Cyanophytes

Stromatolitic laminae with probable filamentous cyanobacteria in a block of Windyfield Chert displaying splash textures

Above: Cross section through crenulated 'stromatolitic' laminae. The darker layers (b) contain relic filaments that may represent cyanobacteria (click on picture for a close-up!). This section is taken from the block of Windyfield chert displaying the geyser vent splash texture seen in previous sections (scale bar = 500Ám).

Introduction

Fossil Record

Morphology

 

Introduction

Cyanophytes are an enigmatic group of prokaryotic organisms that comprise what were formerly known as the 'blue-green algae'. Strictly speaking these are only termed 'algae' because they are organisms that can undergo photosynthesis. They are in fact a large and varied group of bacteria, the cyanobacteria, that possess a type of chlorophyll (chlorophyll a) and are able to photosynthesise in the presence of air and sunlight and produce oxygen. Cyanobacteria, unlike chlorophytes, do not possess chloroplasts. In fact the chloroplast in the cells of eukaryotic algae and plants is actually a symbiotic cyanobacterium. In  cyanobacteria the chlorophyll is carried on specialised membranes within the cells called thylakoids (see inset below).

Diagrammatic section through a prokaryotic cell

Above: Diagrammatic section through a prokaryotic cell.

Many cyanophytes are important in the fixation of atmospheric nitrogen. This process can only occur in the absence of oxygen, therefore certain cyanobacteria possess specially thickened cells or heterocysts which contain an anaerobic microenvironment in which this can take place (see inset below). Cyanobacteria may be unicellular or filamentous and may or may not be colonial (see inset below).

Modern filamentous cyanobacterium

Above: Modern freshwater cyanobacteria forming a chain-like colony of single cells, showing occasional larger, thick-walled heterocysts (h) (scale bar = 100Ám).

Modern cyanobacteria live in many different environments ranging from marine to freshwater habitats and also in soil, on rocks and on plants. A few types of cyanobacteria have a symbiotic relationship with fungi forming lichens.

Certain cyanobacteria help to form rigid biogenic structures such as pisoliths and stromatolites. Both are formed by the organisms growing on a particular surface or substrate to which they gradually bind consecutive layers of very fine sediment and precipitated minerals, eventually forming (especially in stromatolites) a stacked sequence of smooth, wavy and crenulated laminae, often forming mounds. Pisoliths usually form by growth around a moveable object such as a sand grain or shell fragment which may be periodically turned over by current action thus creating an almost concentric layer of laminae.

Stromatolites and pisoliths are locally common in the geological record and in certain areas of the world are being formed today such as at Shark Bay in western Australia. Thermophyllic cyanobacteria also produce stromatolitic textures in hot spring areas where they bind detrital grains and precipitated opaline silica to the substrate (see inset below, and section on The Ancient Environment and Modern Analogues).


Cyanobacterial mat on a modern sinter apron

Above: Cyanobacterial mat on a sinter apron (in this case the bacterium Phormidium) in West Thumb Basin, Yellowstone National Park. The dominantly orange colour is due to the presence of  carotenoids in the photosynthetic bacteria. The view is approximately 1 m across.


 

Fossil Record

Cyanobacteria comprise the earliest forms of life, their fossils having been found in Archaean Precambrian rocks in western Australia dated at almost 3500 million years old. Some of the best preserved fossil cyanobacteria have been found in cherts (including the Early Devonian Rhynie chert) such as in the Late Proterozoic Bitter Springs Chert in Australia. In many cases it is the structures formed by these organisms, primarily pisoliths and stromatolites, that are often preserved in the fossil record, particularly in limestones and carbonate rocks. Both are common and locally well-developed, for example, in the Carboniferous limestones of Ireland and Derbyshire in England and also in the Upper Jurassic Purbeck Limestones of southern England.

The Early Devonian Rhynie chert yields a number of fossil micro-organisms and sedimentary textures that may be attributable to cyanobacteria. Most of the latter occur as well-developed stromatolitic structures that are highly comparable with those formed by the growth of cyanobacterial mats on modern sinter terraces and in hot springs (see heading photograph and sections on Chert Textures and The Ancient Environment and Modern Analogues). However, because of the paucity of well-preserved diagnostic features, only a few probable cyanobacteria have been formally described and named by Kidston and Lang (1921b), Croft and George (1959) and D.S. Edwards and Lyon (1983); Archaeothrix contexta, Archaeothrix oscillatoriformis, Kidstoniella fritschii, Langiella scourfeldii, *Rhyniella vermiformis and Rhyniococcus uniformis. Many other unicellular and multicellular forms of probable cyanobacterial origin are also present but for the lack of sufficient diagnostic features have not been formally described and named. 

For the purposes of this resource we will concentrate on the genus described by Kidston and Lang (1921b), Archaeothrix, the morphology of which is outlined below.

*Note: For the species of probable cyanobacteria described by Croft and George (1959), Rhyniella vermiformis, the generic name was already occupied by the collembolan Rhyniella praecursor.

 

Morphology

Archaeothrix

Two species of Archaeothrix were originally described by Kidston and Lang (1921b); Archaeothrix contexta and Archaeothrix oscillatoriformi. Both consist of simple unbranched filaments comprising discoid cells and possessing probable heterocysts. The difference between the two species is mainly in the diameter of the cells. In A. oscillatoriformi the cells range from 3Ám to 4Ám in diameter whereas in A. contexta the cells are narrower being approximately 2Ám in diameter.

Neither of these cyanobacteria are especially common; A. oscillatoriformi has been found within partially decayed stems of Rhynia (see inset right) and A. contexta has been occasionally found as large masses lying loose within the chert matrix.

 

Archaeothrix oscillatoriformi

Above: Filaments of Archaeothrix oscillatoriformi within a stem of Rhynia gwynne-vaughanii (scale bar = 50Ám).