Salinização causa evolução rápida da tolerância e menor tamanho celular em uma cianobactéria formadora de florações

Abstract

Global changes act as selective forces responsible for inducing species evolution over a short period of time, events that have implications on the structure and functioning of ecosystems, but limited information exists about subsequent ecological effects. Salinization, as a stress factor to freshwater ecosystems, not only represents a threat to biodiversity and key ecosystem functions and services but also exerts selective pressure on species. However, the way species locally adapt to salinization through microevolutionary processes is poorly understood. Thus, we investigated the capacity for rapid evolution to increased salinity and its effects on functional characteristics, growth, and morphology of Brazilian strains of Microcystis aeruginosa (Kütz.). Furthermore, we believe that populations adapted to high salinities will exhibit higher growth rates and smaller cell sizes. To this end, we set up a Common Garden experiment with 4 strains of toxin-producing Microcystis. Prior to the Common Garden, all strains were divided into 3 populations (replicates) and exposed to 2 treatments: addition of NaCl (1.5 g/L) and control (0 g/L NaCl) for a period of 6 to 8 months, resulting in over 100 generations. After this period, each population was exposed to treatments of 0, 1.5, 3, and 5 g/L NaCl with 3 replicates each in a Common Garden design for an additional 7 generations to remove the effects of phenotypic and epigenetic plasticity in the populations. After this stage, morphological and growth characteristics were measured. To test how salinity concentrations and adaptation affect Microcystis growth and morphology, we used Generalized Linear Models (GLMs). Populations exposed to a concentration of 1.5 g/L in the selection stage (adapted) showed higher growth rates compared to those exposed to the control (non-adapted) and grew in all salinization treatments, indicating increased salt tolerance driven by selection on preexisting genetic variation. Two strains, originally growing as individual cells, produced colonies at higher salinities. Colony formation, as well as size, was greater in nonadapted populations, indicating that adapted populations likely do not need to invest in colony formation to survive. The results demonstrate that adaptation to relatively low concentrations (1.5 g/L) of salt also enables tolerance to higher salinities (>3 g/L). We show rapid evolutionary adaptation (microevolution) of a common aquatic primary producer to freshwater salinization over ecological time, with effects on key traits (phenotype) such as cell size. Such adaptation is likely to occur in other cyanobacteria and microalgae species in nature where salinization occurs, with consequences for ecosystem functions still unknown. Our results justify a broader view of the ecoevolutionary implications of salinization-driven selection in aquatic ecosystems.