Blue-green algae are prokaryotic, not eukaryotic. Despite their common name, they are not true algae at all. Scientists now call them cyanobacteria, classifying them as a phylum of bacteria. They are among the oldest living organisms on Earth, with a fossil record stretching back roughly 3.5 billion years.
Why the Name “Algae” Is Misleading
The confusion starts with the name itself. Early scientists observed these organisms growing in water, photosynthesizing like plants, and forming green or blue-green films on pond surfaces. That plant-like behavior earned them the label “blue-green algae.” Over the centuries they also went by myxophyceae, cyanophyceae, cyanophyta, and oxyphotobacteria. It wasn’t until the late 1970s, when microbiologist Roger Stanier and colleagues coined the term “cyanobacteria,” that the scientific community began consistently treating them as bacteria rather than algae.
The “cyano” in cyanobacteria refers to their blue-green color, which comes from light-harvesting pigments called phycobiliproteins. True algae (like green algae, red algae, and diatoms) are eukaryotic organisms with complex, compartmentalized cells. Cyanobacteria share none of that internal complexity. Since Stanier’s reclassification, more than 23,000 published papers have used the term cyanobacteria, and around 5,185 species have been identified so far, with estimates suggesting the total could reach 8,000.
What Makes Them Prokaryotic
The fundamental dividing line between prokaryotes and eukaryotes is the nucleus. Eukaryotic cells store their DNA inside a membrane-enclosed compartment. Prokaryotic cells do not. In cyanobacteria, the DNA sits freely in the cytoplasm in a region called the nucleoid, with no surrounding membrane. They also lack the other membrane-bound organelles found in eukaryotic cells: no mitochondria, no endoplasmic reticulum, no true chloroplasts.
Their cell walls provide another bacterial signature. Cyanobacteria have a Gram-negative cell wall structure, meaning they possess a layer of peptidoglycan (the rigid mesh unique to bacteria) surrounded by an outer membrane containing lipopolysaccharides. Interestingly, their peptidoglycan layer is considerably thicker than that of most other Gram-negative bacteria, giving them a somewhat unusual position among prokaryotes. Eukaryotic plant cells, by contrast, have cell walls made of cellulose, an entirely different material.
Individual cyanobacterial cells are tiny. One well-studied species, Synechocystis, measures just 0.7 to 8 micrometers across. That’s orders of magnitude smaller than most eukaryotic algae cells.
How They Photosynthesize Without Chloroplasts
Cyanobacteria are the only bacteria that perform oxygenic photosynthesis, the same type of photosynthesis plants use to split water molecules and release oxygen. In plants and eukaryotic algae, this process happens inside chloroplasts, which are membrane-bound organelles. Cyanobacteria accomplish the same chemistry without chloroplasts by using internal membrane structures called thylakoid membranes.
These thylakoid membranes sit inside the cell but are not enclosed in a separate organelle. They house two photosystems that capture light energy and drive electron transfer reactions, converting sunlight into chemical energy. At least one ancient species, Gloeobacter violaceous, doesn’t even have distinct thylakoid membranes and instead runs its photosynthetic machinery directly in its outer plasma membrane. This organism is thought to represent an extremely early branch of cyanobacterial evolution.
Their DNA Behaves Differently From Eukaryotic DNA
Cyanobacterial DNA is organized as a circular chromosome, typical of bacteria and distinct from the linear chromosomes found in eukaryotic cells. The DNA condenses into a compact structure in the cytoplasm, and ribosomes and other molecular machinery get pushed to the edges of this nucleoid region.
Researchers using high-voltage cryo-electron tomography have observed something unexpected, though. At certain points in the daily light cycle, cyanobacterial DNA compacts into an undulating rod-shaped body that visually resembles a condensed eukaryotic chromosome. This compaction is transient and changes dramatically as the cell divides, unlike the elaborate, repeatable chromosome condensation seen in eukaryotes. Some scientists have suggested this could represent an early evolutionary version of the chromosome-packing mechanisms that eukaryotes later refined.
The Evolutionary Bridge to Plant Cells
Here’s where cyanobacteria’s prokaryotic identity becomes especially significant: they are the ancestors of every chloroplast in every plant and eukaryotic alga on Earth. Roughly one to two billion years ago, an early eukaryotic cell engulfed a cyanobacterium. Instead of digesting it, the host cell kept it as an internal partner. Over time, that captured cyanobacterium evolved into the chloroplast. This is the endosymbiotic theory of chloroplast origins, first clearly proposed by Mereschkowsky and now supported by extensive molecular evidence.
The genetic fingerprints of this ancient event are everywhere. Only cyanobacteria and chloroplasts share two photosystems and split water to produce oxygen. Many of the genes originally carried by the cyanobacterial endosymbiont have since migrated into the host cell’s nuclear genome, meaning cyanobacterial DNA influences plant biology far beyond the chloroplast itself. Even organisms that have completely lost their chloroplasts during evolution still carry remnants of cyanobacterial genes in their nuclear DNA.
So while cyanobacteria are firmly prokaryotic, their legacy lives on inside every eukaryotic photosynthetic cell. They are, in a sense, both a thing apart from plants and the reason plants exist.
Why Their Prokaryotic Status Matters for Health
Understanding that cyanobacteria are bacteria, not plants, also matters in a very practical sense. Some species produce potent toxins that can contaminate drinking water and recreational waterways during algal blooms. These cyanotoxins fall into several categories with different effects on the body. Some, like microcystins, primarily damage the liver because liver cells have the membrane transporters that absorb them. Others, like anatoxin-a and saxitoxins, are neurotoxins that interfere with nerve signaling and can cause paralysis at high enough doses. Another toxin, cylindrospermopsin, inhibits protein synthesis and can affect the liver, kidneys, and other organs.
The World Health Organization has set a suggested drinking water guideline of 1 microgram per liter for one common toxin variant, microcystin-LR, with a recreational water guideline of 10 micrograms per liter. Skin contact with bloom water can also cause rashes in sensitive individuals. Dogs are particularly vulnerable to cyanotoxins, with guideline values for safe recreational exposure set far lower than for humans. For saxitoxin, the dog-specific recreational water guideline is just 0.02 micrograms per liter, compared to 10 micrograms per liter for people.