Plasticidade morfológica em microalgas induzida por predação por Chironomus cf. xanthus Hempel,1939 (Insecta: Chironomidae)

Abstract

Microalgae and cyanobacteria are photosynthetic microorganisms with different morphologies and may present phenotypic plasticity under predation risk. They are primary producers in aquatic environments and can be predated by innumerous heterotrophic groups, including benthic invertebrates. Among important benthic invertebrates which feed on microalgae, especially green microalgae, diatoms and cyanobacteria, there is the genus of insects Chironomus (Chironomidae, Diptera). Although the morphological variation of microalgae to avoid predation by planktonic invertebrates has been well documented, little is known about the phenotypic plasticity in microalgae predated by those larvae. In this project, we evaluated the morphological plasticity in microalgae as a defense to predation by Chironomus cf. xanthus larvae. We evaluated the morphological alterations in three microalgae strains of the species: Desmodesmus spinosus, since this genus usually presents phenotypic plasticity under predation by other invertebrates; and two microalgae belonging to genera found in benthos, habitat of the larvae: Micrasterias cf. pinnatifida, a green alga (desmid) and Cyclotella sp. a centric diatom. D. spinosus was evaluated directly over predation, however, due to methodological issues, the other algae were evaluated under the extracellular substances released by the larvae. The experiments were performed in triplicates. Under predation, D. spinosus presented significant increase in the coenobium dimensions on the second day, and in the maximum linear dimension (between spines) at the fourth day of growth; in this same period, a higher number of unicellular individuals was observed. Micrasterias biomass and chlorophyll a concentration (determined by cell count and absorbance) increased significantly on the second day in indirect contact with chironomids. In Cyclotella, extracellular substances released by the chironomid larvae (indirect contact), significantly increased the maximum linear dimensions of the individuals after 2 days, the number of filamentous individuals on the fourth day; and the chlorophyll a concentration from the first to the third days of growth. This is the first work showing morphological plasticity in algae predated by chironomids. The knowledge on the morphological plasticity of microalgae under predation can help elucidate the complex taxonomy of these microorganisms. Furthermore, it can help future studies about the interaction between the larva and its prey and its effects on the biota and local productivity.