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Cyanobacterial blooms can be harmful to environmental and human health due to the production of toxic secondary metabolites, known as cyanotoxins. Microcystins (MCs), one of the most widespread class of cyanotoxins in freshwater, have been found to be positively correlated with cyanobacterial biomass as well as with nitrogen and phosphorus concentrations in temperate lakes. However, in tropical water bodies, cyanobacterial density and cyanotoxin correlation to environmental factors is not fully understood. In the present study, we examined the effects of total nitrogen and total phosphorus (TP) concentrations among other environmental parameters on cyanobacterial community structure and MC concentrations in the Dau Tieng reservoir, a tropical, eutrophic water body in Southern Vietnam. Cyanobacterial biomass and MC content were monitored monthly from March 2012 to February 2013, when MCs were present in the Dau Tieng Reservoir. The highest concentrations of intracellular MCs were found in September and February when cyanobacteria biomass reached maximum values, with 2.50 and 2.13 mg MC.L -1 , respectively. Principle component analysis and redundancy analysis showed that MC concentration was positively correlated with the biomass of the cyanobacterial order Chroococcales, whereas TP was the primary abiotic factor influencing cyanobacterial biomass and MC concentrations in the Dau Tieng Reservoir. In addition, Bayesian model average analysis was used to construct a prediction model of MCs using cyanobacterial biomass and environmental variables revealing a suite of useful predictive factors for MCs in the Dau Tieng Reservoir, including water temperature, TP and the biomass of Chroococcales.

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The increase in synthesis and application of silver nanoparticles (AgNPs) in the last decade has resulted in contamination of AgNPs in the aquatic environment. The presence of AgNPs in aquatic environments has posed toxic effects to aquatic organisms and ecological damage. In this study, two tropical microalgae species including the freshwater Scenedesmus sp. and the marine diatom Thalassiosira sp. were employed to examine the toxic effects of AgNPs. The toxic effects were determined by analyzing different end points, such as half maximal effective concentration (EC50), algae growth inhibition, algae cell size, chlorophyll-a content, and total lipid accumulation. The results suggested that AgNPs presented different toxicity mechanisms for microalgae and showed to be more toxic in freshwater than in marine environment. The EC50 values of AgNPs after 72 h for the growth inhibition of Scenedesmus sp. and Thalassiosira sp. were 89.92 ± 9.68 and 107.21 ± 7.43 μg/L, respectively. AgNPs at a certain concentration have resulted in change in cell diameter, reduction in chlorophyll-a content, and enhancement of the total lipid production in the tested microalgae. Thus, local species should be involved in the toxic assessment. This research contributes on understanding the toxicity of AgNPs on freshwater and marine environments.

Eutrophication of surface water has become an environmental concern in recent decades. High concentrations of nutrients, especially nitrogen- and phosphorus-rich species, have contributed to the process of eutrophication, highlighting a demand for effective and economical methods of removing nitrogen and phosphorus from wastewater. This study aimed to investigate the ability of a green microalga species, Scenedesmus sp., to remove nitrogen and phosphorus, as well as chemical oxygen demand (COD) and biochemical oxygen demand (BOD5), from fertilizer plant wastewater. Different microalgae concentrations from 10 mg/L to 60 mg/L were used to assess the growth rate, biomass production, and removal ability. The results indicated that Scenedesmus sp. grew well in the wastewater (with a growth rate from 0.3 to 0.38/day) and produced up to 70.2 mg/L of dry biomass. The algal species was able to remove ammonium (NH4+), nitrate (NO3−), phosphate (PO43−), total phosphorus (TP), COD, and BOD5 with removal rates up to 93%, 84%, 97%, 96%, 93%, and 84%, respectively. Autobioflocculation (AFL) was observed in all cultures with flocculation activity of up to 88.3% in the highest algal biomass treatment. The formation of bioflocculation enhanced the removal of nutrients, COD, and BOD5 from wastewater effluent. The results indicated that wastewater from a fertilizer plant could be used as a cost-effective growth medium for algal biomass. The autoflocculation of microalgae could be used as a more practical approach for wastewater treatment using microalgae to eliminate eutrophication.

The free and covalently bound microcystins (MCs) in 3 fish and 2 bivalves from the Dau Tieng Reservoir in Vietnam were investigated for the first time in the present study. The results showed that all species were contaminated with MCs. Our findings indicate that eating the muscle of fish from the Dau Tieng Reservoir is safe but that eating the bivalves is not safe during toxic cyanobacterial bloom episodes. Environ Toxicol Chem 2017;36:2953-2957. © 2017 SETAC.