Algal Growth Rate Introduction




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Anniken Lydon

6/5/09


Algal Growth Rate
Introduction:
Algal populations that are grown under unregulated conditions exhibit exponential growth characterized by the equation:
Nt=N0et
Where N0 is the initial population size,  is the growth rate, t is the amount of time that has past, and Nt is the population size at time t. Growth rates decrease as conditions within the environment become limiting to growth. In this case, the growth rate deviates from being a linear relationship. The maximum growth rate for a culture is calculated based on the linear part of the growth curve, which occurs before there is any type of limitation on growth. The growth curve will be used in this experiment to calculate the growth rates for the exponential and stationary parts of the growth curve for microalgae. From the calculated growth rates, the population doubling time for different experimental conditions was determined for different light treatments.

The green algae, Dunaliella tertiolecta, was grown under varying light conditions during this experiment. Light treatments included; red light, blue light, green light, and white light. The amount of nutrients provided in this experiment was consistent across all treatments, but was not held at a constant rate over the time of the experiment.

The objective of this experiment was to observe how different colors (wavelengths) of light affected the growth rates for populations of D. tertiolecta.
Methods:
F/2 growing media was prepared ahead of time for algal growth experiments. To prepare this media, seawater was first filtered and then autoclaved to kill anything growing within the water. Then macronutrients, trace metals, and essential vitamins were added to the seawater. The media had all the nutrients dissolved in solution prior to beginning used in the culturing experiment.

Each student in the class was assigned to a specific light treatment and accordingly covered a 250 ml Erlenmeyer with the appropriate filter. Red, blue, and green cellophane was used to filter light for the color treatments. A mesh screen was placed around the white light condition to try and equalize the total amount of irradiance received by each treatment condition.

After the Erlenmeyer flasks were prepared with the filter, 100 ml of the prepared nutrient solution (F/2 media) was added to each Erlenmeyer flask. Then 5.0 ml of the cultured algal cells, D. tertiolecta, was added to each of the treatment conditions. The cultures were then uniformly mixed and an initial count of cell number in solution was performed using a Hemostat counting cell. Counts were recorded at different intervals over the period of a month. At the end of the growth experiment, the data from every student was compiled and analyzed for trends in growth rate.


Analysis:


Figure 1. Exponential growth of D. tertiolecta populations grown under different light conditions
Algal populations from each of the light treatments showed exponential growth (Fig 1). The blue and white light cultures grew faster than the red and green light treatments. The red algal treatment showed more population growth than the green light treatment. The blue light treatment showed the highest growth rate and the greatest final population growth (Fig 1).

The growth for all microalgal populations in the different light treatments remained in the exponential growth phase during the entire extent of the experiment, as indicated by the linear nature of the growth curves on a natural log scale (Fig 2). In this experiment, we did not observe a stationary phase of growth and growth remained exponential. The R2 values for all light treatments appeared to fit closely a line when graph on a natural log scale (Table 1).

The blue light treatment had the highest growth rate =0.1479 per individual in the population per day. Each individual in the white light treatment had a growth rate of =0.1436 per individual in the population per day. The red light treatment had a growth rate of =0.0931 per individual in the population per day. The green light treatment had the slowest growth rate per individual at =0.0785 per individual in the population per day (Table 1).



Figure 2. Natural Log of the population size for D. tertiolecta during the extent of the culuture experiment.

Table 1. Growth rates and doubling time for D. tertiolecta cultures held under varying light conditions.

Light Treatment

Growth Rate

=per individual/ (Nt*days)



R2

Doubling Time (1/) in days

Blue Light


0.1479


0.9344


6.76132522


White Light


0.1436


0.9803


6.963788301


Red Light


0.0931


0.9727


10.74113856


Green Light


0.0785


0.9664


12.7388535


The blue light treatment showed the quickest doubling time (6.76 days); followed closely by the white light treatment that had a doubling time of 6.96 days. The red light treatment had a doubling time of 10.74 days. The green light treatment had the slowest doubling time for all treatments with a doubling time of 12.73 days.


Conclusion:
Dunaliella tertiolecta cultures were grown in four different light treatments for 30 days. All cultures showed exponential growth during the experiment, and it does not appear that any of the cultures reached the point of carrying capacity in the culture flasks. This is indicated by the linear nature of the growth on a natural log scale.

The blue light treatment had the highest growth rate, correlating to the quickest doubling time. The blue light cultures also had the greatest number of individuals at the end of this experiment. This result is as expected, because individuals are only receiving wavelengths of light that can be captured by chlorophyll a and supply proper energy for PS II.

The white light treatment cultures had the second highest growth rates, correlating with the second fastest doubling time in comparison to the other light treatments. The white light treatment ended with the second highest population size at the end of the experiment. The white light treatment cultures received wavelengths of PAR from 400-700nm and thus are able to capture different wavelengths of light to use for photosynthesis. The cultures may not have grown as quickly as the blue light treatment because other wavelengths of light were also being absorbed and transferred to chlorophyll a in the PS II reaction center. The blue light appears much more efficient and productive, as the chlorophyll a is directly absorbing all the blue light necessary for photosynthesis.

The red light treatment cultures had a slower growth rate than the blue and white light treatments, but a higher growth rate and faster doubling time than the green light treatment cultures. The red light energy can be absorbed by chlorophyll a also, but chlorophyll a absorbs much less red light than blue light. Hence, the algae were still able to absorb some light energy for photosynthesis and increase population size.



The green light treatment cultures showed the lowest population growth rates as well as the lowest total populations size at the end of the experiment. The green light treatment also showed the slowest doubling time of all the light treatments. Chlorophyll a does not absorb much energy from green wavelengths of light. Thus the light energy being supplied to the cultures could not efficiently be used for growth. There may have been some accessory pigments that were able to capture small amounts of light in the green wavelengths, but the absorption was likely very little. Since this culture was not being supplied with light energy that it could capture sufficiently, there was a slower population growth rate over the time of this experiment. Blue light appears the most efficient for growing algal cultures if you are strictly attempting to create the highest population size in the shortest amount of time.

These results also indicate that populations of plankton grow differentially within the water column depending upon the amount of light they receive and which wavelengths they receive. Given proper nutrient supplies, you would expect that algal population numbers would theoretically be higher in areas of the water column where blue light penetrates the deepest. This is also dependent upon the species of algae in the water column and the type of pigments the algae have for light harvesting.


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