Lingling Zheng

Seasonal dynamics of water bloom-forming Microcystis morphospecies and the associated extracellular microcystin concentrations in large, shallow, eutrophic Dianchi Lake.

The increasing occurrence of Microcystis blooms is of great concern to public health and ecosystem due to the potential hepatotoxic microcystins (MCs) produced by these colonial cyanobacteria. In order to interpret the relationships between variations of Microcystis morphospecies and extracellular MC concentrations, the seasonal dynamics of phytoplankton community composition, MC concentrations, and environmental parameters were monitored monthly from August, 2009 to July, 2010. The results indicated that Microcystis dominated total phytoplankton abundance from May to December (96%-99% of total biovolume), with toxic Microcystis viridis and non-toxic Microcystis wesenbergii dominating after July (constituting 65%-95% of the Microcystis population), followed by M. viridis as the sole dominant species from November to January (49%-93%). Correlation analysis revealed that water temperature and nutrient were the most important variables accounting for the occurrence of M. wesenbergii, while the dominance of M. viridis was related with nitrite and nitrate. The relatively low content of MCs was explained by the association with a large proportion of M. viridis and M. wesenbergii, small colony size of Microcystis populations, and low water temperature, pH and dissolved oxygen. The extracellular MC (mean of 0.5±0.2μg/L) of water samples analyzed by enzyme-linked immunosorbent assay (ELISA) demonstrated the low concentrations of MC in Dianchi Lake which implied the low potential risk for human health in the basin. The survey provides the first whole lake study of the occurrence and seasonal variability of Microcystis population and extracellular MCs that are of particular interest for water quality monitoring and management.

Heterogeneity of buoyancy in response to light between two buoyant types of cyanobacterium Microcystis

Previous investigations suggested that buoyancy state in response to light differed between individuals within natural populations of cyanobacteria. To understand the mechanisms of heterogeneity of buoyancy in different species/strains in relation to light, two types of colonial Microcystis in different buoyancy behavior were selected and used to compare their photosynthetic activity, gas vesicle volume, ballast mass, and migration at varying light regime. The photosynthesis–irradiance curve and Fv/Fm examination indicated that negatively buoyant strains were more adapted at high irradiance than buoyant ones. Transcription levels of gvp gene and gas vesicle volume decreased in buoyant strains, but increased in negatively buoyant ones at high irradiance. The results indicated that the combination effect of decrease in gas vesicle buoyancy and increase in carbohydrate contributed to the downward migration of buoyant strains, while the significant increase of gas vesicles provided sufficient buoyancy to negate ballast mass, resulting in the upward migration of negatively buoyant ones at high irradiance. In addition, either sinking or floating velocities were elevated in buoyant and negatively buoyant strains, coincidently with the colony enlargement of all strains at high irradiance, respectively. These findings suggest that this heterogeneity was associated with the interplay between gas vesicles, ballast, and colony size. The fact that different species/strains of Microcystis respond diversely to light depending on their physiological conditions presents a good example to understand heterogeneity of buoyancy in the field, and the presence of heterogeneity of buoyancy may be implicated in the dominancy and persistence of Microcystis bloom in ever-changing environment.

Identification of high-lipid producers for biodiesel production from forty-three green algal isolates in China

To identify some desirable algal strains for our future research and/or the production of algae-based biofuel, 43 green algal strains were successfully isolated from Chinese freshwaters, and then incubated in the laboratory bioreactors for the growth and oil accumulation investigations. During a 15 d incubation experiment, the accumulations of their biomass and total lipids, together with the lipid productivities for these green algal strains were systematically investigated and compared. Results indicated that the accumulations of biomass for the 43 algal strains ranged from 0.53 g/L to 6.07 g/L during the experiments, with the highest biomass of 6.07 g/L for green algae Scenedesmus bijuga. The lipid content for the tested algal strains varied from 20% to 51% of the dry biomass at the end of cultivation experiments. Green algae Chlorella pyrenoidosa was one of the best oil producers based on our investigations, with the total lipid content of 51% of dry biomass. Taking the growth rates and the accumulations of intracellular lipids into the consideration, 10 strains were considered to have significant potential for biofuel applications. In addition, the lipid productivities of the selected strains were further investigated.

Applicability of the fluorescein diacetate assay for metabolic activity measurement of Microcystis aeruginosa (Chroococcales, Cyanobacteria)

The fluorescein diacetate (FDA) assay has been widely used to measure metabolic activity in phytoplankton. It was found that FDA fluorescence values did not decrease in some stressed cells, demonstrating that the applicability of the method needs to be assessed further in the context of growth‐influencing conditions. In the present study, changes of FDA fluorescence values were studied in bloom‐forming cyanobacterial Microcystis aeruginosa Kütz cells under stress conditions such as nitrogen (N) or phosphorus (P) deficiency, or darkness and low temperature (10°C), respectively. The results demonstrated that esterase activity decreased immediately in dark‐stressed cells, which correlated with the decline of biomass and photosynthetic activity. Under the other three stress conditions, however, especially at low temperature, the cells lost photosynthetic activity but had the highest esterase activity, which was five times higher than the control group. These findings contrast with the assay criteria that the expression of a stain should reflect the change of photosynthetic activity and that stressed cells should have a lower staining intensity than the control cells. According to these results, the esterase activity response was dependent on environmental factors. Furthermore, higher fluorescence intensity did not mean higher metabolic activity, but a discrepant value indicated a severe stress.

Influence of sunlight on the proliferation of cyanobacterial blooms and its potential applications in Lake Taihu, China

To learn the relationship between sunlight intensity and cyanobacterial proliferations for the further control of the heavy blooms, enclosure experiment were conducted in Meiliang Bay, Lake Taihu by regulating the natural light intensities with different shading ratio (0% (full sunlight), 10%, 25%, 50% and 75% of original natural sunlight intensities) from September to November in 2010. The results indicated that phytoplankton biomass (mean) decreased significantly when the shading ratios reached 50% or more. Higher shading ratios (e.g. 75%) were very efficient in controlling the average and total cyanobacterial bloom biomass, while 50% shading ratio was proven very effective either in controlling the peak value of phytoplankton biomass or postponing the occurrence of cyanobacterial blooms in Lake Taihu. In addition, phytoplankton composition and photosynthesis efficiency were also affected by altering the shading ratios, and in turn, they might also act on phytoplankton growth. Based on the results from the present study, intermediate shading strategies such as regulation of water level or turbidity through the hydrology regulations would probably be an effective and efficient method in controlling cyanobacterial blooms in large and shallow lakes.

Host Re-identification of Cyanophage PP and Its Implications for Host Range and Specificity

In this study,we re-identified the hosts of cyanophage PP. Twenty-three candidate algal strains were tested,and the results indicated that seven strains belonging to Plectonema and Phormidium are the hosts of cyanophage PP,including two previously reported filamentous cyanobacteria,Plectonema boryanum and Phormidium foveolarum (Zhao et al. 2002).

Effect of Microcystis Aeruginosa (Cyanobacteria) Abundance on the Growth and Ingestion of Poterioochromonas sp. (Chrysophyceae)

Poterioochromonas sp., isolated from the mass culture of Microcystis, was a mixotrophic flagellate. It could grow not only in inorganic medium, but also by ingestion and digestion of Microcystis. In this study, the growth and ingestion characteristics of Poterioochromonas sp. was investigated with modified Monod model and Michaelis-Menten model, respectively. The results showed that, the relationship between flagellate growth rate (initial concentration ap 10⁵ cellsldrmL^-1) and prey density (Microcystis aeruginosa FACHB469, *10⁶ cellsldrmL^-1) was denoted by the fomula: growth rate (mu, h^-1)= 0.054* [(x+1.46)/(4.24 +x+1.46)] (r² = 0.919). And the model for ingestion rate was expressed by the equation: Ingestion rate (I, cellsldrflag^-1 h^-1) = 3.3*[x/ (3.76 +x)] (r² = 0.946).

Physiological sensitivity of Haematococcus pluvialis (Chlorophyta) to environmental pollutants: a comparison with Microcystis wesenbergii (cyanobacteria) and Pseudokirchneriella subcapitata (Chlorophyta)

Haematococcus pluvialis is beneficial to human health and is important for commercial use. However, it seldom prevails in permanent freshwater bodies. Increasing environmental pollutants from anthropogenic activity may threaten the wide distribution of H. pluvialis. Here, we quantified and compared the adverse effects of the common pesticides atrazine, pentachlorophenol, malathion, and 3,5-dichlorophenol and the heavy metals Cu(II), Cr(VI), and Cd(II) on H. pluvialis, Microcystis wesenbergii (a freshwater bloom-forming cyanobacterium), and Pseudokirchneriella subcapitata (a standard toxicity test species). We found that H. pluvialis was the species most sensitive to 3,5-dichlorophenol and Cr(VI) exposure and the most tolerant to pentachlorophenol exposure according to IC50, changes in chlorophyll a content, maximum electron transport rates (ETRmax), the quantum efficiency of photosystem II (Fv/Fm), and esterase activity. Haematococcus pluvialis was also the species most sensitive to atrazine according to IC50, chlorophyll a, and ETRmax. Overall, our findings suggest that atrazine, 3,5-dichlorophenol, and Cr(VI) are potential factors limiting the distribution of H. pluvialis. We suggest that H. pluvialis can be a potentially useful bioindicator for evaluating pollutants. Furthermore, ETRmax, FDA assay, and flow cytometry can be combined with Haematococcus to test for toxicity.

Morphology and phylogeny of three planktonic Radiococcaceae sensu lato species (Sphaeropleales, Chlorophyceae) from China, including the description of a new species Planktosphaeria hubeiensis sp. nov.

The family Radiococcaceae sensu lato, defined as colonial autospore-producing mucilaginous coccoid green algae, is widespread in terrestrial and freshwater habitats. Three species of Radiococcaceae sensu lato, including two Radiococcus species and one Planktosphaeria species, were described from China by light and electron microscopy. A new species of Planktosphaeria, Planktosphaeria hubeiensis sp. nov. was erected based on morphological comparisons and genetic analyses. Our phylogenetic analyses indicated that Radiococcaceae sensu lato is polyphyletic, and separated into three lineages. The Radiococcus species did not cluster into a monophyletic group in phylogenetic analyses; therefore the taxonomy of the genus Radiococcus should be revised in the future.

A fluorescence ratio-based method to determine microalgal viability and its application to rapid optimization of cryopreservation

The utility of microalgal biomass and bioproducts depends on long-term maintenance of certain physiological or biochemical features of the species. While unique characteristics may not be durably maintained with general subculture, cryopreservation methods better prevent alterations from desired characteristics. Post-thaw viability is critical to establishing microalgal cultures, and there is a critical need to effectively and rapidly evaluate microalgal viability after the post-thawing process. In the present study, we developed a rapid assay based on the change of fluorescence ratio to determine microalgal viability post-thaw. It was shown that the assessment of microalgal viability by the fluorescence ratio method correlated well with that of the FDA-staining (R2 = 0.978) and regrowth method (R2 = 0.976), demonstrating that the present method could be applied in the high-throughput detection of viability of microalgal strains. Subsequent to establishing this method, we aimed to find out optimal cryopreservation protocol for each strain from a group of 125 microalgal strains. The viability of these strains under different treatments was quickly evaluated by the fluorescence ratio method. Of these strains, 95 attained post-thaw viability over 60%. DMSO was a suitable cryoprotectant for most strains at a concentration ≤10%. Based on the dataset, the relative contribution of 3 variables-genus, cryoprotectants and concentration to post-viability was analyzed with the Random Forest (RF) classification method. All variables together could explain 97.8% of the viability, and type and concentration of cryoprotectant could explain 59.1% in Chlorophyta. This study provided a new approach for viability assay and demonstrated that this method can facilitate to find out the optimal protocols for cryopreservation of microalgal strains.