Xiaoji Fan

Allelopathic interactions of linoleic acid and nitric oxide increase the competitive ability of Microcystis aeruginosa.

The frequency and intensity of cyanobacterial blooms are increasing worldwide with major societal and economic costs. Interactions between toxic cyanobacteria and eukaryotic algal competitors can affect toxic bloom formation, but the exact mechanisms of interspecies interactions remain unknown. Using metabolomic and proteomic profiling of co-cultures of the toxic cyanobacterium Microcystis aeruginosa with a green alga as well as of microorganisms collected in a Microcystis spp. bloom in Lake Taihu (China), we disentangle novel interspecies allelopathic interactions. We describe an interspecies molecular network in which M. aeruginosa inhibits growth of Chlorella vulgaris, a model green algal competitor, via the release of linoleic acid. In addition, we demonstrate how M. aeruginosa takes advantage of the cell signaling compound nitric oxide produced by C. vulgaris, which stimulates a positive feedback mechanism of linoleic acid release by M. aeruginosa and its toxicity. Our high-throughput system-biology approach highlights the importance of previously unrecognized allelopathic interactions between a broadly distributed toxic cyanobacterial bloom former and one of its algal competitors.

Analysis of the Proteome of the Marine Diatom Phaeodactylum tricornutum Exposed to Aluminum Providing Insights into Aluminum Toxicity Mechanisms.

Trace aluminum (Al) concentrations can be toxic to marine phytoplankton, the basis of the marine food web, but the fundamental Al toxicity and detoxification mechanisms at the molecular levels are poorly understood. Using an array of proteomic, transcriptomic, and biochemical techniques, we describe in detail the cellular response of the model marine diatom Phaeodactylum tricornutum to a short-term sublethal Al stress (4 h of exposure to 200 μM total initial Al). A total of 2204 proteins were identified and quantified by isobaric tags for relative and absolute quantification (iTRAQ) in response to the Al stress. Among them, 87 and 78 proteins performing various cell functions were up- and down-regulated after Al treatment, respectively. We found that photosynthesis was a key Al toxicity target. The Al-induced decrease in electron transport rates in thylakoid membranes lead to an increase in reactive oxygen species (ROS) production, which cause increased lipid peroxidation. Several ROS-detoxifying proteins were induced to help decrease Al-induced oxidative stress. In parallel, glycolysis and pentose phosphate pathway were up-regulated in order to produce cell energy (NADPH, ATP) and carbon skeleton for cell growth, partially circumventing the Al-induced toxicity effects on photosynthesis. These cellular responses to Al stress were coordinated by the activation of various signal transduction pathways. The identification of Al-responsive proteins in the model marine phytoplankton P. tricornutum provides new insights on Al stress responses as well as a good start for further exploring Al detoxification mechanisms.

Spatial Variability of Cyanobacteria and Heterotrophic Bacteria in Lake Taihu (China).

Cyanobacterial blooms frequently occur in Lake Taihu (China), but the intertwined relationships between biotic and abiotic factors modulating the frequency and duration of the blooms remain enigmatic. To better understand the relationships between the key abiotic and biotic factors and cyanobacterial blooms, we measured the abundance and diversity of prokaryotic organisms by high-throughput sequencing, the abundance of key genes involved in microcystin production and nitrogen fixation or loss as well as several physicochemical parameters at several stations in Lake Taihu during a cyanobacterial bloom of Microcystis sp.. Measurements of the copy number of denitrification-related genes and 16S rRNA analyses show that denitrification potential and denitrifying bacteria abundance increased in concert with non-diazotrophic cyanobacteria (Microcystis sp.), suggesting limited competition between cyanobacteria and heterotrophic denitrifiers for nutrients, although potential bacteria-mediated N loss may hamper Microcystis growth. The present study provides insight into the importance of different abiotic and biotic factors in controlling cyanobacteria and heterotrophic bacteria spatial variability in Lake Taihu.

Inhibitory effects of tributyl phosphate on algal growth, photosynthesis, and fatty acid synthesis in the marine diatom Phaeodactylum tricornutum

The widely used solvent extractant, tributyl phosphate (TBP), primarily used as a solvent for the conventional processing of nuclear fuel, has come under scrutiny recently due to concerns surrounding potential environmental contamination and toxicity. In this study, we found that, in Phaeodactylum tricornutum, administration of TBP severely inhibited algal cell growth by reducing photosynthetic efficiency and inducing oxidative stress. We further explored the effect of TBP by examining the gene expression of the photosynthetic electron transport chain and its contribution to reactive oxygen species (ROS) burst. Our data revealed that TBP affected both fatty acid content and profile by regulating the transcription of genes related to glycolysis, fatty acid biosynthesis, and β-oxidation. These results demonstrated that TBP did in fact trigger the synthesis of ROS, disrupting the subcellular membrane structure of this aquatic organism. Our study brings new insight into the fundamental mechanism of toxicity exerted by TBP on the marine alga P. tricornutum.

Diuron treatment reveals the different roles of two cyclic electron transfer pathways in photosystem II in Arabidopsis thaliana

Three ecotypes of Arabidopsis thaliana, ecotype Columbia (Wild type, Wt) and two mutants (pgr5 and ndf4), were used to evaluate the effects of diuron on photosynthetic activity of A. thaliana. It was found that diuron adversely affected the fresh weight and chlorophyll content of the plants. Chlorophyll fluorescence studies determined that the pgr5 mutant was more sensitive to diuron than Wt and the ndf4 mutant. Gene expression analysis revealed different roles for the two cyclic electron transfer (CET) pathways, NAD(P)H dehydrogenase (NDH) and proton gradient regulation (PGR5) pathways, in the plant after diuron treatment. For example, a gene in the NDH pathway, lhca5, was activated in the low dose (LD) group in the pgr5 mutant, but was down-regulated in the moderate dose (MD) group, along with two other NDH-related genes (ppl2 and ndhH). In the PGR5 pathway, the pgr5 gene was functional under conditions of increased stress (MD group), and was up-regulated to a greater extent in the ndf4 mutant than that in the Wt and pgr5 mutant. Our results suggest that the PGR5 pathway in plants is more important than the NDH pathway during resistance to environmental stress. Deficiencies in the PGR5 pathway could not be counteracted by the NDH pathway, but deficiencies in the NDH pathway could be overcome by stimulating PGR5.