What is the difference between a bacterium and a cyanobacterium?
Question
I was reviewing some notes after a class on bacterial diversity, and a section about cyanobacteria stood out. I remembered they used to be called blue-green algae, but that seemed off. Looking deeper, it became clear that cyanobacteria, although bacteria, are incredibly unique. From photosynthesis to nitrogen fixation and even their role in Earth’s oxygen history — they’re nothing like typical bacteria.
Answer ( 1 )
Classification and Historical Misunderstanding
Cyanobacteria were once mistaken for algae (“blue-green algae”) due to their photosynthetic traits and resemblance to algae in appearance. However, advances in molecular biology and cellular studies confirmed they are bacteria. The name “cyanobacteria” highlights their bacterial identity and their distinct blue-green pigmentation.
Photosynthetic Differences
The most striking difference lies in their photosynthesis:
Cyanobacteria perform oxygenic photosynthesis, releasing oxygen like plants and algae.
They use water as an electron donor and contain chlorophyll a and phycobilins (phycocyanin, allophycocyanin, sometimes phycoerythrin).
Photosynthetic machinery is organized in internal membranes called thylakoids.
In contrast, other photosynthetic bacteria use compounds like hydrogen sulfide and do not release oxygen (anoxygenic photosynthesis).
Cellular and Structural Characteristics
While both are prokaryotic (lacking membrane-bound organelles), cyanobacteria tend to be:
Larger in size (3–10 µm vs. 0.5–2 µm in most bacteria)
More internally complex
Unique features in cyanobacteria include:
Morphological Diversity
Cyanobacteria exhibit greater diversity in form:
Metabolism and Nutrition
Cyanobacteria are primarily photoautotrophic but may show mixotrophy (using organic carbon in dark conditions). Many can fix atmospheric nitrogen using nitrogenase, either in heterocysts or in standard cells under specific conditions. This unique combination is rare among prokaryotes.
Ecological Roles
– Act as primary producers in aquatic systems – Major contributors to oxygen and carbon fixation – Important nitrogen fixers in soils – Can form symbioses with fungi (lichens), plants, and animals
Other bacteria may be decomposers, pathogens, or symbionts but lack this broad ecological versatility.
Evolutionary Importance
– Responsible for the Great Oxygenation Event (~2.4 billion years ago) – Gave rise to chloroplasts via endosymbiosis in early eukaryotes
Human Relevance
Beneficial:
Used in biofuel and biofertilizer production
Nutritional supplement (e.g., Spirulina)
Harmful:
Cause harmful algal blooms (HABs)
Produce toxins that threaten health and ecosystems
Cyanobacteria stand out within the bacterial domain due to their unique combination of structural complexity, metabolic versatility, and ecological and evolutionary significance.
Source:
The Ecology of Cyanobacteria; Nature Reviews Microbiology