Algae Part I

OTHER ALGAL GROUPS

The green algae are an ancient group of aquatic photosynthetic organisms which gave rise to the land plants. Nowadays, taxonomists consider other algae to be protists rather than plants. The Algae are therefore viewed as an artificial assemblage. There are thought to be about 23,000 species of algae.

The names of the algal groupings used here are traditional botanical groupings which have now been overturned by modern molecular studies.

Click here for a discussion on where algae sit in various classification schemes.

Algae have traditionally been classified on the basis of:-

colour (photosynthetic pigments)
storage material
flagella
cell wall

Molecular approaches focussing on plastid evolution are providing new insights into Algal Evolution but will only be touched on in this course.

Cyanobacteria

These are blue green bacteria which were once called blue green algae. We will not study them in this part of the course. Click here to go on the web and see photomicrographs of a range of blue-greens. As discussed before, these are not even eukaryotes. The fact, however, that they are microscopic and photosynthetic has traditionally led to their receiving consideration in courses like this. Cyanobacteria were probably the first photosynthetic organisms and have been around for about 3.5 billion years. They exist as unicells, colonies or filaments. They have a bacterial type cell wall, no nucleus and no flagella. In addition to bacterial chlorophyll, they have the pigments phycocyanin and phycoerythrin. Blue-greens are responsible for nitrogen fixation on land and in water. Like green algae, they can form lichen symbioses with fungi.

For us, the most significant thing about blue greens is that the first plant cells on earth evolved by the engulfing of blue greens by primitive unicells and these trapped blue-greens in turn became the plastids we know today. This is the endosymbiont hypothesis.

Chlorophytes

These are closest to the land plants and are considered their ancestors as they share
common features:-

photosynthetic pigments - chlorophylls a & b, ß-carotene
cell wall - cellulose-rich
reserves - starch

This is a very diverse group, showing almost the full spectrum of morphological possibilities - from unicells to macroalgae.
In this course, we will look at these in detail.

Algal images courtesy of Dr. Morgan Vis (see his Ohio University Algae Home Page)

Euglenophytes or Euglenids

Euglena is probably the best-known member
of this group of unicellular flagellates.
These Euglen(o)ids resemble green algae in their
photosynthetic pigments (chlorophylls a & b,
ß-carotene) and probably got their plastids second hand from green algae. They differ in that they

Euglena photo used with permission of
Dr. Richard E. Triemer.

store paramylon (a ß1,3 glucan) and fat
lack a cell wall but have a pellicle (protein plates underly the cell membrane)
can be heterotrophic (hence these organisms were once studied by zoologists!)

Euglena can cause algal blooms.
Check out the Euglenoid Project for lots of information on this group.

Phaeophytes

These are the brown algae.

Sargassum on an exposed shoal

These are almost all marine macro-algae.
They contain clorophylls a & c and the brown carotenoid fucoxanthin.
Their reserve, a ß1,3 glucan called laminarin, is stored outside the plastid.

Chrysophytes

These are the golden-brown algae. Link this group in your mind to the brown algae. They share many of their features (same pigments, same storage reserves) but occupy a different niche - they are microscopic and planktonic unlike the browns which tend to be attached seaweeds. In fact, today taxonomists lump the brown and golden-brown algae in a grouping termed the Heterokont Algae.

Diatoms are probably the most
important members of this group- responsible for
20% of global CO₂ fixation.

They have an outer case or frustule made
of silica and are motile.

Click here for a great introduction to diatoms or click below for more details on this group

http://www.bgsu.edu/departments/biology/facilities/algae/index.html
http://hjs.geol.uib.no/diatoms/index.html-ssi
http://www.euronet.nl/users/janpar/diat.html

Award winning picture of a marine diatom by
Wim van Egmond

Rhodophytes

These are the red algae and are mainly marine macro-algae (seaweeds). They have;-

in addition to chlorophyll a, they have red phycoerythrin and sometimes blue-green phycoerythrin, pigments found in blue green bacteria
distinctive plastids without stacked thylakoids
no motile stage (male gametes have no flagella)
as reserve a form of starch (floridean starch)
a complex life cycle of 3 phases (not to be considered in this course)

Red algae have the distinction with the green algae of being one of the oldest algal groups. See Algal Evolution. Some modern classification systems retain the red algae alongside the green algae within the plant kingdom.

Pyrrophytes

This is a diverse group of reddish-brown micro-algae. They inhabit both freshwater and marine habitats.
The most important group within the Pyrrophytes is probably the dinoflagellates or dinophytes.

Dinoflagellates;-
are a weird and wonderful group. Much of the planktonic bioluminescence in the sea is due to these organisms.

they contain chlorophylls a and c₂and carotenoids beta carotene as well as the unique carotenoid peridinin (but half are non-photosynthetic and heterotrophic)
their plastids come from several lineages and they even temporarily use plastids ("kleptoplasts") from organisms they ingest
half are "armoured" with cell wall polysaccharides below the cell membrane (akin to the proteinaceous pellicle of Euglenoids)
they have 2 flagella, one trailing, the other beating transversely, producing a spinning motion like a top; they go through a coordinated daily vertical migration through the water column.
their chromosomes are visible at all stages (not just at cell division like most organisms)
many form symbiotic associations with marine organisms (e.g. corals)
they are responsible for the algal blooms known as "red tides"
"ciguatera poisoning" which can result from eating large fish like barracuda in the northern Caribbean is caused by a dinoflagellate toxin which has accumulated in the fish flesh