A Comparison of the Photosynthetic Rate between Laminaria Digitata and Fucus Vesiculosus

Introduction

My interest in measuring the rate of photosynthesis derives from a biology trip to the Island Svanøy. Both along and up the shore there is a great diversity of organisms. What I found the most interesting, was how the positioning of the organisms affected their morphology. I started to wonder if maybe the position also affects the rate of photosynthesis? In this investigation, the rate of photosynthesis is compared between the brown algae Laminaria digitata and the macroalgae Fucus vesiculosus. L. digitata is mainly positioned lower between the littoral to sublittoral zone, and F. vesiculosus is positioned higher up in the middle shore community.

Photosynthesis is one of the most important biological process on earth. The process which converts light into chemical energy in the forms of sugar, consumes carbon dioxide ( CO2) and liberates oxygen (O2) has give rise to the hospitable environment we know today. Algaes are important photosynthetic organisms around the seashore.

L. digitata is rarely uncovered expect during extreme low tide conditions. Because it is positioned lower down in the shore, light is at minimum. However, it has the highest productivity (photosynthesis) due to long immersion time, enabling the algae to obtain nutrients for growth. L. digitata has a thinner and filamentous structure.

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F. vesiculosus is covered for half the day only. Desiccation problems do occur but is less severe than for organisms higher up on the shore. The position enables the macroalgae to manage stress better and variable temperatures and humidity. But at high tide the light intensity reaching the algae is low. Its structure of pairs of air bladders float with fronds towards the light. F.vesiculosus need high intensity to grow well and its functional-form group is thick leathery.

Main factors affecting the rate of photosynthesis are: light intensity, temperature and carbon dioxide concentration. The algae needs to be fully covered with water for photosynthesis to occur.

Aim

The aim of this biology laboratory experiment is to explore if there is a difference in photosynthetic rate for algaes on the slanted sea shore. By investigating how the rate of photosynthesis differ between the algaes; Laminaria digitata and Fucus vesiculosus as they are located at different positions in the seashore.

Research question

Is there a difference between the rate of photosynthesis between Laminaria digitata and Fucus vesiculosus in seawater with 30 ppt salinity?

Prediction

Null hypothesis: There is no difference between the means of the sample.

Alternative hypothesis: There will be a difference in rate of photosynthesis. Prediction is that Laminaria digitata may have the the faster rate due to its thallus morphology of a thinner structure, thus making it easier for thallus to absorb light. Also, as algaes need to be fully covered in water to successfully undergo photosynthesis. L. digitata may have the faster rate, as it is positioned lower in the shore and is fully covered in water for a longer duration of time.

Variables

Dependent

• Dissolved oxygen concentration: measuring in ppt (± 10% uncertainty as indicated in probe instructions )

Independent

• Laminaria digitata: 36 grams (±0,1g)
• Fucus vesiculosus: 36 grams (±0,1g)

Controlled

• Mass of plants: 6 grams (±0,1g)
• Salinity: 30 ppt ( (± 1%)
• Temperature in the lab: 25 ℃ (± 1%)
• Temperature of water in the beaker: 12 ℃ (± 1%)
• Light level: 712.5 lux (± 10% uncertainty as indicated in probe instructions )

Method

Seawater and the algae were collected from the sea using buckets and knives. Portions of algae were removed with a razor blade. The algae were maintained in buckets fully covered with sea water of 30 ppt salinity. To compare the photosynthetic rate, the amount of dissolved oxygen concentration was measured. The system of measuring was set up of a dissolved oxygen probe, a light level sensor and a temperature sensor. All measurements were collected through the computer programme sparkvue. After the experiment, the algae were put back in the sea.

1. Place a double-beaker between two lamps. Mark the position of the lamps by taping where they are on the table to standardize the beaker’s exposure to light for every experiment.
2. Fill both inner beaker and outer beaker with sea water to prevent overheating of water.
3. Measure concentration of dissolved oxygen present by placing a dissolved oxygen probe inside the beaker, laying on the surface. Measure the light intensity by placing a light level sensor above the system to control the light intensity.
4. Connect the probes and measurements to a computer programme to collect the data.
5. Adjust and cut the algaes to better fit inner beaker and the weighing scale.
6. Dry the algae with paper towel to minimize uncertainty in weighing and then weigh the algae to 6 grams(±0,1g). 6. Place a sample of algae inside the beaker with a tweezer and start the measuring programme immediately as the alage sample is put inside the double-beaker.
7. Keep the sample inside the water for 8 minutes. During the duration use the tweezer to cautiously stir the content to prevent trapping of air bubbles.
8. After 8 minutes, stop the measuring computer programme and pour out the content in an empty bucket.
9. Repeat the above step 9 times for each sample of algae. ( Total 10 times)
10. When all left over water and algae has been collected in the bucket, put it back into the sea.

Equipment and Materials

Apparatus

• Double-beaker
• Buckets
• Interface (PASCO)
• Knives
• 2 Lamps (15W)
• Laptop
• Light level lux sensor ( PASCO)
• Razor blades
• Ring stand
• Scale
• Scissors
• SPARKvue software program
• Timer (± 1 sec)
• Utility clamp
• Dissolved oxygen probe (PASCO)

Materials

• Laminaria digitata: 36 grams (±0,1g)
• Fucus vesiculosus: 36 grams (±0,1g)
• Sea water ( 30 ppt salinity)

Special Considerations

Environmental

No algae were harmed. After the experiment they were put in back into the sea to not further harm the biological system.

Safety

Dressed appropriately when collecting the algae and the seawater because the rocky seashore is slippery. During the setup for the experiment the experiment area was cleared out and safety approved to hinder electricity to get in contact with water.

Results and Discussion

All sample tests shown in table 1 show that there was an increase of dissolved oxygen concentration. Commonly in every sample, the concentration of dissolved oxygen increased, followed by a small decrease and then continued to increase. This can be explained by the correlation between oxygen concentration and temperature in the attached screenshot from one trial from the measuring programme. When initial temperature was 15℃ , the oxygen concentration began to increases but as temperature decreased from 15℃ to stabilize at 12 ℃ the oxygen concentration simultaneously began to decline. When the temperature was stabilized the dissolved oxygen concentration began to increase. This can be explained through the effect temperature has on enzyme activity in photosynthesis.

The significance of the decreased values from every trial was cancelled out when the average values were calculated. Therefore, it is shown as almost no growth in both trendlines in graph 2 as the graphs appear horizontal. Laminaria digitata starts of with a higher concentration of dissolved oxygen than Fucus vesiculosus. However, total percentage increase for all five of F.vesiculosus samples were greater than the total percentage increase for all five of L. digitata trials. The total percentage increase from the mean values are significantly higher for F.vesiculosus (7.01%) than for L.digitata (3.78%).

From the overall look in graph 2 there was a clear increase in dissolved oxygen concentration from 72.18 ppt to 77.24 ppt for F. vesiculosus. The biggest increase is the sharpest part of the graph from 6 minutes to 8 minutes. Between 2 minutes and before 6 minutes there was a very small increase of dissolved oxygen in the beaker, as the curve is close to horizontal. There was also a clear increase in dissolved oxygen concentration for L. digitata from 73.56 ppt to 76.34 ppt. With a similar small increase in growth as F. vesiculosus between 1 minutes to 3 minutes.

Calculated p-value is 0.003795, which is less than 0.05. This indicates that the difference in rate of photosynthesis between F. vesiculosus and L. digitata was significant. The error bars shown in graph 1 do not overlap. Therefore this is another indication that F. Vesiculosus’ data was significantly different from L. digitata.

The prediction of results were not confirmed. There was a difference in rate of photosynthesis between the studied alga. However, it was the uppershore species; Fucus vesiculosus that had a faster rate. A comparison of total percentage increase for all values in table 1, the t-test and the error bar diagram all concludes this. Calculated total percentage increase for the average values in table 2 also show that F. vesiculosus had a faster rate of photosynthesis than Laminaria digitata, as it had greater increased the percentage increase of dissolved oxygen concentration over the same duration of time.

Evaluation of weaknesses with suggested improvements: The small p- value of of 0.003795 indicates that there is evidence against the null hypothesis as the p-value is much smaller than the significance level of 0.05 and even 0.01. Meaning that there is less than 1 in 20 or rather 1 in 100 chance of being wrong. This signifies that the null hypothesis, ” There is no difference between the means of the samples” can be rejected. The results are statistically significant at the 95% confidence level. The error bars also signify that there may have been a significant difference and that the results were most likely not due to chance or sampling errors as the error bars in the samples do not overlap.

However, a bigger sample test would be more reliable to achieve more precise estimates of true population values. According to the results from the data collected Fucus vesiculosus had a faster rate of photosynthesis than Laminaria digitata. As F. vesiculosus is the upper more species it might indicate that upper shore species may photosynthesize faster. Species further up on the slanted shore are completely covered in water for a shorter duration than the lower most species. They are also to a greater extent able to manage variable temperatures and humidity.

But as it has less time available for photosynthesis because limited by desiccation, but still needs to photosynthesize there might have been an adaption in the structure to photosynthesize faster as they do attain less water. According to the competitive exclusion principle, two species can not share the same niche. One will be better adapted than the other and eventually outcompete it. Therefore species show different adaptations for photosynthesis, according to the conditions to which they are best adapted.

For example F.vesiculosus has branches, fronds and air bladders in comparison with L. digitata. Air bladders keep the blade buoyant and near the surface of the water for efficient photosynthesis. High light intensity is an important factor for photosynthesis, especially for F. vesiculosus because L. digitate has more pigments that … But the water will filter off some of the wavelengths of light and thus reduce the light intensity. Therefore the air bladders enables the algae to stay close to the surface in order to absorb the highest wavelength possible. As F.vesiculosus is able to attain higher light intensity due to it floating close to the surface, this may be one of the factors to why the algae resulted in having a faster rate of photosynthesis than L. digitata in the experiment. Because L. digitata did not float and was lower down in the beaker it must have experienced lower light intensity due to the water filtering of some wavelengths.

But, the floating of F. vesiculosus might have caused incorrect data recorded. One systematic errors was the inability to keep F. vesiculosus under water in the beaker. Because of the algae’s floating capacity it would regularly float on the surface so that it was not completely covered, which created bubbles on top. This may have affected the results and thereby lowers the the accuracy of the experiment. As stated by Huppertz : “It is well known that uncovered algae from the intertidal zone show a decrease in photosynthetic activity as a result of a decrease in water content”. This experiment might have reduced the full potential of the algae to undergo photosynthetic activity.

For a better investigation on how the rate of photosynthesis differ up along the shore, a suggested improvement would be to use a greater sample size to record if it showed a similar trend. Using algae from different shores could also increase the accuracy of the experiment as it would include a wider range of algae of the same species. For example, investigating algae from the Atlantic and Baltic sea to examine similar trends and if not, similar it might be due to different salinities, wave actions and other factors. Limitations to the experiment was the few samples of species, only two species were investigated. As there are more species up along the shore, a future investigation may include more species closer and further away from each other.

The best would be to compare several algae and not only two for more reliable results. Those results could help to support this investigation. For example also investigating Fucus serratus, as it does not have the same air bladder like structure as F. vesiculosus. And Pelvetia as it is higher up on the shore. It also important to take into consideration that the age of the alga was unknown. Age of algae can affect rate and activity of photosynthesis. This limitation does also affect the reliability of the investigation.

F. vesiculosus and L. digitata were measured under the same conditions. But as they have different optimal conditions this may also have affected the results. Temperature of 12 ℃ might have favored one more than the other.

Further investigation may also include investigating the algae’s anatomy. In this experiment only the morphology has been discussed, however there may be internal structure’s that also have an affect on the rate of photosynthesis. For instance pigments and the amount of them.

However if these results are applicable for all cases can be discussed due to the factors of errors and the limitations of the experiment. For more accurate results, one may also use another approach to measure the rate of photosynthesis to support the recorded material. For example measuring the decreased dissolved organic carbon concentration or weighing the algae before and after to measure difference in growth. This would also include longer time intervals. Unfortunately, tools to measure photosynthesis activity in other ways as measuring the electron transport rate were not accessible.

Conclusion

The results indicate that there was a difference in the rate of photosynthesis between Fucus vesiculosus and Laminaria digitata used in the investigation.This experiment showed that Fucus vesiculosus, the upper more species had a faster rate. Factors that may contribute to these difference may be the morphology of the thallus and how species structures have adapted to the different positions on the slanted shore.

References

1. https://www.cusd80.com/cms/lib6/AZ01001175/Centricity/Domain/4939/1017Whatisphotosynthesis.pdf [accessed Sep 12 2018].
2. A Field Atlas of the seashore(1998) – Julian Cremona (PDF) Photosynthesis in Marine Macroalgae. Available from: https://www.researchgate.net/publication/227190836_Photosynthesis_in_Marine_Macroalgae [accessed Sep 15 2018]
3. Rate of photosynthesis: environmental factors file:///C:/Users/Student/Downloads/Rate%20of%20photosynthesis%20environmental%20factors%20(1).pdf [accessed Sep 15 2018]