Xiao Tan

Differences in Responses to Darkness between Microcystis aeruginosa and Chlorella pyrenoidosa

ABSTRACT The effect of darkness on the cyanobacterium Microcystis aeruginosa and the green alga Chlorella pyrenoidosa were observed. Cell growth, metabolic activity, chlorophyll a fluorescence, cell size, and maximum quantum yield (Fv: Fm) were determined. M aeruginosa and C. pyrenoidosa both maintained their initial cell abundances while in the dark, but they showed distinctly different physiological responses to darkness. During the dark period metabolic activity decreased slightly, cell size and Fv: Fm decreased significantly, and chlorophyll a increased markedly in M. aeruginosa cells. However, metabolic activity, cell size, and Fv: Fm decreased dramatically, and chlorophyll a decreased trivially in C. pyrenoidosa cells. After re-illumination, metabolic activity, cell size, and Fv: Fm of M. aeruginosa and metabolic activity, chlorophyll a, and Fv: Fm of C. pyrenoidosa recovered significantly. No recovery was found in either chlorophyll a of M. aeruginosa or cell size of C. pyrenoidosa. Compared with C. pyrenoidosa, M. aeruginosa is more capable of withstanding the dark circumstances, which are unfavorable to the growth of alga but usually exists in water and sediment of eutrophic lakes. Its greater tolerance of darkness may be one of the reasons that M. aeruginosa is commonly the dominant species in eutrophic lakes.

Variations of Bacterioplankton Community Composition during Microcystis spp. Blooms in a Shallow Eutrophic Lake

ABSTRACT Variations of bacterioplankton community composition were monitored during Microcystis spp. blooms in Lake Taihu using denaturing gradient gel electrophoresis (DGGE) of amplified DNA fragments from April through November in 2005. Two sampling sites were selected—one in Meiliang Bay and the other in the center of the western region of the lake. The average abundance of Microcystis spp. was higher in Meiliang Bay than in the lake center, while the bacterioplankton community diversity of Meiliang Bay was significantly lower than that of the lake center (P < 0.05). Comparison of DGGE profiles indicated that in Meiliang Bay bacterioplankton communities decreased in band richness and species diversity as Microcystis spp. abundance increased. In the lake center, DGGE band number and Shannon index reached their peaks when Microcystis spp. blooms declined. The Shannon diversity index for the bacterioplankton community was negatively correlated with the Microcystis spp. abundance (r = −0.53, P = 0.10). Changes in composition and biodiversity of the bacterioplankton community in the two regions of the lake were moderately correlated with Microcystis spp. bloom formations, which suggested a possible linkage between bacterioplankton community variations and phytoplankton blooms events.

Interspecific Interaction between Microcystis aeruginosa and Chlorella pyrenoidosa in Different Phosphate Media

ABSTRACT The effects of pyrophosphate and polyphosphate on the growth of Microcystis aeruginosa and Chlorella pyrenoidosa were studied in unialgal cultures and in cultures separated by a dialysis membrane. M. aeruginosa and C. pyrenoidosa were able to similarly utilize pyrophosphate and polyphosphate, but their growth rates and cell densities were low in comparison with those in the orthophosphate controls. C. pyrenoidosa had a higher growth rate than M. aeruginosa in both unialgal and the separated cultures; moreover, C. pyrenoidosa had a greater ability to utilize phosphorus than M. qeruginosa. But because the C. pyrenoidosa population developed early but then declined, M. aeruginosa eventually became dominant in the separated cultures. Therefore, the interaction between M. aeruginosa and C. aeruginosa might be complex because of an allelopathy effect, and nutrient competition is not the only factor determining species dominance in the phytoplankton assemblage.

Recruitment of Phytoplankton from Winter Sediment of Lake Taihu: A Laboratory Simulation

ABSTRACT We collected sediment from Lake Taihu in winter, placed it in BG-11 medium, and then observed the recruitment of phytoplankton to the water column as incubation temperature was increased stepwise in eight increments from 55°C at the start of incubation to 30°C after 70 days. Recruitment of chlorophytes and diatoms was observed above 9°C, but recruitment of cyanobacteria was not detected until 12.5°C. Subsequently, cyanobacteria established dominance above 19.5°C.

Stable Isotope Analyses of Size-Based Microplankton Trophic Structure in Taihu Lake during a Cyanobacterial Bloom

ABSTRACT Variations of δ13C and δ15N in dissolved organic matter and size-fractionated plankton, ranging from 0.7 to 200 μm, were studied in Taihu Lake on two occasions during a cyanobacterial bloom. Paired-sample t-tests indicated that stable isotope ratios of samples collected did not significantly differ from each other on the two sampling dates. The overall correlation between δ13C and δ15 of forty taxa of the plankton community was weak (r2=0.227, N=40), indicating a non-trophic enrichment effect in δ13C. The δ15N values increased along with plankton size. We used δ15 as an index of the trophic level and demonstrated that logarithmic organism size was positively and significantly related to trophic level, indicating that a size-dependent approach is preferable in the analysis of trophic isotope enrichment in microplankton food webs.