Dinâmica temporal do fitoplâncton em um lago raso e mesotrófico (Lagoa Santa, Minas Gerais, Brasil) e a ocorrência de genótipos produtores de toxinas

• Ano de defesa: 2019
• Autor(a) principal: Natália Cristina Vieira de Carvalho
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; Brasil; ICB - DEPARTAMENTO DE BOTÂNICA; Programa de Pós-Graduação em Biologia Vegetal; UFMG
• Palavras-chave em Português: Cianobactérias; Cylindrospermopsis raciborskii; Diversidade; Comunidade fitoplanctônica; Cianotoxinas; Lagos rasos; Fenômenos fisiológicos vegetais; Fitoplâncton; Cylindrospermopsis; Lagos
• Link de acesso: http://hdl.handle.net/1843/40653

The phytoplankton community is very diverse and composed by different taxonomic groups, which include microalgae and cyanobacteria. The community structure is governed by several environmental and biological interactions, and the predominance of one group over another in a given aquatic system is related to certain ecosystem characteristics. In the central lake of Lagoa Santa (Minas Gerais, Brazil), the permanent and stable dominance of Cylindrospermopsis raciborskii, a potentially toxin-producing cyanobacteria species, was recorded in previous studies. This aquatic system is a natural shallow lake, located in an urban area, which has been suffering from environmental degradation. This lake is part of a karst system that has historical and scientific importance. The goal of the present work was to deepen the study of this ecosystem, in order to assess the environmental factors that govern the dynamics of the phytoplankton community, as well as the dominance patterns and the eventual formation of toxic blooms. At the same time, we sought to assess whether the species Cylindrospermopsis raciborskii, present in the lake, had the potential to produce toxins, which had not been measured before, and to evaluate the possible occurrence of other cyanotoxin-producing cyanobacteria. To attain these objectives, monthly sampling was carried out in Lagoa Santa, between 2017 and 2019. The physical and chemical characteristics of the water on the phytoplankton community were evaluated. Molecular approaches, through conventional PCR, were used to detect the presence of genes involved in the potential production of cyanotoxins, such as saxitoxin and microcystin. As a result, the present study revealed changes in the structure of the phytoplankton community, where an alternation of dominance between cyanobacteria and microalgae belonging to the phylum Chlorophyta was found. Differently from previous observations, the cyanobacteria species Cylindrospermopsis raciborskii dominated the community only during part of the study. Some disturbance in the lake must have favored the dominance of chlorophyte species, like Monoraphidium contortum and Botryococcus braunii, defining a period marked by the decrease in community diversity, as demonstrated by the decrease in Shannon's diversity index values. This dominance of chlorophytes occurred in a period of water column mixing, which may have been one of the causes of the observed change in the structure of the phytoplankton community. After this period, however, the cyanobacterial species Microcystis aeruginosa became the most abundant. The stratification and mixing processes of the water column observed in Lagoa Santa may have favored changes in species composition and dominance patterns. A functional traits approach, used in the present study, was important to understand the strategies adopted by each species. Environmental modifications seem to have produced an alternation between r- and K-strategist species over the temporal gradient. Biological interactions, like competitive exclusion to which phytoplankton species are subject, must also have contributed to the changes observed in the community. By molecular approach, we identified the presence of the sxtA gene in this environment, which encodes a multifunctional enzyme that initiates saxitoxin synthesis. The microscopy results suggest that Cylindrospermopsis raciborskii is the main potentially producing species. Furthermore, in a few months of this study, we identified the presence of the mycD gene, which participates in the complex biosynthesis of microcystin, possibly linked to the occurrence of species of the genus Microcystis in the lake. The present study contributed to understand the dynamics of the phytoplankton community of Lagoa Santa, highlighting the importance of the physical and chemical characteristics of the environment, as well as the ecological factors structuring this community. This study also emphasizes the importance of using molecular approaches to detect potentially toxic strains of cyanobacteria in the community.