Diversity Of Soil Organisms – Part 4

Bacteria – Actinobacteria

Actinobacteria is a phylum of Gram-positive bacteria that have a highly diverse morphology, ranging from micrococci (spherical) and rods to branched filaments that resemble fungal hyphae. The bacterial filaments are narrow (diameter from 0.5 to 2 μm) and can be short and rudimentary or extensively branched. Throughout their life cycles, Actinobacteria may combine these different forms. Their reproduction is by fragmentation of hyphae or through the production of spores.

The ecological niche of most Actinobacteria is the aerobic zone in soil. A striking feature of Actinobacteria is the production of extracellular enzymes that degrade complex macromolecules commonly found in soils (e.g. casein, starch, chitin, cellulose and lignocellulose). Furthermore, they synthesise and excrete thousands of metabolites, such as antibiotics. For example, Selman Waksman, one of the most important soil microbiologists, won the Nobel Prize for Medicine in 1952 for his discovery of streptomycin produced by bacteria of the genus Streptomyces. In addition to streptomycin, Streptomyces are capable of producing a wide variety of antibiotics with numerous properties: antibacterial, antifungal, antiviral, antitumor, antiparasitic, insecticide and weed controlling.

Actinobacteria also includes the nitrogen-fixing bacteria of the genus Frankia, which form root symbioses with plants of eight botanical families. Other species belonging to the genera Streptomyces and Corynebacterium are plant pathogens. Animal pathogens are found among other genera and among them, the Mycobacterium avium-intracellulare-scrofulaceum stands out as being lethal for people who have contracted the human immunodeficiency virus (HIV).


Cyanobacteria is a group of bacteria that are able to obtain their energy through photosynthesis. This is possible due to the presence of chlorophyll, being photosynthetic, they manufacture their own food. They are considered one of the most diverse groups of prokaryotes as they vary from unicellular to complex filamentous or branched forms. In some cases they have highly differentiated cells that carry out different functions, so they may be considered as truly multicellular organisms.

Cyanobacteria have the distinction of being the oldest known fossils, more than 3.5 thousand million years old, in fact. The cyanobacteria have been tremendously important in shaping the course of evolution and ecological change throughout Earth’s history. Indeed, the atmospheric oxygen that we depend on was generated by numerous cyanobacteria through photosynthesis.

Furthermore, the photosynthetic structure of plant cells, the chloroplast, evolved from cyanobacterial ancestors. Cyanobacteria also contribute to the health and growth of many plants in another way: they have the ability to convert inert atmospheric nitrogen into ammonia (nitrogen fixation) that plants can use. This process cannot occur in the presence of oxygen, so nitrogen is fixed in specialised cells called heterocysts. These cells have an especially thickened wall that contains an anaerobic environment. Cyanobacteria also form symbiotic relationships with many fungi, forming complex symbiotic organisms known as lichens.

Why does the air smell of soil after rain?

  • The earthy smell after it rains is linked to Actinobacteria.
  • In particular, the molecule responsible for the aroma is known as geosmin.
  • Geosmin is produced by the Gram-positive bacterium Streptomyces, a genus of Actinobacteria, and released when these microorganisms die.
  • The human nose is extremely sensitive to geosmin and is able to detect it at very low concentrations.
  • Geosmin is also responsible for the earthy taste of beetroots.


Ref: Global Soil Biodiversity Atlas p37