PERIPHYTIC DESMIDS IN CORUMBÁ RESERVOIR , GOIÁS , BRAZIL : GENUS Cosmarium CORDA

With the objective of investigating the taxa of Cosmarium, four collection stations along the Corumbá Reservoir were set up. This resulted in the first work done on the genus in the State of Goiás. The collections of the material, of the epilithon type, were carried out in July 1996 and in the period between December 1996 and July 1997. The samples were preserved with 0.5% acetic lugol. The qualitative analysis was done by optical microscopic on non-permanent slides. Twenty-one taxa of the Cosmarium were described, each one illustrated by means of a camera lucida attachment.


INTRODUCTION
The genus Cosmarium (family Desmidiaceae) is one of the largest of those belonging to Placoderm desmids.The characteristics of the specimens such as cell measurements, morphology, length/width relationship, open or shut sinus, amount of pyrenoids, wall forms and presence or not of granules, and lateral and apical view are important for identifying the species and the varieties of populations of Cosmarium.
In their heredity and evolution, many species of Cosmarium have demonstrated a considerable amount of instability in genetic composition.This has resulted in a sometimes confusing variability of morphological details (Prescott et al., 1981), because the genus is probably polyphyletic in origin and any grouping of species is artificial, and because its very numerous taxa are distinguished by illdefined features (Croasdale & Flint, 1988).This is the first work on taxonomic desmids, specifically of the genus Cosmarium, in the State of Goiás.Thus, the purpose of this work was to investigate the taxa of these species in the periphytic community of the Corumbá Reservoir, and to con-tribute to the understanding of the taxonomy and geographic distribution of desmids.

Study area
The Corumbá Reservoir presents a 65 km 2 flooded area of elongated shape and is located in the State of Goiás, having coordinates 17 o 59'S and 48 o 31'W (Fig. l).
The collection stations were distributed along the reservoir, including its lotic, transitional, and lentic areas (Fig. l).

METHODOLOGY
The collection of periphyton (defined as the community of microorganisms living on surfaces of objects submersed in water, in accordance with Wetzel, 1983), of the epilithon type (growing on hard, relatively inert substrata, such as gravel, pebbles, cobbles, and boulders, in accordance with Stevenson, 1996), was done in the littoral zone, in July 1996 (river phase) and in the period from December 1996 to July 1997 (reservoir phase).
The samples were conditioned in a 150 ml recipient, previously humidified with distilled water and placed in ice.In the laboratory, through scraping the substratum, the periphytic material was removed, fixed, and preserved with acetic lugol at 0.5%.
For qualitative analysis of the taxa belonging to the genus Cosmarium (preserved with 0.5% acetic lugol), an average of 10 non-permanent slides per sample were prepared and a 40 and 100X binocular microscope was used.The drawings were obtained through the Nikon 100X binocular microscope, with camera-lucida attachment.Taxonomic comments were made, whenever necessary.Thus, morphologic character descriptions and/or drawings were made only when analyzed by the microscope.Cell measurements (in µm) are indicated as follows: W = width, L = length, and a 10 µm scale is used for the figures are equivalent.
The samples are deposited in the herbarium of the Universidade Estadual de Maringá (HUM) numbering from 8054 to 8089, corresponding to the order in which the 36 samples were collected.

RESULTS AND DISCUSSION
A total of 21 species of Cosmarium were registered for the Desmidiaceae family.According to the taxa surveyed of desmids, Cosmarium was the most abundant genus.The second most common was Staurastrum, with 15 species (Felisberto & Rodrigues, 2002).

Cosmarium baileyi
Cells about as wide as long or wider than long, 28.97-47.27 W, 28.71-44.37 L, 7.83-16.44W at isthmus; median constriction deep with sinus open at the isthmus and shut or slightly closed in the area distal.Semicells 3-lobed with a subrectangular to subcircular form, with prominent and truncate apex and rounded margins with a granulated protrusion in the midregion; in lateral view semicell ovate, with a protuberance on each side near the base; in apical view cell elliptic to oblong; cell wall embellished with granules; chloroplast with 2 pyrenoids.
This taxon was identified to conform with Prescott et al. (1981), who distinguished C. protractum from C. ornatum.Thus, species with prominent apex in which an apical lobe is formed are assigned to C. protractum, whereas those with reniform or subreniform shapes, with apex only slightly elevated, are classified as C. ornatum.
Cosmarium punctulatum Brébisson 1856 var.punctulatum (Fig. 16) Cells 1.07-1.14times longer than wide, or as long as wide, 16.96-20.88 W, 18.27-23.49 L, 5.22-9.13W at isthmus; median constriction deep, sinus closed; semicells oblong to trapezoidal, margins undulates, rounded basal and apical angles and with truncate apex; semicells elliptic in apical view, and semicircular in lateral view; cell wall granulate; chloroplast with 1 pyrenoid.Lopes (1992) mentioned the presence of prominent granules in the central area of the semicells, which were not observed.To Prescott et al. (1981), however, these granules are sometimes smaller and may occur in other areas.

Cosmarium comissurale Brébisson ex Ralfs var. crassum Nordstedt 1870 (Fig. 4)
Cells as long as wide 34 W, 34 L, 10.15 W at isthmus; median constriction deep, sinus widely dilated at the apex; semicells elliptic-pyramidal in frontal view with produced and truncate apex; lateral and central extensions with variable granules variable; chloroplast with 2 pyrenoids.This taxon presents a great morphological likeness with C. ornatum Ralfs.The identification differs with respect to the opening or not of the median sinus.C. ornatum presents sinus open at the isthmus and shut or slightly closed in the distal area, while C. comissulare var.crassum has an open sinus open throughout its extension.The ornamentation pattern of the cell wall is distinct for two species.This taxonomic approach was followed in this work.