Bacteria are divided into two groups according to how they acquire their energy:

1. Self-sustaining or autotrophic bacteria that produce their energy either by exposure to sunlight or by the oxidation of chemical compounds.
2. Secondary or heterotrophic bacteria obtain their energy either by breaking down organic matter or by binding dissolved compounds from the water mass.

Fermentation and respiratory processes produce energy for bacteria

Under oxygen-free or low oxygen conditions, fermentation reactions such as sulphur reduction and denitrification, i.e. the release of nitrogen, produce energy for bacteria. Although inefficient in energy production, denitrification in particular is a key component of the nitrogen nutrient cycle in aquatic ecosystems.

In the upper layers of the water column, the oxygen situation is usually good. As a result, secondary bacteria can base their energy supply on oxygen-powered respiratory processes. The efficiency of such processes is optimal, where growth and reproduction are most efficient.

Cyanobacteria are known as blue-green “algae” 

Generally when people talk about bacteria in the Baltic Sea, they are usually refer to cyanobacteria, commonly known as blue-green “algae”. These are small, single-celled organisms that produce their energy in connection with sunlight, i.e. they are self-sufficient.

Under favourable conditions, cyanobacteria can grow and multiply rapidly. The excessive growth of cyanobacteria in water is popularly called a blue-green algal bloom. This floating population in open water may be composed of many different species. The species composition can vary both temporally and spatially. In addition to this familiar phenomenon, cyanobacteria can also be found on the seabed, attached to various surfaces.

Spirulina bacteria cover rocks and stones with a velvety mat

When they are attached to the bottom and to aquatic plants cyanobacterial growths can be difficult to interpret as bacteria. For example, Spirulina bacteria form a velvety, aniline-red mat. Such structures can be loosely attached to stones or rocks or to colonies of blue mussels. They may also occur on sand.

Although the upper part of a Spirulina mat is the realm of photosynthetic cyanobacteria, underneath a dense mat may be an anoxic patch, which is riddled with other types of bacteria.

Bacterial colonies of the genus Rivularia are found attached to the seabed or to aquatic plants

Cyanobacterial colonies of the genus Rivularia resemble balls of jelly. They commonly grow in shallow waters and are often attached to aquatic plants. The balls are often greenish in colour. They may form long necklace-like structures either on the seabed or between aquatic plants.

Cyanobacteria colonies of the genus Calothrix form familiar slippery mats at the water’s edge

This slimy green bacterium of the genus Calothrix also belongs to the cyanobacteria. It can be found right at the water's edge, in the form of a fuzzy mat. Although these mats look rough when dry, if they get wet, they become very slippery indeed.

Bacteria of the Baltic Sea

• Cyanobacteria or blue-green ”algae”
• Spirulina bacteria
• Rivularia-family bacteria
• Slime mats (Calothrix spp.)