Natalie has an MSc in biology and is currently a researcher at Anglian Water.
Plant vs. Cyanobacteria: What's the Difference?
What Is a Plant?
A strict classification of plants includes any organisms that are a member of the kingdom Plantae. However, there are organisms in other kingdoms (e.g. Protista, Monera, and Fungi) that share similar characteristics to plants.
What Are Cyanobacteria?
Cyanobacteria are photosynthetic, prokaryotic organisms which are green-blue in colour. Metcalf and Codd described cyanobacteria as “common members of the plankton of marine, brackish and freshwaters throughout the world . . . [with] . . . a simple structure at subcellular level and lack a nucleus, a characteristic feature defining them, along with bacteria, as prokaryotes."
Below, we will explore their similarities and differences by comparing their structure and how they perform key functions necessary for life.
Structure of Plant Cells, Cyanobacteria, and Chloroplasts
Cell Type and Structure
One glaring difference between plant cells and cyanobacteria is the cell structure. Plants are eukaryotes, meaning they are multicellular and have membrane-bound organelles; this includes a nucleus that holds their DNA.
Cyanobacteria are prokaryotes, meaning they are single-celled and do not have a nucleus (Staley et al., 2007).
Despite this, they do share similar cellular structures (Figures 1 and 2). They both have cell membranes constructed from a phospholipid bilayer. In addition, they both have a cell wall, but the compositions are different. The cyanobacteria cell wall is made of peptidoglycan, while the plant cell wall is made of cellulose.
Both cyanobacteria and plants follow the central dogma of biology: genetic information encoded in DNA translates into mRNA that encodes for specific proteins necessary for cellular function and maintenance. However, cyanobacteria DNA is circular (plasmid), while plant DNA is tightly wound inside the nucleus (Arjun, K., 2011).
Cyanobacteria are similar to plants in that they both perform oxygenic photosynthesis. This means that they both make their own food from carbon dioxide by using energy from the sun and water as electron donors and releasing oxygen as a byproduct. This process begins with the capture of the sun's light energy in the pigment chlorophyll that gives them their green color.
However, the process occurs in different structures. In plant cells, photosynthesis takes place in the chloroplast, small structures that contain chlorophyll and thylakoids. Cyanobacteria don't have chloroplasts. Instead, the chlorophyll is stored in thylakoids in their cytoplasm. The endosymbiosis theory postulates that cyanobacteria may have evolved into the chloroplasts that exist in plant cells today (Gault and Marler, 2009).
This partly explains the colour difference between them; plants are generally green, while cyanobacteria are green-blue. In addition to chlorophyll, cyanobacteria can also accumulate the pigment phycocyanin to give them a blue tint or the pigment phycoerythrin to give them a reddish tint (Gault and Marler, 2009).
Cyanobacteria reproduce asexually via binary fission, fragmentation, or budding. Although some plants are also capable of asexual reproduction—for example, Chlorophytum produces "runners" that are genetically identical to the "parent"—most reproduce sexually (i.e. fertilisation of seeds). However, plants are also able to reproduce sexually through fertilisation. Because of the high degree of variation, organisms cannot be categorized by reproduction methods alone.
Cyanobacteria can convert inert, atmospheric nitrogen into an organic form (e.g. nitrate or ammonia) that other organisms, including plants, can use. 'True plants' are not able to do this. They can only use the organic form of nitrogen and have to rely on man-made fertilisers or form a symbiotic relationship with diazotrophs (nitrogen-fixing bacteria).
- Metcalf and Codd. (2004), Cyanobacterial toxins in the water environment [Report]
- Gault and Marler. (2009). Handbook on Cyanobacteria: Biochemistry, Biotechnology and Applications. Hauppauge, NY: Nova Science Publishers.
- Staley et al. (2007). Microbial Life, 2nd edition. Sunderland, MA: Sinauer Associates, Inc.
- Arjun, K. (2011). Write a Brief Note on the Structure of Cyanobacteria. Preserve Articles.