·m–2·s–1, the maximum growth rate is observed at 40 μmol·m–2·s–1 (
When the Skeletonema costatum are cultured under green light, the effect of the wavelength on the growth of the microalgae is not significant in the first two days (
By the same method, we study the cell numbers and growth rates of Skeletonema costatum under red light, as
is shown in Figures 4(a) and (b). The growth rates of Skeletonema costatum increase with the increasing light intensity from 20 to 45 μmol·m–2·s–1. When exceeds this light intensity range, the growth rates gradually decrease. The maximum and minimum growth rates of Skeletonema costatum are 0.1953 at 20 μmol·m–2·s–1 and 0.3212 at 45 μmol·m–2·s–1, respectively. The saturated light intensity of Skeletonema costatum under red light is about 45 μmol·m–2·s–1.
Results above show that when cells are exposed to blue, green or red light, the growth conditions of Skeletonema costatum are greatly influence by different light wavelength. For three kinds of monochromatic light, the growth rates of Skeletonema costatum are significantly different. The growth rate under blue light is larger than under green and red light until it reaches saturation light intensity. In blue light condition, the saturation light intensity is about 40 μmol·m–2·s–1, which below the value of saturation light intensity of the green (50 μmol·m–2·s–1) and red light (45 μmol·m–2·s–1).
Spectrum absorption coefficient represents the quantum efficiency of the light photosynthetic effective quantum flux absorbed by microalgae, and it further reflects the efficiency of the light to promote the growth of microalgae. In this work, we estimated the relationship between the growth rates and spectrum absorption coefficient using the equation by [
where λ is wavelength, ∆λ is wavelength interval, is relative spectrum power density, and it is provided by the spectra of light sources. OD (λ) is absorbance, and it can be obtained from the absorption spectrum of microalgae, as is presented in
Based on Equation (2), it is now possible to estimate the spectrum absorption coefficient of Skeletonema costatum in different lights. Spectrum absorption coefficient of blue light is 0.0913 (
In conclusion, effects of light intensity and quality of three kinds of LED monochromatic light (blue, green, and red) on the growth of Skeletonema costatum are investigated. Results indicate that Skeletonema costatum prefer blue and red light to green light and the saturated light intensity is significantly different under three monochromatic
lights. Based on the theory of quantum energy level transition, we analysis the results of the experiment and conclude that the wavelength of the light and the corresponding spectrum absorption coefficient play important roles in the energy level transition of molecular photosynthesis. For further illustrating the role of light in the growth condition of Skeletonema costatum, we obtain the relationship between spectrum absorption coefficient and growth rate or saturated light intensity. In blue light condition, the value of saturated light intensity is the least (40 μmol·m–2·s–1), but the spectrum absorption coefficient is the largest (= 0.0913). In green light condition, the value of saturated light intensity is the largest (50 μmol·m–2·s–1), but the spectrum absorption coefficient is the least (= 0.0868). Hence, saturated light intensities are negative correlation with the spectrum absorption coefficient. It also shows that within the saturated light intensity and the same photosynthetic available quantum flux density, the growth rates is the largest under blue light and the least under green light, therefore, the growth rates of Skeletonema costatum are positive correlation with the spectrum absorption coefficient. Results presented in this work may offer valuable references for estimating the effect of light wavelength on the growth condition of Skeletonema costatum and understanding the mechanism of the red tides occurrence.
This research is funded by Natural Science Foundation of Shandong Province (No. ZR2010DM009) and the Fundamental Research Funds for the Central Universities (No. 201113011).