Chlorophytes

Unicells in a thin section of Rhynie Chert

Above: A number of unicells in Rhynie chert. Some of these displaying cell contents may represent unicellular eukaryotic algae (scale bar = 50Ám).

Introduction

Fossil Record

Morphology

 

Introduction

Chlorophytes are a particular division of eukaryotic algae comprising the green algae. They are characterised by containing two types of chlorophyll, a and b and starch is formed in chloroplasts (see inset below). Green algae may be unicellular but can also form complex multicellular structures such as that seen in the stoneworts or charophytes (see section on charophytes). These non-vascular plants are typically found in freshwater environments.

Diagrammatic section through a eukaryotic cell

Above: Diagrammatic section through a single eukaryotic cell.

 

Fossil Record

Chlorophytic algae are among the oldest known fossils, being recorded from Precambrian rocks primarily from the Ediacara fauna. These Ediacaran fossils are acritarchs and were probably formed by unicellular algae. In fact the discovery of undoubted chlorophytes in the 850 million year old Late Proterozoic Bitter Springs Chert in Australia was the first ever evidence of early eukaryotes. Multicellular chlorophytes are first seen in Cambrian strata where they helped to build algal reefs. Very well preserved chlorophytes are also known from the famous Middle Cambrian Burgess Shale in British Columbia, Canada.

Apart from 'charophytes', a number of other chlorophytes have been found in the Rhynie chert including unicellular and filamentous types. Of these, to date, two filamentous types have been formally described by D.S. Edwards and Lyon (1983); Mackiella rotunda and Rhynchertia punctata. The morphology of these simple green algae is outlined below.

Note: It must be remembered that the affinities of many fossil algae, including those found in the Rhynie chert remain uncertain. This is primarily because modern classifications of algae are based on biochemical and ultrastructural features which are rarely or indeed never preserved in the fossil record (D.S. Edwards & Lyon 1983). Also, positive identification may be compounded by the variable preservation of cell contents.

 

Morphology

Mackiella rotunda

This alga comprises unbranched filaments up to 850Ám in length forming an unattached thallus with no rhizoids. The filaments consist of up to 25 cylindrical cells, each cell typically being more-or-less equal in length and in width (23Ám to 41Ám and 28Ám to 40Ám respectively). The terminal cells of the thallus are rounded and slightly longer than the other cells (29Ám to 49Ám in length). The cell walls are thin and do not show a mucilaginous sheath. The cell contents comprise fine granular material and a dark spherical body, 3.5Ám in diameter, interpreted by D.S. Edwards and Lyon (1983) as a pyrenoid or the chloroplast.

Mackiella was a eukaryotic alga. Because the alga shows unattached filaments that may fragment into short filaments and the cells possess a single pyrenoid, D.S. Edwards and Lyon (1983) tentatively assigned Mackiella to the extant order Ulotrichales.

 

Rhynchertia punctata

This alga consists of an unbranched, unattached thallus comprising multicellular filaments of cells 8Ám to 17Ám in width and commonly twice that in length. The terminal cells of the thallus are rounded and slightly shorter than the other cells. The cells are thin walled and do not show a mucilaginous sheath. The cell contents may be uniform, or with a single dark body (possibly a chloroplast) or may contain many small ovoid bodies. The latter have been interpreted to be reproductive elements, possible zoospores or gametes (D.S. Edwards & Lyon 1983).

Rhynchertia filaments have been found in association with the nematophyte Nematoplexus, primarily within 'gelatinous' areas where the latter appears to have degraded (Lyon 1962). Based on gross morphology, this alga has also been tentatively assigned to the Ulotrichales by D.S. Edwards and Lyon (1983).