Fatores determinantes da ocorrência e abundância de cianobactérias e produção de cianotoxinas em ecossistemas do semiárido

Abstract

Cyanobacteria are prokaryotes, which perform photosynthesis, present in the most diverse types of environments, being able to survive under a range of different environmental conditions. However, some conditions such as high temperatures, high concentrations of nutrients, can favor their development in such a way that they even form blooms, bringing negative consequences for both the system and implications for public health and economic order. Trying to better understand the dynamics of cyanobacteria in semiarid ecosystems, this doctoral work is divided into two chapters. The first one is entitled: Effects of water volume reduction on cyanobacteria and cyanotoxins in semiarid reservoirs impacted by severe drought. In this chapter, the objective was to evaluate the effects of water volume reduction on the composition and abundance of cyanobacteria and the concentration of cyanotoxins in semiarid reservoirs impacted by severe drought. Seven samples were collected in three reservoirs from the Paraíba River Basin (Acauã, Boqueirão and Camalaú) between July 2015 and February 2017. In situ data were collected (transparency, water temperature, pH, electrical conductivity, turbidity, (N-NH4), Nitrite (N-NH3), total nitrogen (TN), soluble reactive phosphorus (SRP), and total dissolved solids, dissolved oxygen and salinity and total phosphorus (PT)), cyanobacteria and cyanotoxins (total microcystins, total saxitoxins and total cylindrospermopsins). The reduction of the water volume modified the physical and chemical characteristics of the reservoirs and consequently the composition and biomass of cyanobacteria. The water temperature in the reservoirs varied around 25ºC, the pH was alkaline and the nutrient concentration was high, with a low total and dissolved N/P ratio. In the regression analysis, the water volume appears as a common variable to the three reservoirs causing negative and positive effects on the biomass of cyanobacteria. For the Acauã and Camaláu reservoirs an inverse relationship is observed and for the Boqueirão reservoir a direct effect, thus, in the first two systems there was increase of the biomass and in the third one a decrease. The Boqueirão and Camalaú reservoirs presented changes in the composition of the species, but for the three systems, we highlight filamentous species such as Cylindrospermopsis raciborskii and Dolichospermum solitarium. Total microcystins and total saxitoxins were detected in the reservoirs throughout the sampled period, with concentrations varying between 2.09 ± 0.06 and 4.80 ± 1.2 μg L-1 and 0.65 ± 0.40 and 1.42 ± 0.30 μg L-1, respectively. The total cylinders were detected during all the months sampled in the Acauã reservoir and values ranged between 1.46 ± 0.04 and 1.83 ± 0.14 μg L-1. In the Boqueirão and Camalaú reservoirs, total cylindrospermopsins were recorded only in June 2016 (1.59 ± 0.37 μg L-1) and February 2017 (2.80 ± 1.12 μg L-1). The water volume reductions can direct different responses regarding the composition and biomass of cyanobacteria, since the physical and chemical characteristics also respond differently to this condition. The detection of microcystin, saxitoxin and cylindrospermopsin generate alerts for continuous monitoring, since the systems are used for public supply and other services such as fish farming and recreation. The second chapter is entitled: Can transposition of a river intensify cyanobacterial blooms and toxin production in recipient reservoirs? Its objective was to verify the effects of the connection between different river basins (transposition of the São Francisco River) in the abundance of the composition and biomass of cyanobacteria and the concentration of cyanotoxins in the reservoir of the semiarid. For this purpose, 10 samples were taken at the Boqueirão reservoir, between July 2015 and February 2017 (before transposition) and June 2017 and August 2018 (after transposition) at three points in the reservoir. In situ data (T°C, pH, electrical conductivity, turbidity, dissolved oxygen, total dissolved solids and salinity) and subsurface water samples for nutrient analysis (ammonia, nitrite, nitrate, reactive soluble phosphorus and total phosphorus), cyanobacteria and cyanotoxins (total microcystins, total saxitoxins and total cylindrospermpsins). There was a difference between the periods before and after the arrival of the waters of the transposition in the Boqueirão reservoir for some characteristics, such as pH and salinity reduction and nutrient concentrations increases. In addition to increasing biomass of cyanobacteria in the post-transposition period, a change in abundance of species composition was verified, and species C. raciborskii and Pseudoanabaena catenata appear as dominant before transposition, with a substitution by D. circinalis occurring after. The concentrations of microcystins increased, as well as the detection of cylinderspermopsin, both coinciding with the period after the transposition, which was the highest cyanobacterial biomass.