Effects of phosphate on arsenate inhibition in a marine cyanobacterium,Phormidium sp

Arsenic and high affinity phosphate uptake gene distribution in shallow submarine hydrothermal sediments

The toxicity of arsenic (As) towards life on Earth is apparent in the dense distribution of genes associated with As detoxification across the tree of life. The ability to defend against As is particularly vital for survival in As-rich shallow submarine hydrothermal ecosystems along the Hellenic Volcanic Arc (HVA), where life is exposed to hydrothermal fluids containing up to 3000 times more As than present in seawater. We propose that the removal of dissolved As and phosphorus (P) by sulfide and Fe(III)(oxyhydr)oxide minerals during sediment-seawater interaction, produces nutrient-deficient porewaters containing < 2.0 ppb P. The porewater arsenite-As(III) to arsenate-As(V) ratios, combined with sulfide concentration in the sediment and/or porewater, suggest a hydrothermally-induced seafloor redox gradient. This gradient overlaps with changing high affinity phosphate uptake gene abundance. High affinity phosphate uptake and As cycling genes are depleted in the sulfide-rich settings, relative to the more oxidizing habitats where mainly Fe(III)(oxyhydr)oxides are precipitated. In addition, a habitat-wide low As-respiring and As-oxidizing gene content relative to As resistance gene richness, suggests that As detoxification is prioritized over metabolic As cycling in the sediments. Collectively, the data point to redox control on Fe and S mineralization as a decisive factor in the regulation of high affinity phosphate uptake and As cycling gene content in shallow submarine hydrothermal ecosystems along the HVA.

Land scale biogeography of arsenic biotransformation genes in estuarine wetland

As an analogue of phosphorus, arsenic (As) has a biogeochemical cycle coupled closely with other key elements on the Earth, such as iron, sulfate and phosphate. It has been documented that microbial genes associated with As biotransformation are widely present in As-rich environments. Nonetheless, their presence in natural environment with low As levels remains unclear. To address this issue, we investigated the abundance levels and diversities of aioA, arrA, arsC and arsM genes in estuarine sediments at low As levels across Southeastern China to uncover biogeographic patterns at a large spatial scale. Unexpectedly, genes involved in As biotransformation were characterized by high abundance and diversity. The functional microbial communities showed a significant decrease in similarity along the geographic distance, with higher turnover rates than taxonomic microbial communities based on the similarities of 16S rRNA genes. Further investigation with niche-based models showed that deterministic processes played primary roles in shaping both functional and taxonomic microbial communities. Temperature, pH, total nitrogen concentration, carbon/nitrogen ratio and ferric iron concentration rather than As content in these sediments were significantly linked to functional microbial communities, while sediment temperature and pH were linked to taxonomic microbial communities. We proposed several possible mechanisms to explain these results.

Metals in cyanobacteria: analysis of the copper, nickel, cobalt and arsenic homeostasis mechanisms

Traces of metal are required for fundamental biochemical processes, such as photosynthesis and respiration. Cyanobacteria metal homeostasis acquires an important role because the photosynthetic machinery imposes a high demand for metals, making them a limiting factor for cyanobacteria, especially in the open oceans. On the other hand, in the last two centuries, the metal concentrations in marine environments and lake sediments have increased as a result of several industrial activities. In all cases, cells have to tightly regulate uptake to maintain their intracellular concentrations below toxic levels. Mechanisms to obtain metal under limiting conditions and to protect cells from an excess of metals are present in cyanobacteria. Understanding metal homeostasis in cyanobacteria and the proteins involved will help to evaluate the use of these microorganisms in metal bioremediation. Furthermore, it will also help to understand how metal availability impacts primary production in the oceans. In this review, we will focus on copper, nickel, cobalt and arsenic (a toxic metalloid) metabolism, which has been mainly analyzed in model cyanobacterium Synechocystis sp. PCC 6803.

Biomethylation and volatilization of arsenic by the marine microalgae Ostreococcus tauri

Ostreococcus tauri is a marine green microalga, recognized as a model organism of the marine phytoplankton assemblage and widely distributed from coastal to oligotrophic waters. This study showed it could tolerate both arsenite and arsenate concentrations of up to 100μM, and cellular As concentration increased significantly (P<0.01) with increasing concentration of As(V) in the medium (0-50μM). It was revealed that As biotransformations were mediated by algal cells. Volatilized As was detected and the ability of As biovolatilization by O. tauri was demonstrated. The reduction of As(V) to As(III) might be the limiting step for As methylation and volatilization from seawater since the treatment with As(III) yielded five times more volatile As as compared to that with As(V). Arsenic biogeochemical cycle in the marine environment might play an important role based on the huge surface area of ocean (71%) and the massive number of marine phytoplankton.

Arsenic speciation and effect of arsenate inhibition in a Microcystis aeruginosa culture medium under different phosphate regimes

To assess the ecological impact of arsenic pollution during cyanobacterial blooms, As speciation and cyanobacterial growth in phosphate-modified Microcystis aeruginosa cultures treated with arsenate were investigated under laboratory conditions. Marked growth inhibition was observed when arsenate was added. The inhibition effect of 1 µM arsenate was lower than that of 10 µM arsenate. Increasing phosphate supply (0-175 µM) in the medium decreased the inhibition of As. In the medium, arsenate, arsenite, and dimethylarsenicals (DMA) occurred under phosphate-deprivation (0 µM) and phosphate-excess (175 µM) conditions. However, only arsenate and DMA were detected under phosphate-limited (1 µM) and phosphate-rich (10 µM) conditions. Moreover, arsenite and DMA concentrations had significantly positive correlation with the biomass production of M. aeruginosa in the existence of phosphate. Arsenic speciation was more significantly affected by phosphate levels than arsenate concentrations. Recovery of total As content in M. aeruginosa culture medium increased with the increasing phosphate supply. The duration of arsenate contamination in the culture of M. aeruginosa had no influence on the variation of As species but affected the concentration of them in the medium under the phosphate-excess condition. This demonstrated that the effect of M. aeruginosa on As speciation was not related to the duration of As contamination under the phosphate-excess condition.

Effects of arsenate on microcystin content and leakage of Microcystis strain PCC7806 under various phosphate regimes

Both arsenic pollution and eutrophication are prominent environmental issues when considering the problem of global water pollution. It is important to reveal the effects of arsenic species on cyanobacterial growth and toxin yields to assess ecological risk of arsenic pollution or at least understand naturally occurring blooms. The sensitivity of cyanobacteria to arsenate has often been linked to the structural similarities of arsenate and phosphate. Thus, we approached the effect of arsenate with concentrations from 10(-8) to 10(-4) M on Microcystis strain PCC7806 under various phosphate regimes. The present study showed that Microcystis strain PCC7806 was arsenate tolerant up to 10(-4) M. And such tolerance was without reference to both content of intra- and extra-cellular phosphate. It seems that arsenate involved the regulation of microcystin synthesis and cellular polyphosphate contributed to microcystin production of Microcystis responding to arsenate, since there was a positive linear correlation of the cellular microcystin quota with the exposure concentration of arsenate when the cells were not preconditioned to phosphate starvation. It is presumed that arsenate could help to actively export microcystins from living Microcystis cells when preconditioned to phosphate starvation and incubated with the medium containing 1 microM phosphate. This study firstly provided evidence that microcystin content and/or release of Microcystis might be impacted by arsenate if it exists in harmful algal blooms.