The Effects Of Extracellular Polymeric Substances On Algal Interface And Toxicity Of Arsenic To Algae

The extracellular polymeric substances (EPS) from green algae play a crucial role on pollutant transport in aquatic ecosystems. As an important toxic pollutant,arsenic is receiving increasing concern for its severe ecotoxic risk. However,the effects of extracellular polymeric substances on algal self-assemble and toxicity of arsenic to algae are unclear. In this study, EPS secreted by Chlorella pyrenoidosa and Chlamydomonas reinhardtii were selected, to investigate the effects of EPS on algal aggregation. Fourier transform infrared spectroscopic (FTIR), dynamic light scattering(DLS), electrophoretic mobility (EPM) and viscometer analysis were applied to analyze the role of EPS on algal self-assemble. Toxicity of As(III) and As(V) under different concentrations to Chlorellayenoidosa was compared,based on endpoints:growth inhibition rate, chlorophyll a (Chl a) concentration, chlorophyll fluorescence parameters of photosystem II (PSII) and ultra-structural morphology of cells. The influence of EPS on the photosynthetic damage of arsenic to Chlorella pyrenoidosa was also analyzed. The following conclusions were obtained:(1) EPS produced by green algae promote algal self-aggregation. EPS are composed of proteins, polysaccharides and other macro-molecules, carrying ionizable functional groups such as hydroxyl (-OH), carboxyl (-COOH), amino (-NH2) groups.Surface of Chlorella pyrenoidosa and Chlamydomonas reinhardtii are both negatively charged, with zeta potential value of -15.5 ±0.6 and -19.2 ±0.2 mV,respectivly. After EPS was removed, the negative surface charge of two algae decreased by 14.4% and 22.1%, indicating higher repulsion between cell particles;while the adhesion abilities decreased by 4.6% and 20.2%, which will impede the algal self-aggregation; and the hydraulic radius of two algae were decreased by 13.6%and 31.0%.(2) The inhibition of arsenic to PSII photosynthetic efficiency of Chlorella pyrenoidosa is influenced by arsenic valence. Half-effective concentration (ECSO) of As(V) were higher than As(III), which indicates that As(III) is more toxic to Chlorella pyrenoidosa than As(V). The chlorophyll a concentration of Chlorella pyrenoidosa treated by As(?) was lower than that treated by As(V). The chlorophyll fluorescence metrics, i.e. maximum quantum yields (F,/Fm), effective quantum yields (Yield),maximum electron transport rate (rETRmax) and the linear section slope (a) treated by 150.0 mg·L-1 As(III) under 96 h were decreased by 77%, 91%, 92% and 85%, which was higher than that exposed to As(V), with dereasing rate of 19%, 50%, 51% and 23%, respectively. The ultrastructural morphology of Chlorella pyrenoidosa exposured to inorganic arsenic was observed by transmission electron microscopy(TEM). Separation of cell wall and cell membrane was observed, as well as the extrusion of accumulated starch granules, diminishing pyrenoid and some lipid droplets. Compared with As (V), ultrastructural morphology of cell treated by As(III)was more serious damaga.(3) EPS enhance the tolerance of Chlorella pyrenoidosa to arsenic toxicity. The maximum quantum yields (Fv/Fm) of cells with or without EPS treated by 112.5 mg·L-1 As(III) after 48 h were decreased by 29.9% and 36.1%, which was higher than exposed to As(V), with decreasing rates of 22.9% and 27.2%, respectively. Result of fourier transform infrared spectroscopy show that bands around 1415 cm-1 shifted to 1394 cm-1 and 1395 cm-1 after adsorption of As(III) and As(V), respectively,suggesting carboxylic groups are relevant to adsorption of As(III) and As(V) on the Chlorella pyrenoidosa cell surface. It is inferred that EPS can moderate arsenic toxicity through the formation of a protective coating and offering effective functional groups, such as carboxyl (-COOH), which could bind inorganic arsenic.