Respostas fotossintéticas de macroalgas lóticas expostas a multiestressores ambientais de origem antrópica
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Data
2021-12-21
Autores
Vilas Boas, Lucas Kortz [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Algas bentônicas são importantes contribuintes para a energia autóctone em ambientes lóticos, além de contribuir para o enriquecimento e estruturação do habitat aquático. De maneira geral, os ambientes aquáticos continentais são historicamente degradados por atividades humanas, das quais, uma das principais é a ocupação e uso de áreas circunvizinhas para o plantio, em larga escala, de espécies de interesse agrícola. A expansão de áreas de agricultura sobre áreas adjacentes a ambientes aquáticos continentais pode afetar tais ambientes de diversas maneiras, entre elas, contaminando-os com pesticidas capazes de agir em organismos não-alvo. Além disso, o aquecimento global, exacerbado pela atividade antrópica, pode afetar a biota dos ambientes aquáticos, influenciando nas características físico-químicas da água e afetando na fisiologia dos organismos. Tais condições constituem um ambiente multi-estressor para os organismos lóticos, especificamente para produtores primários, como as macrófitas e as algas bentônicas. Neste contexto, para se compreender os reais impactos ecológicos desta natureza é fundamental que se avalie os efeitos, diretos e indiretos, de agentes multi-estressores sobre a biota lótica, especialmente, como já dito, sobre os produtores primários, uma vez que estes são, em regra, a base das teias tróficas nestes ecossistemas. Assim, considerando esta lógica, o presente trabalho teve como objetivo avaliar a fotossíntese de espécies de algas bentônicas lóticas após exposição ao herbicida tebuthiuron, um herbicida comumente utilizado em culturas de cana-de-açúcar no Brasil, em conjunto com o aumento de temperatura previsto pelo Painel Intergovernamental de Mudanças Climáticas (IPCC). Três concentrações de tebuthiuron (0,05mg/L, 0,6mg/L e 1,2mg/L) e um controle sem adição do herbicida foram utilizados em conjunto com três temperaturas (21,3 ºC, 23,9 ºC e 26 ºC), calculados a partir das projeções de aumento de temperatura propostas para dois cenários, RCP 4.5 e RCP 8.5, do IPCC. Duas espécies de Chlorophyta (Nitella microcarpa var. wrightii e Oedogonium sp.) e uma espécie de Rhodophyta (Batrachospermum helminthosum) foram testadas. As técnicas de fluorescência da clorofila e da evolução do oxigênio dissolvido foram utilizadas. Todas as espécies foram afetadas negativamente pela exposição ao tebuthiuron, com diminuição de seu rendimento quântico efetivo (Y(II)). No entanto, as duas espécies de algas verdes mostraram respostas negativas apenas nas concentrações acima de 0.6 mg/L, enquanto B. helminthosum apresentou uma maior sensibilidade, com efeito negativo aparente mesmo na concentração mais baixa. Considerando a temperatura, N. microcarpa não mostrou efeitos negativos expressivos relacionados a esse parâmetro, provavelmente por se tratar de uma espécie de alga verde comumente encontrada em locais com alta irradiação e, consequentemente, temperatura mais elevada. Oedogonium sp., por outro lado, apesar de descrita como uma alga com altos níveis de crescimento em uma grande amplitude de temperaturas, teve sua taxa de fotossíntese líquida (NPR) reduzida tanto no cenário RCP 4.5 quanto no RCP 8.5. B. helminthosum, uma espécie comumente descrita como aclimatada a locais de baixa irradiância e consequentemente a menores temperaturas, apresentou sinais de estresse no cenário RCP 8.5, porém teve sua NPR aumentada no cenário RCP 4.5. Alguns efeitos sinérgicos foram observados entre a exposição ao tebuthiuron e ao aumento de temperatura, como em N. microcarpa em que uma diminuição mais expressiva da taxa de transporte de elétrons (ETR) foi causada pelo tebuthiuron no cenário RCP 8.5 e em B. helminthosum com sua NPR reduzida em maior intensidade pelo tebuthiuron nos dois cenários de temperatura testados. A capacidade de ação do tebuthiuron nesses organismos não-alvo se dá justamente pelo seu modo de ação no fotossistema II, interrompendo o fluxo de elétrons próximo à plastoquinona B. Considerando a elevada toxicidade ambiental do tebuthiuron, seu uso descontrolado e o concomitante aumento de temperatura causado pelo aquecimento global, os resultados do presente estudo sugerem que alguns dos cenários multi-estressores testados podem gerar grandes impactos à comunidade de organismos fotossintetizantes de água doce, possivelmente impondo, inclusive, severos efeitos negativos nos demais níveis tróficos, dado os reflexos de um potencial “efeito cascata”.
Benthic algae are important contributors to the autochthonous energy input in lotic environments, in addition to contributing to the richness of the habitat. In general, freshwater environments are historically degraded by human activities, with practices such as the occupation of neighboring areas for the large scale cultivation of species of agronomic interest standing out. The expansion of agricultural areas to zones surrounding freshwater ecosystems may affect these environments in several ways, among them, with the contamination with pesticides, capable of acting on non-target organisms. In addition, global warming, increased by anthropogenic activities, may affect freshwater organisms by altering the physicochemical properties of water and influencing their physiology. Such conditions constitute a multi-stressor environment for lotic organisms, specifically for primary producers, such as macrophytes and benthic algae. In this context, in order to understand the ecological impacts in such circumstances, it is crucial to assess the direct and indirect effects of multi-stressor environments on lotic organisms, especially on primary producers, considering their role in the basis of the food web. Therefore, this study aimed to evaluate the photosynthesis of lotic benthic algae species after exposure to different concentrations of the herbicide tebuthiuron, an herbicide commonly used in sugarcane crops in Brazil, along with the temperature increase predicted by the Intergovernmental Panel on Climate Change (IPCC). Three concentrations of tebuthiuron (0.05mg/L, 0.6mg/L and 1.2mg/L) and a control without herbicide were used in conjunction with three temperatures (21.3 ºC, 23.9 ºC and 26 ºC ), calculated from projections of temperature increase proposed by the IPCC RCP 4.5 and RCP 8.5 scenarios. Two species of Chlorophyta (Nitella microcarpa var. wrightii and Oedogonium sp.) and one species of Rhodophyta (Batrachospermum helminthosum) were tested. The chlorophyll fluorescence and dissolved oxygen evolution techniques were used. All species were negatively affected by exposure to tebuthiuron, with a decrease in their effective quantum yield (Y(II)) when exposed to the herbicide. However, the two species of green algae showed negative responses only at concentrations higher than 0.6 mg/L, while B. helminthosum displayed greater sensitivity, with an apparent negative effect even at the lowest concentration. Considering the temperature, N. microcarpa did not show significant negative effects related to this parameter, probably related to being commonly found in places with high irradiation and, consequently, higher temperature, as most green algae. Oedogonium sp., on the other hand, despite being described as an alga with high growth levels over a wide temperature range, had its net photosynthetic rate (NPR) reduced in both RCP 4.5 and RCP 8.5 scenarios. B. helminthosum, a species commonly described as adapted to low irradiance environments and, consequently, lower temperatures, showed signs of stress in the RCP 8.5 scenario, but had its NPR increased in the RCP 4.5 scenario. Some synergistic effects were observed between the exposure to tebuthiuron and temperature, with N. microcarpa showing a higher decrease in its electron transport rate (ETR) caused by tebuthiuron in the scenario RCP 8.5 and B. helminthosum with its NPR reduced in a greater intensity by tebuthiuron in the two tested temperature scenarios. Tebuthiuron's ability to act on these non-target organisms is precisely due to its mode of action in photosystem II, blocking the electron flow close to the site of plastoquinone B. Considering its high environmental toxicity, its unrestrained use simultaneous with the increase in temperature caused by global warming may generate great impacts on the community of freshwater photosynthetic organisms, possibly affecting other trophic levels due to a potential “cascade effect”.
Benthic algae are important contributors to the autochthonous energy input in lotic environments, in addition to contributing to the richness of the habitat. In general, freshwater environments are historically degraded by human activities, with practices such as the occupation of neighboring areas for the large scale cultivation of species of agronomic interest standing out. The expansion of agricultural areas to zones surrounding freshwater ecosystems may affect these environments in several ways, among them, with the contamination with pesticides, capable of acting on non-target organisms. In addition, global warming, increased by anthropogenic activities, may affect freshwater organisms by altering the physicochemical properties of water and influencing their physiology. Such conditions constitute a multi-stressor environment for lotic organisms, specifically for primary producers, such as macrophytes and benthic algae. In this context, in order to understand the ecological impacts in such circumstances, it is crucial to assess the direct and indirect effects of multi-stressor environments on lotic organisms, especially on primary producers, considering their role in the basis of the food web. Therefore, this study aimed to evaluate the photosynthesis of lotic benthic algae species after exposure to different concentrations of the herbicide tebuthiuron, an herbicide commonly used in sugarcane crops in Brazil, along with the temperature increase predicted by the Intergovernmental Panel on Climate Change (IPCC). Three concentrations of tebuthiuron (0.05mg/L, 0.6mg/L and 1.2mg/L) and a control without herbicide were used in conjunction with three temperatures (21.3 ºC, 23.9 ºC and 26 ºC ), calculated from projections of temperature increase proposed by the IPCC RCP 4.5 and RCP 8.5 scenarios. Two species of Chlorophyta (Nitella microcarpa var. wrightii and Oedogonium sp.) and one species of Rhodophyta (Batrachospermum helminthosum) were tested. The chlorophyll fluorescence and dissolved oxygen evolution techniques were used. All species were negatively affected by exposure to tebuthiuron, with a decrease in their effective quantum yield (Y(II)) when exposed to the herbicide. However, the two species of green algae showed negative responses only at concentrations higher than 0.6 mg/L, while B. helminthosum displayed greater sensitivity, with an apparent negative effect even at the lowest concentration. Considering the temperature, N. microcarpa did not show significant negative effects related to this parameter, probably related to being commonly found in places with high irradiation and, consequently, higher temperature, as most green algae. Oedogonium sp., on the other hand, despite being described as an alga with high growth levels over a wide temperature range, had its net photosynthetic rate (NPR) reduced in both RCP 4.5 and RCP 8.5 scenarios. B. helminthosum, a species commonly described as adapted to low irradiance environments and, consequently, lower temperatures, showed signs of stress in the RCP 8.5 scenario, but had its NPR increased in the RCP 4.5 scenario. Some synergistic effects were observed between the exposure to tebuthiuron and temperature, with N. microcarpa showing a higher decrease in its electron transport rate (ETR) caused by tebuthiuron in the scenario RCP 8.5 and B. helminthosum with its NPR reduced in a greater intensity by tebuthiuron in the two tested temperature scenarios. Tebuthiuron's ability to act on these non-target organisms is precisely due to its mode of action in photosystem II, blocking the electron flow close to the site of plastoquinone B. Considering its high environmental toxicity, its unrestrained use simultaneous with the increase in temperature caused by global warming may generate great impacts on the community of freshwater photosynthetic organisms, possibly affecting other trophic levels due to a potential “cascade effect”.
Descrição
Palavras-chave
Temperatura, IPCC, Pesticida, Tebuthiuron, Fluorescência, Ecotoxicologia, Pesticide, Temperature, Ecotoxicology, Fluorescence