More Seaweeds: Green Algae

Above a filament of green algal cells dividing. These unbranched filaments were growing on mud (and stones) at the bottom of a salt marsh. This alga is a species of Rhizoclonium, probably Rhizoclonium riparium which is abundant in such habitats. Note the green chloroplasts which occupy much of the peripheral cytoplasm of the cell. Green Algae or chlorophytes generally lack sufficient additional pigments to mask the green color of the chlorophyll in the chloroplasts, but  some species are colored brown due to accessory pigments (e.g. the terrestrial orange-brown Trentepohlia that grows on the bark of trees).

The chloroplasts of Rhizoclonium are disc-shaped and apparently united by connecting strands into a reticulum or network, as is evident in these photomicrographs. Each may contain a single pyrenoid surrounded by a shell formed from two starch grains acting as hemispherical caps enclosing the pyrenoid to form a lens-shaped structure (some of which are visible as bright round granules in the micrographs above). A pyrenoid is a structure containing the all-important enzyme Rubisco (RubisCO or RuBisCO, rubulose bisphosphate carboxylase/oxygenase) the most abundant enzyme involved in photosynthesis. reserve carbohydrates (starch in this case) is deposited near the pyrenoid, both inside the chloroplasts in green algae.

Above: the typical life-cycle of a sexual species of Rhizoclonium. The life-cycle is either asexual, or diplohaplontic in sexual forms as illustrated here. Diplohaplonitc means that there is an alternation of generations between haploid (with n chromosomes) and diploid (with 2n chromosomes) individuals. The life-cycle is also isomorphic, meaning the haploid and diploid individuals are morphologically very similar - both are unbranched (simple) filaments in this case, anchored by a basal rhizoid.

The life-cycle is as follows:

1, 2) One or more individual cells in a diploid filaments (sporophytes) mature into zoosporangia and divide their protoplasts into diploid quadriflagellate zoids (zoospores) which swim around inside the zoosporangium before escaping through a pore (3) in the sidewall (or the end-wall on a cell at the end of a filament. 'Quadriflagellate' means they possess 4 flagella for locomotion. Note the zoids are haploid: they are produced by a meiotic cell division (a reduction division that halves the number of chromosomes) and so are also meiospores. These zoids are pyriform (pear-shaped).

4) The zoid settles and attaches to a substrate and gives rise to a haploid filament (gametophyte, 5). When ripe, one or more cells of the filament develops into a gametangium (6) and its protoplast divides up into a number of haploid sexual zoids or gametes. The gametes have two flagella and so are biflagellate zoids and are more spherical than the pyriform quadriflagellate zoids. The gametes swim around inside the gametangium before exiting through a pore (7).

The gametes are all similar in morphology (isogametes) so there are no distinct male and female gametes and a pair of gametes (even from the same gametangium) may fuse in a process called syngamy (fertilization, 8) in which the protoplast from one cell flows into the other and the nuclei fuse to form a diploid zygote (9).

The zygote germinates into a germling (10) which develops an anchoring cell or rhizoid at the basal end (this cell lacks chlorophyll) and gives rise to a new filament of diploid cells (1) to complete the cycle.

There are no plasmodesmata connecting the protoplasts through the crosswall between adjacent cells. Each cell of Rhizoclonium is a multinucleate compartment (the alga is said to be siphonocladous). Mitosis is closed, involves centrioles as the microtubule-organising centers and the telophase spindle persists (resulting in a dumbbell shaped nucleus). There is no (visible) cytoplasmic streaming in these cells.

Green algae from the bottom of a salt marsh channel. The fibrous form is Rhizoclonium.

Above: The cyanobacterium Oscillatoria often grows with Rhizoclonium.

Chlorophyta (Green Algae)

The biology of green algae, including the well-known green seaweed Ulva lactuca, the Sea Lettuce, is outlined in algal bodies.

The chlorophytes or green algae are the most diverse algal group but it is a paraphyletic group. This means that it has a common ancestor but does not include all of that ancestor's descendants, since green plants also derive from the same ancestor, at least according to molecular studies. Green plants are separated from green algae not just because many of them are better adapted to life on land but because green plants develop from an embryonic stage (and so are also called embryophytes) whereas algae do not have embryos.We say that green plants and green algae are different grades (separated according to their obvious differences) but they belong to the same clade (a group which is monophyletic, that is containing a common ancestor and all of its descendants).

To further clarify the difference between a grade and a clade, mammals form a clade, as far as we can tell: they are all derived from a common ancestor and include all the descendants of that ancestor (it is a monophyletic group). However, reptiles do not form a clade, since mammals and birds are descended from them, so although they have a common ancestor this grouping does not include all of its descendants and so the reptiles form a paraphyletic grade. Note that clades can be nested within one-another: the mammalian and bird clades are nested within the larger clade that includes reptile, mammals and birds. Clades are more useful for exposing evolutionary relationships between organisms, and are the aims of modern taxonomy, but grades are still useful for classifying organisms based on apparent differences we can all observe and so should also remain.

A third type of group also exists in taxonomy: a polyphyletic group. such a group includes organisms that have different ancestors but share some feature in common due either to convergent evolution or because they inherited the same character from an ancestor outside the group. For example, the term 'Jellyfish' is used by zoologists to refer to Scyphozoans, such as the Lion Mane's Jellyfish, but not other jelly-like cnidarians like the Portuguese man o' war which is a siphonophore (and a member of the Hydrozoa) - see jellyfish. However, colloquially it is reasonable to refer to both as 'jellyfish' due to their gelatinous bodies (even though the gelatinous material or mesoglea is more abundant in scyphozoans). Personally, I prefer to use descriptive terms like 'jellyfish' or 'worm' colloquially to describe particular body forms than as precise taxonomic terms. Another example is furnished by the rhinoceroses and elephants, once grouped together as 'pachyderms' due to their thick skin, but they evolved this thick skin independently and rhinoceroses are more closely related to horses than they are to elephants.

Below: the green alga Cladophora.

Cladophora belongs to the same sub-division of the green alga as Rhizoclonium and has certain features in common, such as the reticulum of connected discoid chloroplasts containing pyrenoids encased in a pair of lens-shaped starch grains. Thus the multinucleate cell compartments look similar to those of Rhizoclonium except that the filaments are distinctly branched. This form was found attached to rocks on a rocky coastline. They are temperate and tropical and live either attached to rocks or free-floating. The life-cycle is basically similar to that of Rhizoclonium.

Cladophora vagabunda is a well-studied species (a detailed summary is given by Van den Hoek, Mann and Jahns, 1995. Algae - An introduction to phycology, Cambridge university press).

This alga occurs in intertidal rock pools and salt marsh ponds. The newest lateral branches occur towards the apex of the main filament. The axes, both the main axis and branches, are indeterminate (unlimited); that is they keep growing by cell division at the apex followed by cell elongation until an axis becomes fertile. Once a frond is fertile, the outer cells of the branches mature into zoidangia (gametangia in the gametophyte, zoosporangia in the sporophyte) and growth of the apex terminates. The apical cell swells, and divides into zoids which escape through a terminal pore in the cell wall; other distal cells in the branch similarly produce zoids, each releasing them through a lateral pore in the uppermost side-wall. However, branches may continue to elongate by intercalary growth: that is more basal cells along the filament divide to produce new cells. This can give rise to filaments bearing few branches, although new lateral branches may form.

In unfavorable conditions some cells may develop into thick-walled resting spores or akinetes, which germinate in more favorable conditions to produce a new filament.

Each branch recapitulates the main axis, that is it repeats the pattern: apical growth and formation of lateral branches. Branched rhizoids sprouting from the main axis and the bases of side-branches, anchor the body to the substrate, though in still waters the alga may occur as free-floating masses with branching filaments up to 1 m long.

Single-celled, other filamentous and colonial green algae have been described to some extent in Algal Bodies and in Pond Life. more complex levels of organization includes sheets and tubes of cells, as seen in Ulva and Enteromorpha.

Below: cells in the frond of Ulva. The biology of Ulva lactuca (Sea Lettuce) is outlined in Algal Bodies.

Above: Fucus serratus, a brown seaweed, with the green Ulva lactuca (s.l.) and some red algae (possibly Cerastium). The fronds of Ulva are beautiful bright green membranous leaf-like structures only 2-cell layers thick. The morphology is highly variable, as the frond may be entire or divided up into strap-like lobes. Morphologically this is distinguished by the tubular green fronds of Enteromorpha, below (Enteromorpha clathrata):

As the name suggests, the fronds are inflated gut-like tubes (water has entered the gas-filled tubular frond above, through the cut end and the air bubble makes the hollow nature of the tube very apparent). Enteromorpha intestinalis may reach 60 cm in length and is found in rock pools on the upper shore. Like Ulva, it is attached by a disc-like holdfast. The smaller form shown here was found attached to stones and plants on a slat marsh. Note the short unbranched anchoring rhizoid. This morphological classification of Ulva and Enteromorpha may be useful in the field, but genetic analyses strongly suggest that are not distinct genera and some 'Ulva' are actually 'Enteromorpha' and vice-versa. Individual species are also highly variable in morphology.

Above: Enteromorpha clathrata (= Ulva clathrata).