Characteristics and Diversity of Cyanobacteria in Periphyton from Lentic Tropical Ecosystem , Brazil

This study aimed to perform the taxonomic survey of the species of periphytic Cyanobacteria in a lentic tropical environment, seeking to contribute to the knowledge on biodiversity and their distribution. This study was conducted at the Samambaia Reservoir, which is located at Federal University of Goiás, Goiânia, Central West region of Brazil. In general, the water of the Samambaia Reservoir is characterized as more turbid and the increased biochemical oxygen demand as well as a higher concentration of coliforms in the rainy season. Twenty six samples were collected, ten in the dry season (09/2010 and 11/2010; 07/2012) and ten in the rainy season (01/2011 and 03/2011; 01/2013). Periphyton was collected from stems of aquatic plants (Cyperaceae), which were in the marginal area of the reservoir. With the floristic survey, 38 taxa of Cyanobacteria were identified in periphyton of the Samambaia Reservoir. Phormidium (family Phormidiaceae) and Aphanocapsa (Merismopediaceae) are among genera with the highest species richness. Regarding morphological types, many species of filamentous cyanobacteria were identified, followed by colonial taxa. In relation to the frequency of occurrence and the seasonal period, 14 taxa were classified in the rare category, 12 in common category and 12 in constant category. Regarding the seasonal period, Cyanophyceae were the greatest wealth in the rainy season (97.4%).


Introduction
Brazil is the country that has the largest amount of freshwater in the world, and the greatest river system in Goiás is one of the most important water divisors and contributors in the "composition" of water resources of the country, with tributaries and palm swamps which join to form the three major Brazilian watersheds [1].
Despite the significance of the amount of freshwater to Goiás Cerrado, there is little research on periphytic communities [2]- [8].
In these periphytic communities, there are important photosynthetic microorganisms, such as Cyanobacteria (blue algae), which are found in the community of periphytic algae or planktonic.These microorganisms become more populous because they are capable of forming massive growth of blooms with adverse effects on sanitation, which are aggravated when toxic cyanobacterial species are present [9].However, the knowledge of their morphological variability (important for their identification) is still little known, which requires a thorough revision [10].
The basic morphology comprises unicellular, colonial and multicellular filamentous forms.Unicellular forms, such as Chroococcales, have spherical, ovoid or cylindrical cells, and in some species the colony forms are constituted by a cell aggregate held together by the slimy matrix secreted during the growth of the colony [13].
Filamentous morphology is the result of repeated cell divisions occurring in a single plane at right angles to the main axis of the filament and consisting of a chain of cells is called a trichome, which may be straight or coiled [14].
Filamentous species in the order Oscillatoriales, with unseriated and unbranched trichomes, are composed of essentially identical cells.Other orders with a filamentous organization (Nostocales and Stigonematales) are characterized with trichomes having a heterogeneous cellular composition, which may be differentiated into heterocysts (vegetative cell somewhat larger and rounder shape, diminished pigmentation, thicker cell envelopes, and usually prominent cyanophycin granules at poles adjacent to other cells) and akinetes (vegetative cell larger, thicker cell envelopes, containing many reserve granules) [14] [15].
Therefore, this study aimed to execute the taxonomic survey of the unicellular, colonial and multicellular filamentous species of periphytic Cyanobacteria in a lentic tropical environment.

Materials and Methods
The study area (16˚35'S, 49˚16'W, Figure 1) is located in Campus II of the Federal University of Goiás in Goiânia-GO, Central West region of Brazil.
The Samambaia Reservoir was constructed in 1972 by damming the stream Samambaia [16]; it has an average depth of 3.5 m and was characterized as an environment oligo to mesotrophic.This reservoir has been built to the purpose of fish farming experiments of Federal University of Goiás (UFG) and later went on to supply water for the entire university campus through the company of Goiás Sanitation S/A (Saneago).However, there are significant points of sources and diffusion of pollutants.
The characterization of drought and rain was based on climatological daily data of rainfall obtained through newsletters from the Rainfall Station of Goiânia, located in the School of Agronomy and Food Engineering, UFG, distant about 600 m of the study area.
In general, the water of the Samambaia Reservoir is more turbid with the increased of biochemical oxygen demand, as well as a higher concentration of coliforms in the rainy season (Table 1, [8]).The climate in the study area is considered as tropical by according to Köppen classification [17].
Periphyton was collected from stems of aquatic plants (Cyperaceae) that were in the marginal area of the reservoir (Three petioles per sample point), five sampling points in the study area.Twenty samples were collected, ten in the dry season (24/09/2010 and 04/11/2010; 07/2012) and ten in the rainy season (06/01/2011 and 02/03/2011; 01/2013).
The periphytic substrate material was removed with the aid of a razor blade, followed by distilled water jets.After the scraping, the material was transferred to vials, preserved and fixed with Transeau solution.
The samples were deposited in the herbarium collection of the Federal University of Goiás, Goiânia-GO under the numbering 47324 -47349.
The samples were analyzed between slide and cover slip in the preparation of about 10 temporary slides per sample.The capture images of subjects was processed with the aid of the optical microscope ZEISS brand (model AXIOSKOP 40) and a digital camera coupled by the optical system of the microscope.
Identification of taxa was made by the morphological and morphometric characteristics of the cells, which  were obtained by the optical microscope at the increase of 40 to 100 times, and consultation based into the specialized literature, including articles, reviews and monographs.The frequency of occurrence, in percent, was calculated considering the presence and absence of the taxon and divided by the total number of taxa sampled.The categories used were: Rare (≤10%), Common (˃10% ≤30%) and Constant (˃30%).

Results and Discussion
With the taxonomic analysis, we identified 38 taxa of Cyanobacteria in periphyton of the Samambaia Reservoir (Table 2).Phormidium and Aphanocapsa were the most specious genera (Table 2).
For periods, the higher taxonomic richness (97.4%) was evaluated for the rainy season.Regarding morphological types, many species of filamentous cyanobacteria were identified, followed by colonial taxa.
In relation the frequency of occurrence and the seasonal period, 14 taxa were classified in the rare category, 12 in common category and 12 in constant category (    In the specimens found, we observed a thick mucilage, different from the material analyzed by Azevedo and Sant'Anna [18] and Sant'Ana et al. [19], which are mentioning an inconspicuous mucilage hyaline in this species.Filaments solitary, regularly coiled, 22.2 μm in distance between the coils, not constricted at the septum (not shown).Pale bluish-green color.Cells possibly longer than wide (1.7 μm in width), homogeneous structure with irregularly disposed granules, without aerotopes.Apical cells rounded.Sheaths absent or facultatively present, fine and colorless.
The absence of aerotopes was also documented by Komarek and Anagnostidis [20].

Figure 1 .
Figure 1.Map showing the location of the research area Samambaia Reservoir in Goiás State.

Table 2 .
Frequency of occurrence of the species found in the sampling in both study periods of the Samambaia Reservoir.* = absence of species; x = presence of species; R = rare; Cm = common; Cs = constant; Freq.= Frequency in %.Cells heteropolar, solitary or in groups, elongated, attached by one end to substrate (usually by indistinct, hyaline, gelatinous pad), and in groups or in stellate clusters.Cells (5.2 -7.4 × 2.2 µm) are oval to cylindrical and rod-shaped, rounded at the apex, slightly and shortly attenuated or rounded at the base.