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The allelopathic effect of Prymnesium parvum (Prymnesiophyta), which produces toxins with haemolytic, ichthyotoxic and cytotoxic properties, was investigated on a natural plankton community. Under controlled conditions, 3 laboratory bioassays were performed by adding cell-free filtrate from a P. parvum culture into different size fractions (<150, <100 and 20 to 150 µm) of a natural Baltic Sea plankton community. The effect of P. parvum cell-free filtrate was determined by measuring chlorophyll a, cell numbers (phytoplankton, ciliates, bacteria), carbon ( 14 C) uptake by phytoplankton and the incorporation of 3 H-leucine by bacteria. P. parvum cell-free filtrate affected the whole phytoplankton community, resulting in a decrease in both chlorophyll a and carbon uptake. Furthermore, the plankton groups present in the community exhibited different sensitivity to the cellfree filtrate. While growth of cyanobacteria and dinoflagellates was inhibited, that of diatoms and ciliates was not only completely suppressed, but no cells were present at the end of the experiment in the bottles with P. parvum filtrate. In all experiments, therefore, cyanobacteria and dinoflagellates were the most resistant groups, which led to their dominance in the treatments with filtrate compared to controls. Bacterial production was also negatively affected by P. parvum filtrate. The results show that compounds released by P. parvum induce changes in the plankton community structure, killing other members of the marine food-web, especially other phytoplankton (allelopathy), and suggest that secreted compounds of P. parvum are inhibitory to potential grazers (ciliates). It is proposed that allelopathy is an important process in the ecology of P. parvum.KEY WORDS: Prymnesium parvum · Allelopathy · Natural plankton community · Phytoplankton · Ciliates · BacteriaResale or republication not permitted without written consent of the publisher

Guanabara Bay is the second largest bay in the coast of Brazil, with an area of 384 km2. In its surroundings live circa 16 million inhabitants, out of which 6 million live in Rio de Janeiro city, one of the largest cities of the country, and the host of the 2016 Olympic Games. Anthropogenic interference in Guanabara Bay area started early in the XVI century, but environmental impacts escalated from 1930, when this region underwent an industrialization process. Herein we present an overview of the current environmental and sanitary conditions of Guanabara Bay, a consequence of all these decades of impacts. We will focus on microbial communities, how they may affect higher trophic levels of the aquatic community and also human health. The anthropogenic impacts in the bay are flagged by heavy eutrophication and by the emergence of pathogenic microorganisms that are either carried by domestic and/or hospital waste (e.g., virus, KPC-producing bacteria, and fecal coliforms), or that proliferate in such conditions (e.g., vibrios). Antibiotic resistance genes are commonly found in metagenomes of Guanabara Bay planktonic microorganisms. Furthermore, eutrophication results in recurrent algal blooms, with signs of a shift toward flagellated, mixotrophic groups, including several potentially harmful species. A recent large-scale fish kill episode, and a long trend decrease in fish stocks also reflects the bay’s degraded water quality. Although pollution of Guanabara Bay is not a recent problem, the hosting of the 2016 Olympic Games propelled the government to launch a series of plans to restore the bay’s water quality. If all plans are fully implemented, the restoration of Guanabara Bay and its shores may be one of the best legacies of the Olympic Games in Rio de Janeiro.

We investigated the effects of cell-free filtrates of the cyanobacteria Nodularia spumigena, Aphanizomenon sp. and Anabaena sp. on a brackish-water plankton community (<150 µm). In a laboratory experiment, we monitored the concentration of chlorophyll a and cell numbers of bacteria, phytoplankton and ciliates in the experimental units treated with cyanobacterial filtrates and the control medium. Cyanobacterial filtrates altered the cell numbers of many phytoplankton groups; they stimulated both colonial (Snowella spp.) and filamentous cyanobacteria (Pseudanabaena spp., Anabaena spp., Aphanizomenon sp., N. spumigena), a chlorophyte (Oocystis sp.), a dinoflagellate (Amphidinium sp.) and nanoflagellates, but inhibited cryptophytes. The filtrates also increased the numbers of bacteria at the beginning of the experiment, whereas they had no effect on ciliate abundances. Our results suggest that cyanobacteria may enhance the effects of eutrophication by stimulating various phytoplankton groups. The specific compounds causing this stimulation remain to be studied. On the other hand, cyanobacteria also seem able to reduce the biomass of certain phytoplankton species, via an allelopathic mechanism.KEY WORDS: Cyanobacteria · Nodularia · Aphanizomenon · Anabaena · Allelopathy · Baltic SeaResale or republication not permitted without written consent of the publisher