Distinguishing bloom-forming Species

Differences in colour and morphology are visible to the naked eye, but many cell and reproductive characteristics on which species identifications are based require microscopic examination.


The red, brown and green colour is the first characteristic used for the identification of cyanobacteria and algae. It is reliable and identifiable but care must be taken with some cyanobacteria and red algae which may resemble a brown alga when sunlight degrades the red pigment, phycoerythrin, to a brown colour. Algae stranded and dehydrating on the shore should never be used for identification as the dominant pigment often degrades to reveal the underlying chlorophyll pigment before it also degrades.

Thallus morphology

The thallus is the relatively simple body or growth form of a cyanobacterium or alga. Algae exhibit an enormous diversity in morphology, but bloom-forming species typically have either a sheet-like (1) or filamentous thallus (2-6).

Filamentous thalli

Filaments may be unbranched or threadlike (2) or branched (3-6). Without a microscope it is often difficult to determine whether thalli of fine filaments are a mass of tangled unbranched filaments or branched filaments. Morphology is a less reliable character when large thalli, produced during blooms, detach and are fragmented by waves.

Branched filaments

Branching patterns are varied and can include:

  • many branches from all sides of the main axes (3)
  • branched at long intervals (4)
  • repeatedly forked into two equal branches (5)
  • with branches mostly from one side of the main axes (6).

Cell structure

Cyanobacterial and algal thalli are composed of cells which can only be seen under a microscope. Various species have filaments composed of either a single row or many rows of cells. Sheet-like thalli of Ulva species are composed of two layers of similar looking cells.

Reproductive structures

The different cyanobacterial and algal groups reproduce in different ways and the structures they produce can be used to further differentiate species. However, microscopic examination is necessary to visualise the reproductive structures and therefore will not be discussed in this brochure.