Abstract: The effects of light intensity and salinity on algal growth were investigated under three light intensities (3000 lx, 6000 lx and 12000 lx) and seven salinity levels (10, 15, 20, 25, 30, 35 and 40) in a disposable incubator. The results showed that light intensity had a significant effect on algal growth (p < 0.05), salinity had a highly significant effect on algal growth (p < 0.01), and the interaction between light intensity and salinity had no significant effect (p > 0.05). Regardless of the salinity condition, among the light intensities of 3000 lx, 6000 lx and 12000 lx, the highest algal cell density and the best growth were observed at 6000 lx. Among the seven salinity gradients, the density of algal cells was the highest and the growth was the best at a salinity of 30, regardless of the light conditions. The most suitable light intensity for algal growth was 6 000 lx, the salinity was 30, and the maximum density of algal cells was 65.93 × 104 /mL, the maximum growth rate was 0.3679, and the average growth rate was 0.1553 under this condition.
Keywords: algae; light intensity; salinity; growth rate; many factors inducing algal outbreaks of red tides, and its law is also very complicated. Nutrient salts, trace elements and interspecific competition are some of them, but undoubtedly, environmental factors such as temperature, salinity and light conditions are one of the important factors. At present, there are more reports on the study of temperature, salinity, pH and other single factors on algal growth, while there are not many reports on light and the interaction with salinity [13]. Therefore, we experimentally investigated the growth characteristics of algae under three different light conditions and seven salinity gradients, with a view to exploring the effects of light, salinity environmental factors and their interactions on the growth of algae, and then provide basic research information for the formation mechanism of algal red tide.
1 Materials and methods 1. 1 Experimental materials Algae species: Algae species were provided by the Algae Species Bank of the Biological Research Laboratory of the National Marine Environmental Monitoring Center. The culture solution was f /2, and the seawater used in the experiment was sand-filtered seawater, which was filtered with a 300-mesh sieve silk to remove impurities and then boiled and sterilized. The incubation temperature was (20 ± 1) ℃, and the light/dark ratio was 12 L ︰ 12 D. Algae species in the exponential growth period were taken for the experiment.
Instruments: GXZ-380B intelligent light incubator, OLYMPUS microscope, LDZX-50KBS vertical pressure steam sterilizer. The culture containers were sterilized by autoclaving and chemical methods, and the vessels used were soaked in 5% HCI overnight, dried and prepared for use.
1.2 Experimental methods The experiments were conducted using one-time culture method and two-factor experimental design, with three light intensities of 3000 lx, 6000 lx and 12000 lx, seven salinities of 10, 15, 20, 25, 30, 35 and 40, totaling 21 combinations of conditions, and four parallel groups for each experimental group. The salinity 10-30 test group was prepared by mixing pure water and seawater proportionally, and the salinity 35 and 40 test groups were prepared by adding sea crystals to seawater. A 250 mL triangular flask with 200 mL of sterilized test water with f /2 culture solution was used to inoculate the algae at exponential growth period, and then cultured in an incubator at a temperature of (20 ± 1) ℃ and a photoperiod of 12L: 12 D. The initial number of algal cells was set at 5000/mL (the initial number was 50 at the light intensity of 3000 and 12000 lx, as measured). The initial number of algal cells was set at 5000/mL (5040/mL at 3000 and 12000 lx, and 5070/mL at 6000 lx). During the experiment, the bottle was shaken three to four times a day, and 1 mL of samples was taken at regular intervals every 3 d. After fixation with iodine solution, the number of cells was counted under a 10 × 40 OLYMPUS microscope to determine the cell number of the algae.
Under certain environmental conditions, the intensity of light determines the rate of algal photosynthesis. Within the saturating light intensity, the photosynthetic rate of algae accelerates with the increase of light intensity, and above the saturating intensity, the photosynthetic rate of algae decreases until it stops. Algae biological response to light intensity and other environmental response with the different species and there are great differences in temperature conditions, Chlorella optimal light intensity of 10,000 lx; green Duchenne's optimal light intensity of 5,000 lx, light increased to 10,000 lx, the growth rate instead of a significant decline; marine protozoa of the optimal light intensity is relatively low, the growth rate of the largest light intensity of 3000 lx, if the light intensity is greater than 6000 lx, the growth rate is significantly reduced.
3 Conclusion (1) The effects of three light intensities (3000 lx, 6000 lx and 12000 lx) and seven salinity gradients (10, 15, 20, 25, 30, 35 and 40) on the growth of the algae were studied in a one-time incubation method and a two-factor experimental design. The results showed that light intensity had a significant effect on algal growth (P < 0.05), and salinity had a highly significant effect on algal growth (P < 0.01), while the interaction between light intensity and salinity did not have a significant effect (P > 0.05).
(2) Algae could grow under 3000-12000 lx of light and 10-40% salinity. However, the most suitable light intensity for algal growth was 6000 lx and salinity was 30. Under this condition, the maximum number of algal cells reached 65.93 × 104 /mL, the maximum growth rate was 0.3679, and the average growth rate was 0.1553.
(3) The growth characteristics of algae are different between different origins and lineages, and algae isolated from the same location may also have different growth characteristics. In addition, different experimental methods and conditions between researchers may also lead to differences in the experimental data. Therefore, attention should be paid to these matters when comparing data.
(4) Algae grow better under stronger light (12000lx) and higher salinity (40) than under lower light (3000 lx) and lower salinity (10), i.e., algae are more prone to form red tides in the sea area under strong light in summer.
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