Station 4: Supergroup Archaeplastida

Red algaes are mainly filamentous or leafy. Besides Chlorophyll a, these have red and blue pigments to give various colors. Red algae are delicate seaweeds and are important sources of gelling agents like Agar.  While this group is related closely to plants it is still distinct from the chlorophytes and plants in a number of ways.  One of the key features missing in this group are flagellated cells. Another difference is that red algae contains red and blue pigments in their chromatophores that are lacking in green plants.

Red algae have a long history of use as a source of nutritional, functional food ingredients and pharmaceutical substances. They are a source of antioxidants including polyphenols, and contain proteins, minerals, trace elements, vitamins and essential fatty acids. Traditionally red algae are eaten raw, in salads, soups, meal and condiments.

Genus Polysiphonia

Polysiphonia is a genus of filamentous red algae with about 19 species on the coasts of the British Isles and about 200 species worldwide, including Crete in Greece, Antarctica and Greenland. Its members are known by a number of common names.

Green Algae include Chlorophytes and Charophytes. These are most common in freshwater. These have Chlorophyll a and b, store food as starch and have cellulose in cell walls. All characteristics are common with plants. Algae lack multicellular sex organs.  They have various forms: unicellular-Chlamydomonas; filamentous-Spirogyra; leafy - Ulva or colonial - Volvox with daughter colonies inside. Land plants originated from charophytes (the clade Charophyta will be covered with the rest of the plants).

Genus Chlamydomonas

Chlamydomonas is a genus of green algae consisting of about 325 species all unicellular flagellates, found in stagnant water and on damp soil, in freshwater, seawater, and even in snow as "snow algae". Chlamydomonas is used as a model organism for molecular biology, especially studies of flagellar motility and chloroplast dynamics, biogeneses, and genetics.

Genus Pandorina

Pandorina is a genus of green algae composed of several cells, held together at their bases to form a sack globular colony surrounded by mucilage. The cells are ovoid or slightly narrowed at one end to appear keystone- or pear-shaped. Each cell has two flagella with two contractile vacuoles at their base, an eyespot, and a large cup-shaped chloroplast with at least one pyrenoid.

The colonies co-ordinate their flagellar movement to create a rolling, swimming motion. Pandorina shows the beginnings of the colony polarity and differentiation seen in Volvox since the anterior cells have larger eyespots.

Charophyta is a group of freshwater green algae. The terrestrial plants (the Embryophyta) most likely emerged within Charophyta, possibly from terrestrial unicellular charophytes.  This is an important group in the understanding of plant evolution.

Charophyta are complex green algae that form a sister group to the Embryophyta. The chlorophyte and charophyte green algae and the embryophytes (plants) form a clade called the Archaeplastida, that is united among other things by:

• the absence of phycobilins (red and blue pigments present in red algaes)

• the presence of chlorophyll a and chlorophyll b

• cellulose in the cell wall

• the use of starch, stored in the plastids, as a storage polysaccharide

The charophytes and embryophytes share several traits that distinguish them from the chlorophytes, such as the presence of certain enzymes, lateral flagella (when present), and, in many species, the use of phragmoplasts in mitosis. 

One of the features that can be seen clearly in Charophytes is a structure called a pyrenoid.  Pyrenoids are sub-cellular micro-compartments found in chloroplasts of many algae, and in a single group of land plants, the hornworts. Pyrenoids are associated with the operation of a carbon-concentrating mechanism (CCM). Their main function is to act as centres of carbon dioxide (CO2) fixation, by generating and maintaining a CO2 rich environment around the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). [You saw this enzyme before in the Phylogenetic Analysis lab because it is used to look at relationships in plants]

In this group there are unicellular and multicellular members.  The unicellular ones are often referred to as Desmids.

Genus Closterium

Closterium is a genus of unicellular charophyte algae and is a member of the desmids.

Closterium cells are crescent-shaped or elongate and lack spines. Some are quite straight and needle-like, while others are much broader with curved ends. The ends of the cell are usually tapered and may be pointed or rounded. Each cell has a two axial, ridged chloroplast with at least one pyrenoid. Occasionally there are more than two chloroplasts per cell. The nucleus is located in the center of the cell between the chloroplasts. 

Terminal vacuoles at the cell tips hold vibrating crystals of barium or calcium sulfate, the function of which are unknown. Brownian motion causes these microscopic particles to move erratically due to the impacts of collisions with the surrounding liquid molecules in which they are suspended.

Genus Spirogyra

Spirogyra (common names include water silk, mermaid's tresses, and blanket weed) is a genus of filamentous charophyte green algae, named for the helical or spiral arrangement of the chloroplasts that is characteristic of the genus. It is commonly found in freshwater habitats, and there are more than 400 species of Spirogyra in the world. Spirogyra measures approximately 10 to 100 μm in width and may grow to several centimeters in length.

Genus Nitella

Nitella, one of the genera of charophyte green algae, grows submerged in water. Nitella is a multicellular algae and can be confused with plants but lacks many of the features of plants (true roots, leaves, vascular tissue, flowers, etc.)  

Nitella’s root-like rhizoids, which attach the plants to the muddy substrate, are colorless and well-developed. The shoot-like central axis or stipe is ridged with regularly-spaced nodes or joints.  Nitella species are typically deep, translucent green, and very smooth and delicate.