Photosynthetic Rates of S. Obliquus After Exposure to Different Light Colors

Abstract

The purpose of this experiment was to compare the rate of photosynthesis of algal beads in carbon dioxide (CO2) indicator using a red cellophane wrapped lamp and a normal lamp as the light source. The algal beads were separated equally, placed in cuvettes, and positioned 25 cm from the lamps. The CO2 indicators’ absorbances were recorded by the spectrophotometer in specific intervals lasting a duration of 30 minutes. Higher absorbance readings resulted in darker color of the CO2 indicator, and higher pH. It was concluded that the algal beads under the cellophane lamp had a higher CO2 concentration compared to those under the normal lamp, demonstrating a lower photosynthetic rate. The algal beads under the normal lamp had a higher photosynthetic rate, because the CO2 indicator had turned more basic, as shown through the increased absorbance readings. The cellophane wrap used on the lamp impeded but did not completely halt photosynthesis in the algal beads. Introduction Photosynthesis is a metabolic process which harvests energy in the form of light and converts it to sugar (glucose) with the input of carbon dioxide (CO2) and water (H2O) as key reactants.

Oxygen (O2) is a secondary product in the process of photosynthesis. This process is an energy transferring process because autotrophs, also known as primary producers, convert light energy into glucose. The primary autotrophs are plants, but there are also protist primary autotrophs, such as algae, phytoplankton, and prokaryotic primary autotrophs, such as photosynthetic bacteria [6]. The glucose produced by these organisms are used by heterotrophic organisms which rely on the consumption of glucose to fuel further metabolic processes. Factors that impact the photosynthetic rate include light intensity, carbon dioxide concentration, and temperature. When light intensity increases, carbon dioxide concentration increases, and temperature increases to its optimum state, enabling the photosynthetic rate to increase as well. The experimental question created prior to this experiment was: “How would wrapping a lamp light in red cellophane affect the photosynthetic rate of the algal beads when compared to the control (white lamp light)?” It was hypothesized that the algal beads placed under the lamp wrapped in red cellophane would have a lower photosynthetic rate than the algal beads under the white lamp light due to lower light intensity. The independent variable in the experiment was light color, because it was manipulated. One lamp emitted a white light, while the other was covered in red cellophane wrapping. Light color and intensity play a key role in the earth’s ecological systems.

Photopigments found in plants are used by chloroplasts to harness the sun’s light rays and engage in photosynthesis. When a plant is exposed to only one color it cannot trigger the other necessary photopigments used in photosynthesis, thus decreasing the photosynthetic rate. Sunlight contains the full light spectrum and allows plants to perform photosynthesis at an optimum rate. Aquatic plants do not have the same access to sunlight as terrestrial plants. Aquatic plants are dependent on other factors to promote photosynthesis; Rather than using CO2, they rely upon HCO3 as an alternative reactant to initiate photosynthesis. These plants also have smaller, thinner leaves than terrestrial plants and cannot capture the same amount of sunlight, but have chloroplasts located in the surface layer of leaves making it more cost effective [7]. Cellular respiration is not affected by light intensity, because light is not required for cellular respiration. In this experiment, algal beads (S. obliquus) were used to perform photosynthesis. S. obliquus are most commonly found in freshwater environments and are classified as a unicellular green alga. These alga do not move and simply attach themselves to environmental structures like rocks. They are popular to use in studies concerning future energy sources because of their light-dependent hydrogen production [8]. They are also suitable organisms to study photosynthetic rates. This experiment is significant because it can show the importance of light color and how differences in wavelength can affect photosynthetic rates. This is indicated by the absorbance readings from the CO2 indicator. A new understanding about light color and its effects on photosynthetic rates of algal beads can be discovered, providing broader implications about how light color and intensity may impact other photosynthetic organisms.

Materials and Methods

The materials required in this experiment were two lamps of the same brand and type. It was critical that the lamps were the same brand, because different lamps can emit light at different intensities, affecting the algal beads ability to perform photosynthesis. This should be kept constant. The manipulated variable of the experiment is the light color being emitted. Therefore, red cellophane was used to wrap one of the lamps, while the other lamp was not wrapped, serving as the control. The twenty S. Obliquus algal beads were the photosynthetic organisms used to measure the effects of light color on photosynthetic rate. Two cuvettes were needed to mix the algal beads with 2 mL of CO2 indicator in cuvette. A spectrophotometer was used to measure the absorbance values of the CO2 indicator. In the experiment, two lamps were used as agents influencing the algal beads’ ability to perform photosynthesis. One lamp was covered with a red cellophane wrap and the other was left to emit the ordinary white light. As previously mentioned, the independent variable of the experiment was lamp light color used to act as a photosynthetic agent. 2 mL of CO2 indicator was added to each cuvette. Each lamp was placed 25 cm away from the cuvette and the spectrophotometer was set to a 550 nm wavelength. The absorbance were recorded every five minutes for each cuvette. Visual changes to the CO2 indicator were also recorded. This was done for six separate intervals lasting a combined duration of 30 minutes [9]. The results were then displayed using a scatterplot graph and a table to show the relationship between photosynthetic rate and light color being emitted over time. The data displays also made the difference between the photosynthetic performances of the algal beads under the two lamps more apparent.

The absorbance for the cuvettes containing the algal beads and the CO2 indicator were measured every five minutes for a total of 30 minutes. This was done with the spectrophotometer set at a 550-nanometer wavelength to ensure that it would be sensitive enough to concentration changes in CO2 [5]. The scatterplot graph above displays the absorbance values of CO2 indicator in the cuvettes and the change over time. The specific data points are shown and a trendline was created to assist in deciphering whether or not the different light color affected the photosynthetic rate of the algal beads (S. Obliquus). The initial rate of the reaction for the algal beads under the normal lamp was different than those under the red cellophane wrapped lamp. These were calculated by using a simple equation: the change in absorbance (final minus initial) divided by the time needed to see this change occur [9]. The algal beads under the normal lamp had an initial rate of reaction of 0.001 while the algal beads under the cellophane wrapped lamp had an initial rate of reaction of 0.0002. These numbers indicate a difference in CO2 production and photosynthetic performance between the algal beads under the different light conditions. In addition to quantitative data, observation changes were recorded as well. Bubbles were observed coming from the cuvette placed under the cellophane wrapped lamp. The color of the CO2 indicator in the cuvette under the normal lamp seemed to have become darker while the indicator under the cellophane wrapped lamp seemed to have stayed relatively consistent.

Discussion

The results of the experiment demonstrate that the algal beads under normal lamp conditions performed more photosynthesis than the algal beads under the red cellophane lamp. The data displays a steeper increase in absorbance over time for the algal beads under the normal light conditions. As absorbance increases, the CO2 in the solution decreases, because it is being used for photosynthesis. This is because CO2 is a key reactant in photosynthesis, necessary for the process to occur. Although photosynthesis was still being performed by the algal beads under the cellophane lamp, the photosynthetic rate was lower, as seen in lower absorbance readings. Bubbles from the algal beads were observed in the solution, indicating a release of oxygen, which is one of the products of photosynthesis. The initial rates of the reaction also serve as evidence to support the fact that the photosynthetic rate of the algal beads under the normal lamp conditions was higher. The normal lamp had a 0.001 initial of the reaction, compared to the cellophane wrapped lamp, which had a 0.0002 initial rate of reaction. This higher reaction rate under the normal lamp condition indicates that the rate of photosynthesis was higher over the recorded time frame. Cellular respiration could not be tracked directly, but likely occurred, because photosynthesis creates glucose from light. This glucose is then used as a reactant in the cellular respiration reaction. Plants can perform photosynthesis and cellular respiration while animals can only perform the latter. These results indicate that in order for plants to perform photosynthesis, it is vital for them to have access to a sufficient amount of sunlight. Artificial light sources that emit different colors hinder photosynthetic performance. Photosynthesis is necessary in keeping organisms alive, including humans who are dependent on the products of this process. During photosynthesis, oxygen is a released byproduct which allows organisms to engage in the vital process of cellular respiration. Another byproduct is glucose, which is also necessary for animals to respire, as the glucose is broken down in the mitochondria during cellular respiration releasing energy. This energy is then used to carry out metabolic activities necessary to animal life. If the process of photosynthesis is hindered by pollutants that obstruct photosynthetic organisms from having access to sunlight, life on Earth can potentially face extinction. It was hypothesized that the algal beads under the red cellophane lamp would underperform relative to the algal beads under the controlled conditions due to lower light intensity.

The hypothesis stated was supported by the results of the experiment. The photosynthetic rate of the algal beads under the control was higher as shown through the initial rate of reaction calculations and the data displays. They both performed photosynthesis but did so at different rates when identifying the differences in absorbance values recorded. Sources of error for this experiment could have been initial rate of reaction calculation errors and spectrophotometers not being correctly calibrated. If groups chose to not be meticulous when recording values, the data could show a different trend. The rates of the reactions may seem closer than they truly are, and the absorbance values may also show no indication of change. The conclusions made from these results can then be incorrect. The experiment could have been improved by trying to minimize human error as much as possible, which would yield more accurate recorded values, calculations, and conclusions. To further explore the topic on how photosynthetic rates can be affected, the temperature of a solution containing organisms capable of performing photosynthesis can be manipulated. This would have real world implications. The scientific community has a consensus that climate change is occurring, and that Earth’s temperatures are increasing up to potentially catastrophic levels. By manipulating temperature, a prediction can be made about how rates of photosynthesis performed would change in a hotter environment.