Botanic Gardens Trust, Sydney, Australia


Preserving freshwater algae

Storage and preservation

Algae can be stored initially in a bucket, jar, bottle or plastic bag, with some water from the collecting site. The container should be left open or only half filled with liquid and wide shallow containers are better than narrow deep jars. Note that glass is reportedly not satisfactory for some Chrysophyta and other algae of acidic waters due to its inherent alkalinity damaging cells. However, glass phials are commonly used to collect algae. If refrigerated or kept on ice soon after collecting most algae can be kept alive for short periods (a day or two). If relatively sparse in the sample, some algae can continue to grow in an open dish stored in a cool place with reduced light (traditionally a south-facing window in the Southern Hemisphere).

For long-term storage, specimens can be preserved in liquid (see below), dried, or made into a permanent microscope mount (preferably all three). Even with ideal preservation, examination of fresh material is sometimes essential for an accurate determination. Motile algae particularly must be examined while flagella and other delicate structures remain intact.

Liquid preservation

Commercial formalin (which is a solution of 40% formaldehyde), diluted between 1/10 and 1/20 with the collecting solution, is the most commonly used fixative. Note that formaldehyde is thought to be carcinogenic and all contact with skin, eyes and air passages should be avoided. FAA (by volume, 40% formaldehyde 1: glacial acetic acid 1: 95% alcohol 8: water 10) or 6-3-1 (by volume, water 6: 90% alcohol 3: 40% formaldehyde 1) solutions give better preservation results for some of the more fragile algae, whereas the standard alcohol and water mix (e.g. 70% ethyl alcohol or industrial methylated spirit) will ruin all but the larger algae.

Algae can be kept in diluted formalin for a number of years, but the solution is usually replaced by 70% ethyl alcohol with 5% glycerin (the latter to prevent accidental drying out).

Lugol's solution is commonly used for short-term (e.g. a few months, but possibly a year or more) storage of microalgae. Dissolve one gram of iodine crystals and two grams of potassium iodide in 300 ml of water. Use three drops of this solution in a 100 ml sample (it should look like very weak tea).

Dried herbarium specimens

Dried herbarium specimens can be prepared by 'floating out' similar to aquatic flowering plants. Ideally, fresh specimens should be fixed prior to drying. Most algae will adhere to absorbent herbarium paper. Smaller, more fragile specimens or tangled, mat-forming algae may be dried onto mica or cellophane. After 'floating out', most freshwater algae should not be pressed but simply left to air dry in a warm dry room. If pressed, they should be covered with a pieces of waxed paper, plastic or muslin cloth so that the specimen does not stick to the drying paper in the press.

To examine a dried herbarium specimen add a few drops of water to the specimen. After a minute or so the specimen will swell and lift slightly from the paper. Carefully remove a small portion of the specimen with forceps or a razer-blade.