Light sensing final match : Daphnia vs. light sensor

Light sensing final match : Daphnia vs. light sensor

 

You might be wondering what science students do ! Here is a good example of how interesting research can be learned. We are four students from the CRI (Center for Research and Interdisciplinary) who are involved in a seminar called Biosensors. This week, we had the opportunity to work on light sensors. You probably know that some electronics are used every day to measure light ! But what if some biological organisms could replace those compounds ? Well, we found out little planktonic crustaceans called Daphnia, are sensitive to light .

 

 

They are used in research to study heart beats as a function of temperature. But what interested us was their response to light. Our project deals with the comparison in sensing light intensities between LDR (light dependent resistor) and Daphnias. Which one of LDR or Daphnia is the best sensor ? Our team LUMinomex tried to answer this question by focusing on accuracies.

 

Figure 1 : Visual protocol

 

Our goal was to stimulate Daphnias with different blue light (their favorite one !) intensities to understand if those crustaceans could serve as good light sensors.  To do so, we designed a protocol divided into several steps. First we made two simple linear electronic circuits: one for our LDR (Light Dependent Resistor) and the other one with exchangable resistors to make blue LED’s intensity vary. The LDR circuit was linked to an Arduino, a little electronic platform. Through a computer code we were able to store the capted values.

Secondly, we made a laboratory experiment using 12 transparent tubes with 15 Daphnia per tube. We then illuminated the tube from the top and counted how many Daphnias came towards the light.

Results

We compared our two sensors with help of four different light intensities: complete darkness, low intensity, intermediate intensity and brightness. According to those light variations, both sensors should theoretically present a linear, increasing response.

 

Figure 2. Sensor responses to varying intensities

As we can see on Fig.2, values of the electronic light sensor increase as the light intensity grows. On the other hand, Daphnia seem to remain quite constant in their number as intensity varies.

Conclusion

    The LDR presents different values for a specific light intensity, meaning it is not precise. Precision means that measurements remain exactly the same when repeated, it shows a certain stability. However, linearity of the curve shows the accuracy of the LDR. Accuracy represents how the measure is close to the reality. As for Daphnia, the curve is almost constant, it means they detect the blue light but they are not sensitive to light intensity. So they are not good biological light intensity sensors but they could probably be used as light detectors.

Discussion

However, we faced several issues during our experimentation. In fact, we had to deal with tiny living beings, which swim very fast, making them hard to catch and count in their tubes. What’s more, they are quite ephemeral and some of them died during our experiments. Another problem that we pointed out, is that the LDR was quite randomly positioned, and not placed directly under the blue LED. If we wanted to improve the experiment, we would first design a more stable device, with fixed sensor and light. Also, we would develop a more precise technique to count Daphnia as well as a way to keep them alive.

If you want to know more about Daphnia light response, go check this interesting article: Stephen C. Stearns, Light responses of Daphnia pulex, Laboratory of Limnology, University of Wisconsin, 07/1975 , V.20 564-570

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