Filamentous green algae, extracellular alkaline phosphatases and some features of the phosphorus cycle in ponds

Filamentous green algae (FGA) may reach high biomass and play a very important functional role in productivity and nutrient cycling in the different water bodies. Their extracellular alkaline phosphatase activity may be an important player in the phosphorus cycle. Currently, there is intensive development of green algae in various freshwater and marine waterbodies, which creates problems for people's activities and necessitates its investigation. Filamentous green algae in four Chinese and Crimean (Russia) shallow freshwater ponds were in focus of this study. The dissolved phosphorus fraction in pond water, algal pigment level, activity and kinetic properties of alkaline phosphatase were evaluated in water column and cell membrane of filamentous green algae. Microalgal taxa were identified in the plankton samples. Species composition and density of FGA in the studied ponds were different. Two ponds had more than 50 % coverage of a water surface by FGA and its wet biomass more than 100 g∙m-2. Two others were with wet biomass less than 2 g∙m-2. In ponds with low FGA biomass, the soluble reactive phosphorus concentration exhibited considerably low level with less than 10 µg∙L-1, and the dissolved organic phosphorus comprised the largest phosphorus fraction, averaging 23.1 µg∙L-1 and ranged from 20.8 to 25.4 µg∙L-1. However, in ponds with high FGA biomass, particulate phosphorus was the major component, which contributes 45.8 % and 56.7 % of total phosphorus, respectively. Size fractionation of extracellular alkaline phosphatase activity in water column expressed spatial heterogeneity, which corresponded with biomass of FGA. The response of extracellular alkaline phosphatase activity to different phosphate concentration in water column was completely distinct from that in the cell membrane of FGA, the last of which represented the significantly inhibition effect to high phosphate concentration. The significant inhibition of alkaline phosphatase activity in cell membrane of FGA by phosphate in water may validate that FGA growth was limited by phosphorus. The contradiction between a low concentration of soluble reactive phosphorus and high FGA biomass may indicate that there was high speed nutrient cycling, probably, due to the alkaline phosphatase activity. Excreting exo-alkaline phosphatases, FGA, microalgae and bacteria accelerate phosphorus cycling through different mechanisms, and this may increase their development. In ponds with high FGA biomass, many of bacteria are responsible for regeneration of nutrients, which then consuming by FGA. Those bacteria also may concurrently restrict a microalgae development, such as unicellular Chlorophyta species. As an example, Cladophora provides habitat for different species of epibionts (bacteria and microalgae, primarily diatoms), and sustains of strong mutualistic alga-bacterium interactions. Therefore, the problem of excessive FGA growth should not be considered in isolation, but in a whole-ecosystem context.