Science Buddies: "Ask an Expert"

Hello!

I'm a Junior in HighSchool, and I'm currently planning my final Senior lab project, and would really appreciate some help! I was very much inspired by the experiment titled Oil Spills in ScienceBuddies, and developed my own question. Essentially I'll be changing the amount of crude oil in a saline solution and see what effect this will have on growth of a specific algae. I would have ideally completed my lab this year, but due to the pandemic will have to do it next year.

I'm current struggling on figuring out how exactly to measure my dependent variable. So, I looked at a Hemocytometer and a Colorimeter, and decided to track the growth every other day for 2-4 weeks. Based on my research of the Colorimeter, I'm going under the assumption that an increase in absorption rate would lead to an increase in growth rate. But I feel like this is somewhat flawed as the crude oil will change how much light will pass through as its quite yellowish/black in color. Is this thinking correct? Do you have any ideas on an alternative to measure the growth? I think it's important to note that I've never used these devices before - it would be my first time.

Also, thinking about my controlled variables, I'm not quite sure how to control gas exchange, as the crude oil might entirely cover the algae, not allowing it to breathe.

Thank you for taking the time to read this. I'd appreciate any help you could offer!

Have a great day.
Benji

Hi Benji, sounds like a cool project!

First, I'm assuming your colorimeter is one for chemistry - you insert cuvettes full of control or sample liquid, and measure how much visible light is absorbed.
So a colorimeter is measuring absorption, but this isn't necessarily "an increase in absorption leading to an increase in growth". In a control sample of algae (no oil), increased absorbance implies an increase in growth. But in high oil samples absorption will increase primarily because of the extra oil, which is probably not going to lead to increased growth.

This leads us to your point, the oil in each sample will be what we call a 'confounding variable' when using the colorimeter.
So you would need to have an 'oil control" sample for each level of oil, meaning a sample that only has the oil in the saline, and no algae. The difference between these measurements would tell you how much light the algae alone is absorbing. These oil controls will be considered negative controls, and you need one for each oil level. But as you noted, once the solution is black from oil, you won't be able to tell what's happening with the algae.

Also, and this is nitpicky, I realize, but all the machine can tell you is how much light gets through the sample, which in your case implies the amount of light hitting the algae. But the machine can't necessarily tell you, for example, if your algae are alive or dead.

So colorimetry will give you interesting data, but may not work for all samples, so you should collect a second type of data as well.

My next questions are:
How big are these algae? That is, how many of the smallest squares does each one cover, when you put them on a hemocytometer?

Do these algae grow in clumps? If so, can they be separated by shaking or pipetting up and down?

Will you have access to an analytical scale? Like one that has a little platform in the middle and glass doors that slide closed on either side?

What about hand held pipettes?

I'm thinking that counting/weighing your algae may be your best bet to look at growth, especially with higher oil concentrations.
Let me know answers to the above and we'll go from there.

Regarding the variable of gas exchange, you are correct, the oil is going to interfere with this. Oil doesn't just block out light - it smothers living things. So gas exchange isn't something you can control for. Instead, it is a dependent variable, meaning it depends on, or will change with, differing levels of oil (independent variable). You could maybe measure gas exchange by checking CO2 levels in your samples, and this would let you hypothesize about why the algae are living or dying in a particular sample. (Gas exchange vs light availability).

Write back, and we'll get this solidified for you!