Examining freshwater algae
Microscopes & microscopy
Examining fresh material
Observations (preferably including drawings or photographs) based on living material are essential for the identification of some genera and a valuable adjunct to more leisurely observations on preserved material for others. The simplest method is to place of drop of the water including the alga onto a microscope slide and carefully lower a coverslip onto it. It is always tempting to put a large amount of the alga onto the slide but smaller fragments are much easier to view under a microscope. Start by observing the algae at lower magnification (X40 or X100) and move sequentially up if necessary. Microalgae may be better observed using the 'hanging drop method': place a few drops of the sample liquid on a coverslip and turn it over onto a ring of paraffin wax, liquid paraffin or a 'slide ring'.
A permanent slide is a valuable addition to wet and dry herbarium specimens. Analine blue (1% aqueous solution with 4% molar HCl), Toluidine blue O (0.05% aqueous solution) and Potassium permangenate (2% aqueous KMnO4) are useful stains for macroalgae (different stains suit different species) and Indian Ink is a good stain for highlighting mucilage and some flagella-like structures.
After staining for 30 seconds to five minutes (depending on the material), rinse in water, then add a drop or two of 10% corn syrup solution (Karo™ corn syrup with the addition of 2% phenol) to a small piece of the algae placed on a microscope slide then carefully lower the coverslip. (A corn syrup solution of 5% or less may be required for the more fragile species.) Add drops of 40% corn syrup solution at the side of the coverslip as the liquid underneath the coverslip evaporates. Once ‘set’ (i.e. solid but often still sticky), the sides of the coverslip can be readily sealed with nail polish.
Glycerine solution (75% glycerine, 25 % water) is another useful mounting agent and should be introduced in a similar manner to the corn syrup, i.e. starting with a very dilute solution and building up to 100% glycerine. Sealing with nail polish is essential.
These two mountants are unsuitable for most unicellular algae which should be examined fresh or in temporary mounts of liquid-preserved material.
Magnifications of between 40 and 1000 times are required for the identification of all but a few algal genera. A compound microscope is therefore an essential piece of equipment for anyone wishing to discover the world of algal diversity. Student microscopes with 10X eyepiece and 4X-10X-40X objectives are available for $410-$520 and such a microscope would be suitable for identifying all algae in this guide. An oil immersion 100X objective, available for c. $90, would be a useful addition, particularly when identifying to species level. A camera lucida attachment is helpful for producing accurate drawings while an eyepiece micrometer is important for any species-level identifications. Phase-contrast or interference (e.g. Nomarski) microscopy can improve the contrast for bleached or small specimens, but are available only on more expensive microscope systems.
A dissecting microscope providing magnifications up to 40 or 50 times is a useful aid but is secondary to a compound microscope. High quality dissecting microscopes cost between $1500 (without light source) to fully integrated systems with a built-in light source at about $3500. Dissecting microscopes costing less than $1500, either of lower quality or with a reduced range of magnifications, may be suitable for some purposes. An adequate 20X or 40X system can be bought for $250-$320.
Scanning and transmission electron microscopes are beyond the reach of all but specialist institutions but are an essential tool for identifying some of the very small algae. None of the algae illustrated here require electron microscopy for identification.