Tuesday, August 14, 2012

Green Algae

The school where I work has an outdoor classroom that invites nature into our urban schoolyard and creates a peaceful outdoor space for students to learn about life.  We have trees, wildflowers, and a pond, each hosting their share of associated organisms.  I'll be doing some nature blogging about the outdoor classroom for use by teachers at our school, and I'm posting the first outdoor classroom blog post here:

Welcome!  I'm excited about starting an outdoor classroom blog.  I can't wait to see what we find in our little urban nature oasis.  The subject of our first post is green algae.

If you look under the surface of the pond, you see a very busy ecosystem indeed!  There are plants, fish and insects, and those are just the visible organisms.  There are way more microscopic organisms than big ones, which I'll save for a future post.

Green algae growing just below the surface of our pond.
The strangest macroscopic (big enough to see) organism is the filamentous green alga that forms clouds of soggy green cotton candy.  But what on earth are green algae?  They seem a lot like plants: they photosynthesize, they have cell walls, and they have chloroplasts (green structures that photosynthesize).  There are also some features of green algae that we don't usually associate with plants: they live entirely under water; they don't form roots, leaves or stems; and some of the microscopic ones can swim!  Scientists have wavered a bit about whether algae are actually plants or protists.  The discovery of how to sequence DNA has allowed algal geneticists to confidently classify green algae as plants, though the other colors of algae (red, brown and blue-green) are not classified as plants.  I have had to re-learn my green algae taxonomy!

Interconnected strands of green algae pulled up from under the surface.
Our green alga (alga, hard g, is singular, and algae, soft g, is plural) is a filamentous type, meaning it grows in long strands.  The filaments (strands) of green algae are only one-cell thick, which is surprising considering how tough they are.  Each algal cell is cylindrical like a soup can, and the filament is arranged like an infinite strand of soup cans glued end to end.

Green algae out of the water.
The color of green algae comes from the pigment chlorophyll, just like in other plants.  Chlorophyll is the molecule that can catch light to allow plants to use its energy to build food.  Green algae cells contain structures called chloroplasts that hold the chlorophyll plus all the other machinery needed to conduct photosynthesis.  All plants' cells contain chloroplasts.

I looked at the algae from our pond using a microscope that magnified what I saw by a factor of 100.  In the picture below, you can see the cell wall between adjacent algal cells, just above the pointer.  Cell walls are rigid structures made of cellulose (a strong, rigid molecule), and they give plant cells their shape.  Paper is made by starting with plant material and getting rid of everything but the cellulose, meaning that paper is essentially squished, dried plant cell walls.
The pointer rests on a filament of green algae.  A broken alga releases its cell contents. 100x
Another interesting thing in the picture above is the broken cell.  A chloroplast is slipping out of the broken plant cell in the center of the picture.

Below is another view under the microscope, with one normal filament and one filament with shrunken cell contents.  The chloroplasts and other structures have been compressed into a central structure in each cell.  The strange filament may be undergoing reproduction or it could be stressed, but either way, you can see the beautiful cylindrical shape of each individual cell.

Shrunken cell contents in the lower filament allow you to see the cell walls.  The round object is an air bubble. 100x
In our pond, the algae seem to be growing quickly.  There is ample sun for food, plus decomposing leaves and insect/fish excrement providing nutrients, the equivalents of vitamins in our food.  When you see a pond with lots of algae in it, you can assume that there are a lot of nutrients in the water, either from natural sources or from pollution.

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