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Responses of photosynthetic rates, determined by oxygen evolution using the light and dark bottles technique, to different temperatures, irradiances, pH, and diurnal rhythm were analyzed under laboratory conditions in four charophyte species ( Chara braunii Gmelin , C. guairensis R. Bicudo , Nitella subglomerata A. Braun and Nitella sp.) from lotic habitats in southeastern Brazil. Parameters derived from the photosynthesis versus irradiance curves indicated affinity to low irradiances for all algae tested. Some degree of photoinhibition, [ β = -(0.30-0.13) mg O2 g -1 dry weight h -1 ( µ mol photons m -2 s -1 ) -1 ], low light compensation points (I c = 4-20 µ mol photons m -2 s -1 ) were found for all species analyzed, as well as low values of light saturation parameter (I k ) and saturation (I s ) 29-130 and 92-169 µ mol photons m -2 s -1 , respectively.Photoacclimation was observed in two populations of N. subglomerata collected from sites with different irradiances, consisting of variations in photosynthetic parameters (higher values of α , and lower of I k and maximum photosynthetic rate, P max , in the population under lower irradiance). The highest photosynthetic rates for Chara species were observed at 10-15 ° C, while for Nitella the highest photosynthetic rate was observed at 20-25 ° C, despite the lack of significant differences among most levels tested. Rates of dark respiration significantly increase with temperature, with the highest values at 25 ° C. The results from pH experiments showed highest photosynthetic rates under pH 4.0 for all algae, suggesting higher affinity for inorganic carbon in the form of carbon dioxide, except in one population of N. subglomerata , with similar rates under the three levels, suggesting indistinct use of bicarbonate and carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for most algae tested, which was characterized by two peaks: the first (higher) during the morning (07.00-11.00) and the second (lower) in the afternoon (14.00-17.00). This suggests an endogenous rhythm determining the daily variations in photosynthetic rates.

SUMMARY Responses of photosynthetic rates, determined by oxygen evolution using the light and dark bottles technique, to different temperatures, irradiances, pH, and diurnal rhythm were analyzed under laboratory conditions in four charophyte species (Chara braunii Gmelin, C. guairensis R. Bicudo, Nitella subglomerata A. Braun and Nitella sp.) from lotic habitats in southeastern Brazil. Parameters derived from the photosynthesis versus irradiance curves indicated affinity to low irradiances for all algae tested. Some degree of photoinhibition, [β= ‐(0.30–0.13) mg O2 g−1 dry weight Ir1 (μmol photons m−2 s−1)−1], low light compensation points (Ic= 4–20 μmol photons m−2 s−1) were found for all species analyzed, as well as low values of light saturation parameter (Ik) and saturation (Is) 29–130 and 92–169 μmol photons m−2 s−1, respectively. Photoacclimation was observed in two populations of N. subglomerata collected from sites with different irradiances, consisting of variations in photosynthetic parameters (higher values of a, and lower of Ik and maximum photosynthetic rate, Pmax, in the population under lower irradiance). The highest photosynthetic rates for Chara species were observed at 10–15°C, while for Nitella the highest photosynthetic rate was observed at 20–25°C, despite the lack of significant differences among most levels tested. Rates of dark respiration significantly increase with temperature, with the highest values at 25°C. The results from pH experiments showed highest photosynthetic rates under pH 4.0 for all algae, suggesting higher affinity for inorganic carbon in the form of carbon dioxide, except in one population of N. subglomerata, with similar rates under the three levels, suggesting indistinct use of bicarbonate and carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for most algae tested, which was characterized by two peaks: the first (higher) during the morning (07.00–11.00) and the second (lower) in the afternoon (14.00–17.00). This suggests an endogenous rhythm determining the daily variations in photosynthetic rates.

Photosynthetic characteristics (assessed by chlorophyll fluorescence and O 2 evolution) were analysed monthly during one year in a tropical population of Nitella cernua from southeastern Brazil (20 o 50 32" S, 49 o 26 15" W). Parameters derived from photosynthesis-irradiance (PI) curves by fluorescence (high values of the photosynthetic parameter, I k , and lack or low values of photoinhibition, β) suggested adaptation to high irradiance, whereas those by O 2 evolution showed a different pattern (low values of I k and compensation irradiance, I c , high values of photosynthetic efficiency, α, and photoinhibition). Parameters from PI curves by O 2 evolution suggested light acclimation: I k and maximum photosynthetic rate, P max (as rETR, relative electron transfer rate) increased, whereas α became significantly lower under higher irradiance (winter). This pattern is an adjustment of both number and size of photosynthetic units. Photosynthetic performance assessed by fluorescence revealed two seasonal periods: higher values of P max (rETR), I k and nonphotochemical quenching from October to March (rainy season), and lower values from April to October (dry season). Temperature responses were observed only in summer, but temperature optima were different between methods: peaks of net photosynthesis occurred at 20 o C, whereas rETR increased towards higher temperatures (up to 30 o C). Dark respiration increased with higher temperatures. Current velocity had a stimulatory effect on photosynthetic rates, as suggested by positive correlations with P max (rETR) and α. pH experiments revealed highest net photosynthetic rates under pH 4.0, suggesting higher affinity for CO 2 than HCO 3 -. This broad range of responses of photosynthetic characteristics of this N. cernua population to irradiance, temperature, and pH/inorganic carbon reflects a wide tolerance to variations in these environmental variables, which probably contribute to the wide distribution of this species. Key words: charophytes, irradiance, Nitella, pH, photosynthesis, seasonal dynamics, temperature Características fotossintéticas de uma população tropical de Nitella cernua (Characeae, Chlorophyta): Foram analisadas mensalmente, durante um ano, as características fotossintéticas (por meio de fluorescência da clorofila e evolução de O 2 ) de uma população tropical de Nitella cernua do sudeste do Brasil (20 o 50 32" S, 49 o 26 15" W). Parâmetros derivados das curvas fotossíntese-irradiância (FI) por fluorescência (alto valor do parâmetro de saturação de luz, I k , e ausência ou baixa fotoinibição, β) sugeriram adaptação a alta irradiância, enquanto aqueles por evolução de oxigênio mostraram adaptações à sombra (baixo valores de I k e de irradiância de compensação, I c , altos valores de eficiência fotossintética, α, e β). Parâmetros das curvas FI por oxigênio sugeriram aclimatação à luz: I k e taxa fotossintética máxima, F max (analisada como rETR, taxa relativa de transporte de elétrons) aumentaram e α foi significativamente menor sob irradiância mais alta (i...

The present work aimed to carry out traditional ecotoxicological and ecosystemic tests in microcosms for the evaluation of the use of glyphosate herbicide adopted to control floating aquatic macrophytes in aquatic ecosystems. Ecotoxicity tests were performed using standardized methodology and standard species of different trophic levels. Although the active ingredient glyphosate is toxic, the toxicity of its commercial formulation is higher. The inert ingredient of the Roundup Original ® composition may be responsible for the higher commercial compound toxicity. Experiments in microcosms indicated that the use of the formulation is possible, in considering tested concentration and controlling the spray process, without causing toxic effect on the aquatic ecosystem. Complementary studies evidenced the risk of using the product in aquatic environments without vegetation cover, being not recommended the herbicide application in a place without floating aquatic macrophytes cover.