Microalgae-Based Growth-Inhibition Bioassay and In Situ Sediment Bioaugmentation in Urban Water Bodies of Costa Rica: A Pilot Study

Diffuse pollution affects urban water bodies worldwide, hindering management and restoration and increasing the need for complementary detection tools and sustainable bioremediation. This pilot study evaluated an adapted microalgae growth-inhibition assay (based on OECD Test No. 201) in a closed system and assessed microalgae-assisted sediment bioaugmentation in a lotic system. Two aquatic environments in Costa Rica’s Greater Metropolitan Area were studied: La Paz Park lagoons (closed) and the Toyogres River (lotic). In the lagoons, pollution levels and ecological impacts were analyzed during dry and rainy seasons using physicochemical analysis, microscopy, and 16S rRNA amplicon sequencing. Growth inhibition of Chlorella sp., Scenedesmus sp., and Anabaena sp. was tested with lagoon water samples. Chlorella sp. was subsequently immobilized in clay matrices and used for sediment bioaugmentation in the river. In the lagoons, chemical analyses of composite samples showed organic contaminants (VOCs, BTEX, trihalomethanes, antibiotics, and neonicotinoids) below the limits of quantification, whereas several metals were quantified at seasonally variable concentrations; aerobic conditions prevailed and microscopy indicated a sparse microbial community. The microalgal bioassay detected sublethal growth inhibition, which was transient in the dry season and persistent in the rainy season. In the river, Chlorella sp. bioaugmentation was associated with reduced sediment toxicity in the dry season but not in the rainy season, and changes in the abundance and diversity of sediment microorganisms were not statistically significant. In conclusion, microalgae-based bioassays are a promising complement to chemical analyses for pollution monitoring, and microalgal bioaugmentation warrants further replicated study as a sediment restoration strategy.