MISCELLANEOUS PUBLICATION 955, U.S. DEPT. OF AGRICULTURE 



some form of reserve food, like starch grains, sugar crystals, or oil 

 droplets. 



Pigmentation and the nature of the photosynthate, or kind of re- 

 serve food, are usually correlated in the various groups of algae. Fre- 

 quently pigmentation is used as a basis for dividing algae into the 

 larger categories of classification. On this basis there are blue-green, 

 grass-green, yellow-green, brown, and red algae, and all contain 

 chlorophyll, carotene, and xanthophyll. All these groups, except the 

 grass-green algae, have additional pigments that mask the green of the 

 chlorophyll. Other groups of algae contain the euglenoids, crypto- 

 monads, and dinoflagellates. 



Generally seven or eight algal groups are recognized by taxonomists, 

 and these groups currently are considered to be of "division" rank. 

 In all, there are probably about 18,000 described species. A good 

 taxonomic review is that by Papenfuss (#4), and detailed references 

 to taxonomy and morphology may be found in Smith (27) and Fritsch 



Algae are of considerable practical importance. The free-floating- 

 forms, or phytoplankton, occur in great numbers and are the main 

 source of food, directly and indirectly, for fish and other aquatic 

 animals. Since oxygen is a byproduct of photosynthesis, algae supply 

 oxygen to their aquatic environs during the day. However, if they are 

 present in great numbers, they can be undesirable. Pond scum and 

 red tide are examples, as well as slime accumulations at drinking 

 fountains, in reservoirs, and at swimming pools. 



The diatoms, members of the division or phylum Chrysophyta, are 

 of interest because of their siliceous cell walls, or shells. They occur 

 in large numbers in both fresh and salt water, and when they die their 

 shells fall to the bottom of the lake or ocean. Great beds of these shells 

 have been built up and are called diatomaceous earth. Diatomaceous 

 earth has several uses, such as a filtering agent, insulating material, 

 polishing powder, and a carrier for nitroglycerin in dynamite. 



The most important groups of algae economically are the seaweeds, 

 consisting of the brown algae with their giant kelp and of the red 

 algae with their agarophytes, or agar producers. Although seaweeds 

 have been used as sources of soda ash, potash, iodine, and food, their 

 chief importance today arises from the phycocolloids that can be 

 extracted from them. According to Idson (15), U.S. annual output 

 of seaweed colloids increased from $2 million in 1945 to about $10.3 

 million in 1955. Gross sales for the industry are expected to be $20 

 million per year by 1975. 



The three commercial seaweed colloids are agar, carrageenin, and 

 algin. Several others, including fucoidan, laminoran, hypnean, 

 eucheuman, furcellaran, agaroid, porphyran, and iridophycan, have 

 little or no known commercial importance as yet, although some are 

 said to be easily obtainable. Agar and carrageenin come from red 

 algae, principally Gelidium and other genera for the former and 

 Ghondrus for the latter. Algin is obtained from brown algae, or kelp, 

 and a major source is members of the genus Macrocystis. 



An extensive treatment of the phycocolloids and their uses can be 

 found in the book edited by Whistler and BeMiller (28), in which 

 several chapters are devoted to products from red and brown algae. 



