An experimental study on the influence of the bloom-forming alga Gonyostomum semen (Raphidophyceae) on cladoceran species Daphnia magna

Abstract

-The effect of the unicellular, bloom-forming alga Gonyostomum semen (Raphidiophyceae) on the survival rate and body size of Daphnia magna was tested under experimental laboratory conditions. Using samples from four humic lakes with a long history of Gonyostomum blooms, we exposed D. magna for 72 h to various Gonyostomum treatments which included homogenized biomass (frozen and fresh), live cell populations as well as lake water separated from the concentrated biomass of live cells. Filtered lake water and the chlorophycean alga Stichococcus bacillaris population (homogenized biomass or live cells) we used as controls. Our study revealed that (1) frozen homogenized G. semen biomass in the concentrations typical for blooms was not harmful for Daphnia and appeared to have a nutritive effect because it supported its growth; however, Daphnia mortality occurred after exposure to fresh and highly concentrated cell homogenate containing high amount of mucilage; (2) it is unlikely that living Gonyostomum cells excrete extracellular substances harmful for Daphnia; however, dense live Gonyostomum population that formed mucilaginous aggregates immobilized Daphnia and increased its mortality. The results suggest that various interactions between G. semen and D. magna take place and may play an essential role in natural freshwater ecosystems.Keywords: algal blooms / mucilage / crustaceans / toxicity Résumé -Une étude expérimentale de l'influence de l'algue formant des efflorescences Gonyostomum semen (Raphidophyceae) sur l'espèce de cladocère Daphnia magna. L'effet de l'algue unicellulaire, formant des efflorescences, Gonyostomum semen (Raphidiophyceae) sur le taux de survie et la taille corporelle de Daphnia magna a été testé dans des conditions expérimentales de laboratoire. En utilisant des échantillons de quatre lacs humiques avec un long passé d'efflorescences de Gonyostomum, nous avons exposé D. magna pendant 72 h à différents traitements de Gonyostomum qui incluent : la biomasse homogénéisée (congelée et fraîche), des populations de cellules vivantes ainsi que l'eau du lac séparée de la biomasse concentrée de cellules vivantes. L'eau filtrée du lac et une population d'algues chlorophycées Stichococcus bacillaris (biomasse homogénéisée ou cellules vivantes) ont été utilisées comme témoins. Notre étude a révélé que : (1) la biomasse congelée homogénéisée de G. semen dans les concentrations typiques d'efflorescences n'était pas nocif pour Daphnia et semblait avoir un effet nutritif car elle a soutenu sa croissance, cependant, la mortalité de Daphnia a eu lieu après une exposition à un homogénat frais et hautement concentré de cellules contenant une grande quantité de mucilage ; (2) il est peu probable que les cellules vivantes de Gonyostomum excrètent des substances extracellulaires nocives pour Daphnia, cependant, une population vivante dense de Gonyostomum vivante qui a formé des agrégats mucilagineux a immobilisé Daphnia et a augmenté sa mortalité. Les résultats suggèrent que diverses interactions entre G. se...

Vertical distribution of expansive, bloom-forming algaeGonyostomum semenvs.plankton community and water chemistry in four small humic lakes

“…Such phenomenon is observed inside blooms of other species, when ambient concentrations of inorganic nutrient forms may be reduced or even depleted due to algal incorporation into its biomass (Heisler et al, 2008). Thus, our results may serve as further example of the influence of G. semen on the lake ecosystem, as it was earlier revealed or suggested in case of plankton, benthic invertebrates or fish (Trigal et al, 2011;Angeler and Johnson, 2013;Karosiene et al, 2014;Pęczuła et al, 2017). Further research including laboratory tests as well as day/night nutrient dynamics within the lake ecosystems dominated by the species may make this topic more clear.…”

“…The alga was previously considered inedible due to its large dimensions which are above the preferred size range for many filterfeeding zooplankton as well as the presence of trichocysts, which eject threads of mucilage in a mechanical stress (Cronberg et al, 1988;Havens, 1989). Nevertheless, recent studies had showed that G. semen mucilage is not harmful for Daphnia magna; moreover, disintegrated cells of the alga may serve as nutritive component enhancing daphnids body growth (Pęczuła et al, 2017). Also, many recent experimental studies revealed that Gonyostomum may be directly grazed by zooplankton, including rotifers (A. priodonta), cladocerans (Daphnia magna, Daphnia pulicaria, Holopedium gibberum) or copepods (Diaptomus oregonensis, Eudiaptomus gracilis) (Williamson et al, 1996;Lebret et al, 2012;Johansson et al, 2013;Björnerås, 2014).…”

“…Trophic relations between G. semen and zooplankton species have been studied recently, mainly testing the algae as a food source, sometimes with contradictory results (Williamson et al, 1996;Lebret et al, 2012;Johansson et al, 2013;Björnerås, 2014). There are also few studies on the impact of G. semen blooms on plankton communities, benthic invertebrates or fishes in the ecosystem scale (Trigal et al, 2011;Angeler and Johnson, 2013;Karosiene et al, 2014) or in the laboratory studies (Pęczuła et al, 2017). Nevertheless, the knowledge on biotic and abiotic relations with G. semen in the context of its vertical distribution is still scarce.…”

Influence of environmental factors on vertical distribution of zooplankton communities in humic lakes

“…Large amount of seston also could limit growth and reproduction of zooplankton (Muller-Navarra and Lampert, 1996;Nix and Jenkins, 2000). According to other authors G. semen during blooms form mucilaginous aggregates which may serve as mechanical traps for large daphnids and in this way may enhance organisms' mortality (Pęczuła et al, 2017). Thus the food quality seems to be an important factor that limits the success of large zooplankton in brown water woodland ponds (Nix and Jenkins, 2000).…”

“…Many of the mixotrophs could form dense blooms in humic lakes. For example, Gonyostomum semen (Ehrenberg) Diesing is a large flagellate that can form dense blooms in dystrophic lakes of the Wigry National Park (Pęczuła et al, 2017). Like many other flagellates, the species is able to swim, therefore it is often found unevenly distributed in the water column.…”