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Although microcystins (MCs) are the most commonly studied cyanotoxins, their significance to the producing organisms remains unclear. MCs are known as endotoxins, but they can be found in the surrounding environment due to cell lysis, designated as extracellular MCs. In the present study, the interactions between MC producing and the non-producing strains of Microcystis aeruginosa, PCC 7806 and PCC 7005, respectively, and a green alga, Desmodesmus subspicatus, were studied to better understand the probable ecological importance of MCs at the collapse phase of cyanobacterial blooms. We applied a dialysis co-cultivation system where M. aeruginosa was grown inside dialysis tubing for one month. Then, D. subspicatus was added to the culture system on the outside of the membrane. Consequently, the growth of D. subspicatus and MC contents were measured over a 14-day co-exposure period. The results showed that Microcystis negatively affected the green alga as the growth of D. subspicatus was significantly inhibited in co-cultivation with both the MC-producing and -deficient strains. However, the inhibitory effect of the MC-producing strain was greater and observed earlier compared to the MC-deficient strain. Thus, MCs might be considered as an assistant factor that, in combination with other secondary metabolites of Microcystis, reinforce the ability to outcompete co-existing species.

In lakes, cyanobacterial blooms are frequently associated with green algae and dominate the phytoplankton community in successive waves. In the present study, the interactions between Microcystis aeruginosa PCC 7806 and Desmodesmus subspicatus were studied to clarify the probable ecological significance of algal secondary metabolites; focusing on the role of cyanotoxin ‘microcystin-LR’ (MC-LR). A dialysis co-cultivation technique was applied where M. aeruginosa was grown inside and D. subspicatus was cultured outside of the dialysis tubing. The concentration of the intra- and extracellular MC-LR and the growth of two species were measured at different time points over a period of one month. Additionally, the growth of the two species in the culture filtrate of one another and the effect of the purified MC-LR on the growth of the green alga were studied. The results indicated that the co-existing species could affect each other depending on the growth phases. Despite the early dominance of D. subspicatus during the logarithmic phase, M. aeruginosa suppressed the growth of the green alga at the stationary phase, which coincided with increased MC production and release. However, the inhibitory effects of Microcystis might be related to its other extracellular metabolites rather than, or possibly in addition to, MC. Keywords: Interspecies interactions, Co-cultivation, M. aeruginosa, D. subspicatus

El pesticida neonicotinoide imidacloprid se encuentra entre los agroquímicos más vendidos en todo el mundo. Debido a su amplio uso en mezclas con diferentes disolventes y co-adyuvantes, estudiar el impacto ambiental de las formulaciones comerciales derivadas se ha convertido en obligatorio. En este estudio se utilizaron ensayos ecotoxicológicos de laboratorio para cuantificar el impacto del ConfidorR 20SL (formulación que contiene imidacloprid) en los compartimentos terrestre y acuático. Los efectos letales y subletales de las dosis recomendadas de aplicación del producto fueron evaluadas en los invertebrados terrestres Eisenia foetida y Folsomia candida mientras que la toxicidad de los lixiviados de los suelos contaminados se evaluó en los organismos acuáticos modelo Daphnia magna y Raphidocelis subcapitata (anteriormente Selenastrum capricornutum). La exposición a concentraciones ambientalmente relevantes de imidacloprid no causo mortalidad en las lombrices de tierra (CL50 de 4.23 mg de imidacloprid por kg de suelo seco) pero altero los patrones de comportamiento y reproducción (valores de CE50 de 0.43 y 1.40 mg de imidacloprid por kg de suelo seco en los ensayos de alejamiento y reproducción respectivamente). Los efectos en los colembolos F. candida fueron despreciables. El imidacloprid presento una lixiviabilidad moderada, con tasas de recuperación en los extractos acuosos que fueron del 25.4 al 50.4% de la cantidad presente en los suelos y concentraciones de 13.05 a 71.8 µg por litro. Las pruebas estándar de ecotoxicidad acuática no fueron capaces de detectar toxicidad aguda o crónica en los organismos de ensayo. Sin embargo, las concentraciones de insecticida en los extractos fueron lo suficientemente grandes como para representar una amenaza letal para otros organismos acuáticos no estándar The neonicotinoid pesticide imidacloprid is among the top sold agrochemicals worldwide. Due to its widespread use in mixtures with different solvents and co-adjuvants, studying the environmental impact of its derived commercial formulations has become mandatory. In this study we used laboratory ecotoxicological tests to quantify the impact of the imidacloprid-containing formulation Confidor R20SL on the terrestrial and aquatic compartments. Lethal and sublethal effects of recommended application doses of the product were assessed on standard terrestrial invertebrates Eisenia fetida and Folsomia candida whereas the toxicity of leachates from contaminated soils was evaluated in the aquatic model organisms Daphnia magna and Raphidocelis subcapitata. The exposure to environmentally relevant concentrations of imidacloprid caused no mortality to earthworms (LC50 of 4.23 mg imidacloprid kg-1 dry soil) but altered their behavior and reproduction patterns (EC50 values for avoidance and reproduction tests of 0.43 and 1.40 mg imidacloprid kg-1 dry soil, respectively). Effects on collembolans F. candida were negligible. Imidacloprid presented moderate leachability, with recovery rates that ranged from 25.4 to 50.4% of the amount present in soils and concentrations in water extracts from 13.05 to 71.8 µg L-1. Standard aquatic ecotoxicity tests were not able detect chronic or acute toxicity in standard test organisms. Nonetheless, concentrations of the insecticide in water extracts were high enough to pose a lethal threat to several other non-standard aquatic organisms