|Other Abstract||Poterioochromonas sp. strain DO-2004, isolated from mass culture of Microcystis, can not only grow autotrophically, but also live on Microcystis. The present study attented its growth and ingest behavior along with different nutrition patterns, and focused further on the interaction between Poterioochromonas sp. and Microcystis in the following aspects: how fast was the growth rate of Poterioochromonas sp. when feeding on Microcystis; was Poterioochromonas sp. sensitive to toxic Microcystis; what was the environmental conditions for the growth of Poterioochromonas sp.. The answers to these questions may help illustrate the implications of this organism in dynamic of Microcystis bloom and explore the potential role of Poterioochromonas sp. as biological control agent.
The results showed that this golden alga has size-based prey selectivity. Poterioochromonas sp. were able to swallow preys, whose diameters varied from 1 to 10 μm, such as M. aeruginosa, Synechocystis sp.,Gloeocapsa alpicola, Chroococcus sp., Chlorella vugaris, Chlamydomonas reinhardtii. And Poterioochromonas sp. could swallow not only the immobile algae but also mobile ones when the size was suitable. Poterioochromonas sp. could swallow short catenarian and filiform organisms (eg: Anabaena flos-aquae and Phormidium mucicola) whose total length did not exceed 10 μm.
In different nutrition patterns, the growth rates of Poterioochromonas sp. were expressed in the decreasing order as mixotrophy, autotrophy, phagotrophy and osmotrophy. Feeding preys had a significant effect on the growth rate. Poterioochromonas sp. in mixotrophy (0.252 ± 0.019 d-1) reproduced more quickly than that in autotrophy (0.151 ± 0.012 d-1) when feeding on Microcystis aeruginosa FACHB469. And Poterioochromonas sp. could not maintain its growth only by obligate phagotrophy and osmotrophy. The shape and ultrastructure of Poterioochromonas sp. were diverse during the ingestion and digestion of prey.
The growth rate of autotrophic Poterioochromonas sp. was affected by light intensity and temperature significantly, while the correlation between growth rate and pH was not significant. When providing preys, the growth of Poterioochromonas sp. was accelerated significantly. The growth and ingestion rate of Poterioochromonas sp. increased hyperbolically with prey concentration. When there was commensurate prey, the correlation between the growth of mixotrophic Poterioochromonas sp. and light intensity was significant, while the correlations between the growth and temperature and pH were not significant. The negative correlation between the ingestion rate and pH was much significant, while the correlations between the ingestion rate and temperature and light intensity were not significant. In a word, this Poterioochromonas has the ability to live and graze Microcystis in many culture conditions.
In order to investigate whether Microcystis have negative effects on Poterioocromonas, the activity of several antioxidant enzymes and ultrastructures of autotrophic and mixotrophic Poterioocromonas sp. were compared. The results showed that both toxic and nontoxic Microcystis could accelerate the growth of Poterioocromonas. The contents of MDA and GSH between mixotrophic and autotrophic Poterioocromonas were not different significantly, while SOD and CAT in mixotrophy were much higher than those in autotrophy. The number of chloroplasts of mixotrophic Poterioocromonas increased and mitochondria became larger. In conclusion, both toxic and nontoxic Microcystis could accelerate the growth of Poterioocromonas sp. DO-2004.
When added into high concentration of preys (107 cells•ml-1), Poterioocromonas sp. (initial concentration: 103 cells•ml-1) reproduced quickly. Unlike most daphnids and rotifers, Poterioochromonas sp. was not sensitive to toxic Microcystis. The growth rates of Poterioocromonas were among 0.079 - 0.128 h-1 for different conditions. The highest growth rate (0.128 h-1) was observed when feeding on Gloeocapsa alpicola in aeration. On the other hand, the biomass of preys decreased quickly. Chlorophyll a concentration decreased to 17-30% at 120 h for the aerated groups, to 48-75% at 120 h for the unaerated groups.
The ultrastructure of Poterioocromonas sp. changed a lot during this special process. The chloroplasts of most predators became shrunken or disappeared in the first few days, and there were also many large osmiophilic globules located in the cytoplasm at the same time. When Poterioochromonas sp. entered a “stationary growth phase”, their chloroplasts could appear again, and the large osmiophilic globules disappeared simultaneously.
Field experiment in mini raceway ponds also confirmed that Poterioochromonas sp. affects the dynamic of Microcystis bloom significantly. In the treated ponds Poterioochromonas sp. increased quickly from 8.70×102 cells•ml-1 to about 7.7×104 cells•ml-1 in 3 d and M. aeruginosa PCC7806 decreased on the contrary. At the end of the experiment the residual Microcystis in the treated ponds was less than 5% of the control. This experiment demonstrated that Poterioochromonas sp. was able to grow and ingest Microcystis in the field and accelerate the disappearance of Microcystis.
The present study had also shed light on the possibility of utilizing this organism, together with other agents, for the control of Microcystis bloom.|