Prokaryotic Algae Cells: Function, Definition & Features

Instructor: Jeremy Battista
We can separate many of the kingdoms and domains of biological taxonomy based on the type of cells in the living organism. In this case, we will be looking at prokaryotic blue-green algae cells.

Prokaryotic vs. Eukaryotic Organisms

Scientists classify biological organisms into different domains. For example, humans, plants, and animals, are sorted into the Eukarya domain. To be classified in this domain, organisms must have cells that contain a nucleus and membrane-bound organelles.

Organisms that do not have a centralized nucleus or membrane-bound organelles are known as prokaryotes and are sorted into a different domain. Prokaryotes are typically single-celled organisms, such as bacteria and archeabacteria (Latin for ancient bacteria). In this lesson, we are going to explore blue-green algae, which are considered prokaryotic even though they share some of the characteristics of plants in the Eukarya domain.

What Are Blue-Green Algae?

Blue-green algae is the common name used for the phylum (category) of bacteria known as Cyanobacteria. To remember the scientific name, just remind yourself that cyan is a blue-green hue used to describe the color of the bacteria. The algae part of the common name can be a little misleading since most algae are considered plants, which are classified as eukaryotic organisms. But as stated earlier, cyanobacteria are prokaryotic, so there is a huge difference in their structure compared to the structure of plants.

Cyanobacteria are amazing creatures (yes they are alive; remember, they are bacteria, which is a living organism). They are found in nearly every single possible environment or habitat that we know of - from the hottest deserts to the deepest oceans to the highest mountains. Cyanobacteria are extremely versatile and have adapted to live essentially anywhere.

They are similar to plants in that they go through photosynthesis, albeit in a very different manner. Cyanobacteria lack chloroplasts, which are the membrane-bound organelles containing the chlorophyll responsible for photosynthesis in plants. However, cyanobacteria utilize other pigments, such as phycocyanin, to process photosynthesis. You see, cyanobacteria actually take an electron from water or hydrogen sulfide so that they can use the phycocyanin to capture sunlight.

They then use carbon dioxide, the electrons from water (or hydrogen sulfide), and phycocyanin to produce sugars for themselves to use. In plants, this process takes place in the chloroplasts, but cyanobacteria do not have chloroplasts, so instead, the process takes place in some of the folds of the bacteria. If we look at photosynthesis in plants, we find another organelle in chloroplasts called the thylakoid, which holds the chlorophyll. It is also here in the thylakoid membrane that we have photosytems I and II. These are where the reactions for photosynthesis take place.

Going back to cyanobacteria, we find that the organisms have combined photosystems I and II in order to work through photosynthesis. Many of these cyanobacteria still have thylakoids so the photosynthetic process takes place here. The photosynthesis process is still somewhat similar to that of plants, except it all occurs in one photosystem rather than two.

Nitrogen Fixation

In photosynthesis, the organism takes in and uses carbon dioxide as one of its components. Cyanobacteria has the ability to take in nitrogen as well as carbon dioxide for use in processing and making food.

Here we see atmospheric nitrogen being converted into ammonia and nitrates. The end products are NH4 (ammonia) and NO3 (nitrates).
Nitrogen Cycle

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