SOME NEW AND INTERESTING SPECIES OF TREBOUXIA, A GENUS OF LICHENIZED ALGAE

AHMADJIAN, VERNON. (Clark U., Worcester, Mass.) Some new and interesting species of Trebouxia, a genus of lichenized algae. Amer. Jour. Bot. 47(8): 677-683. Illus. 1960. Four species of the phycobiont Trebouxia, 3 of which are new, have been described in this paper. On the basis of this study, the algal genus has been divided into 2 main groups depending on the position that the chromatophore assumes during its bipartitions and on the shape of the cells. Growth rates have been calculated and growth curves drawn for the various species. In some cases, a physiological difference was shown between morphologically identical forms. In several instances, the same algal symbiont was isolated from widely separated lichen species, a fact which is contrary to earlier claims of the high specificity of the lichen fungus to its algal component. THE GENUS Trebouxia is by far the most common phycobiont of lichens. This algal genus is found in lichens which exist under practically all the various environmental conditions of nature, from the frozen wastes of the polar regions to areas of persistently high temperature: it is an algal genus found in both freshwater and maritime lichens. Trebouxia is an extremely hardy alga, able to withstand wide ranges of temperature and prolonged periods of desiccation (Mish, 1953; Lange, 1953; Ahmadjian, 1958). Despite the frequency of its occurrence in lichens, the cytological details regarding this genus are markedly few. The species of Trebouxia described by earlier workers such as Chodat (1913), Waren (1918-'19), and Jaag (1929) have been distinguished, largely, on the basis of such variable, inconstant characteristics as the color, size, shape, texture and growth intensity of the colonies while growing on organic nutrient agar media. Although these characteristics are indeed important in some cases, they should not, for the most part, be utilized as primary distinguishing attributes but rather should supplement observations of cellular organization. A good example of the extreme to which the nomenclature of lichen algae can be carried because of slight colony shape differences presented by individual clones grown on a nutritive organic agar medium is shown by Werner (1954), viz.: "Cystococcus (Eucystococcus) xanthoriae parietinae Let. em. R.G. Werner:-Cells subspherical, attaining a diameter of 22-25,u. ssp. rugosus R.G. Werner: acervuli rugosi. var. humilis R.G. Werner: acervuli complanati. 1 Received for publication February 1, 1960. Portion of a thesis submitted to the Graduate School of Arts and Sciences, Harvard University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. For her assistance with the mathematical computations, I thank Mrs. June M. Ahmadjian. For their help with the Latin descriptions of the new species. I would like to thank Dr. I. Mackenzie Lamb, Director, Farlow Herbarium, Harvard University, and Dr. Harry Smith, Institute of Systematic Botany, Uppsala University, Uppsala, Sweden. The help of Dr. Harold C. Bold in the preparation of this paper is gratefully acknowledged. var. vermiculatus R.G. Werner: acervuli plus minus vermiculati. var. crispus R.G. Werner: acervuli passim crispi. var. emarginatus R.G. Werner: acervuli marginibus dificientibus. ssp. squamosus R.G. Werner: acervuli saepius squamosi. ssp. minimnus (Waren) R.G. Werner: acervuli minutissimi, plani laevesque, sed in aetate tuberosi." All the above mentioned algal forms were isolated from specimens of the lichen Xanthoria parietina. The practice of naming an algal symbiont for the lichen from which it is isolated, for example, Cystococcus (=Trebouxia) xanthoriae parietinae, should be discontinued. The only reason for doing it earlier was that each lichen was supposed to possess its own specific alga. This belief has now been disproved (Raths, 1938; Zeitler, 1954; Ahmadjian, 1959b), and any continuation of this practice would create a hopelessly confused nomenclature. Since the algal symbionts of lichens are generally regarded as genetically independent organisms, they should be treated as such. MATERIALS AND METHODS.The isolation of lichen algae and the methods and media used in their investigation have been discussed in an earlier paper (Ahmadjian, 1959a); thus, only a brief description of the procedural techniques and materials is given here. The algae were isolated from the lichen thallus by means of a micropipette. Each isolate, which consisted of a single cell, was trans. ferred through successive drops of sterile water on a slide until it was free from surrounding contamination, and then inoculated onto an agar medium containing organic nitrogen and carbon compounds. The media generally used for the growth of the colonies were glucose-asparagine agar (pH 6.10), glucose-peptone agar (pH 6.70)2 and a stock soil2 These media were used by the earlier investigators (Chodat, 1913; Waren, 1918-'19) of lichen algae and found to encourage rapid growth of these normally slow-growing algal forms. The descriptions of the many new species made by these workers were based on the size and shape of the algal colony formed on the types of solid, nutritive organic media listed above.