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Suspended particulate matter (SPM), an important component of the natural water environment, can act as a carrier of many pollutants that affect aquatic organisms. In the present study, the effect of SPM obtained from Jinjiang Estuary on the physiological, biochemical, and photosynthetic properties of typical freshwater algae (Chlorella pyrenoidosa) was investigated. The results showed that under different concentrations of SPM treatment, the superoxide dismutase (SOD), catalase (CAT) activities, and malondialdehyde (MDA) content of C. pyrenoidosa increased, but the soluble protein content decreased. SPM with different particle sizes had less effect on SOD of C. pyrenoidosa, but showed a promoting effect on CAT and MDA as well as soluble protein content. In terms of photosynthetic activity, high concentrations (70, 90 mg/L) and small particle sizes (0–75, 75–120 μm) of SPM had a greater effect on the chlorophyll a content of C. pyrenoidosa. In addition, different concentrations of SPM had no significant effect on the potential photosynthetic activity of PS II (Fv/F0) and the maximum quantum yield of PS II (Fv/Fm), but the inhibition of the initial slope (alpha), the maximum photosynthetic rate (ETRmax) and the semi-light saturation point (Ik) increased with the increase of SPM concentration. Fv/F0, ETRmax, and Ik of C. pyrenoidosa showed some degree of recovery after inhibition in the presence of SPM of different particle sizes. [ABSTRACT FROM AUTHOR]

The effects of suspended particles (SP) of different concentrations and sizes on the secretion of exopolysaccharide (EPS) by Chlorella pyrenoidosa (C. pyrenoidosa) and Microcystis flos-aquae (M. flos-aquae) in Jinjiang Estuary, Fujian Province, China were studied in co-cultures of microalgae and SP. The results show that the SP concentration has an "inhibit–promote–inhibit" effect on the secretion of EPS by C. pyrenoidosa, whereby there is an optimal concentration of SP corresponding to the largest amount of EPS secreted by C. pyrenoidosa. Low concentrations had no significant effect on the secretion of EPS from M. flos-aquae (p > 0.05), whereas higher concentrations had an inhibitory effect. The C. pyrenoidosa EPS content was found to be significantly decreased in response to SP of small particle sizes (0–75 and 75–120 μm) and significantly increased for SP of large particle sizes (120–150 and 150–500 μm). Small particle sizes (0–75 and 75–120 μm) were found to be beneficial to the secretion of EPS from M. flos-aquae, and the promotion of EPS secretion gradually decreased with the increase in SP particle size. However, when the particle size was too large (120–150 and 150–500 μm), SP had no significant effect on EPS secretion. This study is helpful for understanding the microalgae EPS secretion response to SP and provides a scientific basis for studying the mechanism of EPS secretion by algae and the effect of SP on eutrophication of the estuary. [ABSTRACT FROM AUTHOR]