Planktothrix agardhii versus Planktothrix rubescens: Separation of Ecological Niches and Consequences of Cyanobacterial Dominance in Freshwater

Abstract

Cyanobacteria dominate lakes under diverse trophic conditions. Of these, two harmful filamentous cyanobacterial species, namely Planktothrix agardhii and P. rubescens, occupy completely different ecological niches in which they can form dense populations. In the present study, we investigated the effects of environmental conditions on the growth and vertical distribution of these species in lakes of different trophic statuses. Moreover, we underscored certain inconveniences in the assessment of the ecological status of lakes according to the European Union Water Framework Directive. The highest biomass of P. agardhii was recorded in eutrophic lake at a depth of 0.5–1 m, under high light intensity. Meanwhile, the highest biomass of P. rubescens, at which the deep chlorophyll maximum was recorded, was observed in mesotrophic lakes at a depth of 11–12 m, often below the euphotic zone under very low light intensity. P. rubescens, but not P. agardhii, exerted a strong allelopathic effect on the diversity and biomass of phytoplankton. Moreover, both species utilised different dissolved nitrogen fractions for their growth; specifically, P. agardhii used ammonium nitrogen, whilst P. rubescens used nitrate nitrogen. Furthermore, dissolved phosphorus produced a potentially limiting effect on P. rubescens growth. Overall, the tested Polish PMPL, German PSI, and Estonian phytoplankton indices were indeed useful in the assessment of the ecological status of lakes, albeit limited to the eutrophic lake with a high biomass of cyanobacteria (P. agardhii) in the upper water layers. However, problems arose in the proper assessment of lakes with a high biomass of cyanobacteria (P. rubescens) with a deep chlorophyll maximum outside the range of the euphotic zone. In such cases, two of the tested indices, namely the Polish and German indices, allowed sample collection from the euphotic layers, which significantly affected the number of samples included in the calculation. Consequently, the correct calculation of the ecological status of the lake was uncertain. Only the Estonian index allowed for a sample collection from two to three thermal layers of water, including the bloom layer of P. rubescens. Hence, the Estonian index offered the best fit for calculations.

Allelopathic Potential of the Cyanotoxins Microcystin-LR and Cylindrospermopsin on Green Algae

“…As a result of these changes, the ratio between the mainly represented taxonomic groups of algae and cyanobacteria, forming the phytoplankton in freshwater basins, has also changed in recent years. It has been reported that cyanobacteria are a dominant group in many water bodies [ 56 , 57 , 58 , 59 , 60 , 61 ]. In some water bodies, there is even a displacement of the Chlorophyta, which traditionally occupy the first place in the phytoplankton of freshwater basins, by representatives of cyanobacteria [ 57 , 60 , 62 ].…”

The Elucidation of Planktothrix Cyanopeptide Profiles using Mass Spectrometry-Based Metabolomics

“…Cyanobacteria strains from the Planktothrix genus are the main focus of this investigation. P. agardhii (formerly Oscillatoria agardhii) and P. rubescens (formerly Oscillatoria rubescens) possess similar morphological organization, featuring trichome structures [64][65][66] . However, morphological taxonomic distinction is possible by their differing photosynthetic pigments.…”

“…P. agardhii characteristically presents as a blue-green hue generated from its phycocyanin production. P. rubescens prominently generates red phycoerythrin, a phenotype that P. agardhii lacks entirely 61,66 . Both filamentous species are devoid of heterocysts and possess gas vesicles that facilitate buoyancy migration in aquatic environments.…”

“…Both filamentous species are devoid of heterocysts and possess gas vesicles that facilitate buoyancy migration in aquatic environments. Buoyancy control allows the cyanobacteria to vertically migrate throughout the water column to occupy a position with optimal light, nutrient and temperature, as environmental conditions shift 66 . The two species inhabit distinct niches according to their light intensity and temperature preferences.…”

“…P. agardhii favors upper water layers with increased light intensity and temperature, characteristic to shallow eutrophic lakes. Alternatively, P. rubescens occupies deeper water layers with light deficient conditions and lower temperatures in mesotrophic lakes 61,66 . This niche division suggests these two species may have discrete responses to heightened light intensity, considering the more sensitive photoinhibition in P. rubescens than in P. agardhii 66 .…”