Algae population density methods

The growth of algae populations are easy to observe in both natural lakes and Lake-in-a-Tube microcosms. Any number of LIAT experiments ask some form of the question: "What conditions make the water turn green with algae?" And though we can readily detect very subtle differences in algae population density (approx. 500,000 to 1,000,000 algae cells/mL) with the naked eye in LIAT microcosms, we can various methods to quantify algae density more precisely.

Here you can get a visual overview of three different methods for determining algae cell density. All three have been rigorously classroom tested and can accommodate various levels of classroom resources from very-little to fully-equipped college laboratories. All three methods which are described in full in my "The Lake-in-a-Tube Companion" book.

A simple experiment

A great starting lab experiment addresses the question "Does nutrient concentration affect algae population growth?" By establishing LIAT microcosms that vary only in the concentration of nutrient media added to them and measuring algae cell density over time. Mine looked like this after three weeks:

I choose to include two replicates of each treatment nutrient concentration (5% and 20%) as well as a negative control containing no nutrients (0%).

I can get the data to answer the study question by quantifying algae cell density by any of the three methods below.

Image courtesy of wikimedia commons

Cell count via hemocytometer

The most precise method of quantification is counting algae cells in a special microscope slide called a hemocytometer at 400X magnification. This method provides direct observation of individual algae cells over a grid that is etched into the slide.

The images below are examples from cell counts within one medium-sized squares (i.e. highlighted red in the diagram to the left) taken from the experiment. Counts from ten of these squares are combined and converted to cells/mL.

0% nutrient media, replicate 1

5% nutrient media, replicate 1

20% nutrient media, replicate 1

Spectrophotometry of algae cells in solution

This method uses the absorbance of light by algae pigments to determine algae cell density. A standard curve (available in my 2025 ABLE paper or in The LIAT Companion book) describes the relationship between these two variables. While this method is less precise than direct cell counts using a hemocytometer, it is faster.

0% nutrients replicate 1

0% nutrients replicate 2

5% nutrients replicate 1

5% nutrients replicate 2

20% nutrients replicate 1

20% nutrients replicate 2

Comparison of LIAT microcosms with a standard dilution of algae cells

This method uses a set of linear dilutions of concentrated R. subcapitata of known algae densities in nutrient media. These are then visually compared to LIAT microcosms. While this is the least precise method, it requires very little specialized equipment.

Images below show front and rear views of experimental replicates of each treatment flanked by the dilutions that are most similar in algae cell density.

0% nutrients replicates 1 & 2

5% nutrients replicates 1 & 2

20% nutrients replicates 1 & 2

Resulting data from each method

Because each quantification method generates algae cell density data in the same units, they can be used interchangeably.

In the table that follows, you can see how these methods compare.

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