New insights into algal-bacterial sludge granulation based on the tightly-bound extracellular polymeric substances regulation in response to N-Methylpyrrolidone.

Algal-bacterial granular sludge (ABGS) system is promising in wastewater treatment for its potential in energy-neutrality and carbon-neutrality. However, traditional cultivation of ABGS poses significant challenges attributable to its long start-up period and high energy consumption. Extracellular polymeric substances (EPS), which could be stimulated as a self-defense strategy in cells under toxic contaminants stress, has been considered to contribute to the ABGS granulation process. In this study, photogranulation of ABGS by EPS regulation in response to varying loading rates of N-Methylpyrrolidone (NMP) was investigated for the first time. The results indicated the formation of ABGS with a maximum average diameter of ∼3.3 mm and an exceptionally low SVI5 value of 67 ± 2 mL g-1 under an NMP loading rate of 125 mg L-1 d-1, thereby demonstrating outstanding settleability. Besides, almost complete removal of 300 mg L-1 NMP could be achieved at hydraulic retention time of 48 h, accompanied by chemical oxygen demand (COD) and total nitrogen (TN) removal efficiencies higher than 90 % and 70 %, respectively. Moreover, possible degradation pathway and metabolism mechanism in the ABGS system for enhanced removal of NMP and nitrogen were proposed. In this ABGS system, the mycelium with network structure constituted by filamentous microorganisms was a prerequisite for photogranulation, instead of necessarily leading to granulation. Stress of 100-150 mg L-1 d-1 NMP loading rate stimulated tightly-bound EPS (TB-EPS) variation, resulting in rapid photogranulation. The crucial role of TB-EPS was revealed with the involved mechanisms being clarified. This study provides a novel insight into ABGS development based on the TB-EPS regulation by NMP, which is significant for achieving the manipulation of photogranules.

Nitrification-denitrification co-metabolism in an algal-bacterial aggregates system for simultaneous pyridine and nitrogen removal.

Photosynthetic oxygenation in algal-bacterial symbiotic (ABS) system was mainly concerned to enhance contaminant biodegradation by developing an aerobic environment, while the role of nitrification-denitrification involved is often neglected. In this study, an algal-bacterial aggregates (ABA) system was developed with algae and activated sludge (PBR-1) to achieve simultaneous pyridine and nitrogen removal. In PBR-1, as high as 150 mg·L-1 pyridine could be completely removed at hydraulic residence time of 48 h. Besides, total nitrogen (TN) removal efficiency could be maintained above 80%. Nitrification-denitrification was verified as the crucial process for nitrogen removal, accounting for 79.3% of TN removal at 180 µmol·m-2·s-1. Moreover, simultaneous pyridine and nitrogen removal was enhanced through nitrification-denitrification co-metabolism in the ABA system. Integrated bioprocesses in PBR-1 including photosynthesis, pyridine biodegradation, carbon and nitrogen assimilation, and nitrification-denitrification, were revealed at metabolic and transcriptional levels. Fluorescence in situ hybridization analysis indicated that algae and aerobic species were located in the surface layer, while denitrifiers were situated in the inner layer. Microelectrode analysis confirmed the microenvironment of ABA with dissolved oxygen and pH gradients, which was beneficial for simultaneous pyridine and nitrogen removal. Mechanism of nitrification-denitrification involved in pyridine and nitrogen removal was finally elucidated under the scale of ABA.

Unraveling the intracellular and extracellular self-defense of Chlorella sorokiniana toward highly toxic pyridine stress.

A bottleneck of microalgae-based techniques for wastewater bioremediation is activity inhibition of microalgae by toxic pollutants. The defense strategies of Chlorella sorokinana against toxic pyridine were studied. Results indicated that pyridine caused photoinhibition and reactive oxygen species overproduction in a concentration-dependent manner. The 50% inhibitory concentration of pyridine (147 mg L-1) destroyed C/N balance, disrupted multiple metabolic pathways of C. sorokinana. In response to pyridine stress, ascorbate peroxidase and catalase activities increased to scavenge reactive oxygen species under pyridine concentrations lower than 23 mg L-1. At higher pyridine concentrations, the activation of calcium signaling pathways and phytohormones represented the predominant defense response. Extracellular polymeric substances increased 3.6-fold in 147 mg L-1 group than control, which interacted with pyridine through hydrophobic and aromatic stacking to resist pyridine entering algal cells. Unraveling the intracellular and extracellular self-defense mechanisms of microalgae against pyridine stress facilitates the development of microalgal-based technology in wastewater bioremediation.

Interactive effects of temperature and salinity on the survival, oxidative stress, and Na+/K+-ATPase activity of newly hatched obscure puffer (Takifugu obscurus) larvae.

Obscure puffer (Takifugu obscurus) is an anadromous fish widely distributed around the coastal and inland rivers in East Asia. T. obscurus often encounters fluctuations in temperature and salinity. This study aimed to investigate the effect of the interactions of temperature and salinity on survival and oxidative stress response of newly hatched T. obscurus larvae. A combination of three temperatures (19, 25, and 31 °C) and three salinities (0, 10, and 20 ppt) was applied for 96 h under laboratory conditions. The newly hatched larvae could not tolerate 31 °C for 96 h. No death was recorded at other temperatures during this experiment. Malondialdehyde concentrations increased significantly after 6 h of exposure to high salinity (10 and 20 ppt) and then decreased until the end of the experiment at each temperature. The highest superoxide dismutase activity was observed under the exposure to 20 ppt for 24 h at 31 °C. Na+/K+-ATPase activity significantly increased as salinity increased, especially at low temperatures. With the prolong of exposure time, the integrated biomarker response (IBR) values showed an increase until 48 h and then declined at 96 h in most treatments. The largest IBR value appeared when larvae were exposed to the highest temperature and salinity for 24 h. Our study indicated that high temperature with high salinity may negatively affect the early development of T. obscurus and their combined effects should be considered in the larvae culture.

Allelopathic inhibition of juglone (5-hydroxy-1,4-naphthoquinone) on the growth and physiological performance in Microcystis aeruginosa.

The elimination of cyanobacteria blooms has become an urgent concern in aquatic environmental protection. Allelopathic control is considered a potential approach because of its exclusive and ecological safety properties. The present study evaluated the allelopathic effect of juglone, a derivative from the genus Juglans, on the toxic Microcystis aeruginosa. Juglone at 3.0-9.0 mg L-1 notably depressed the cell proliferation of M. aeruginosa. The cell abundance treated by 9.0 mg L-1 juglone decreased by 75% after an 11-day exposure. The antioxidant enzyme activity (SOD and CAT) in juglone groups increased remarkably, suggesting juglone-induced oxidant stress in the M. aeruginosa cells. Juglone exposure enhanced the intracellular and extracellular microcystin contents per cell. Nonetheless, the total amount of microcystins in the juglone-treated cyanobacterial system did not increase because of the decreased cell abundance. These results indicated the application potential of juglone for M. aeruginosa extermination.

High temperature promotes the inhibition effect of Zn2+ on inducible defense of Scenedesmus obliquus.

Morphological defense is assumed to be an effective anti-grazer strategy in phytoplankton. Scenedesmus obliquus, a globally widespread freshwater chlorophyte, can form colonies in response to the infochemicals of herbivorous zooplankton and survive in coexistence with grazers. However, the inducible defense response is often disturbed by abiotic or biotic factors, especially under the increasing global warming and environmental pollution. In this study, two nonlethal environmental factors, namely, elevated temperature and environmentally relevant Zn2+ concentrations, decreased colony formation of S. obliquus induced by Daphnia grazing infochemicals. Elevated temperature (30 °C) reduced the inducible colony size and shortened the maintenance time of defensive colonies. Decreased colony size was detected with increased Zn2+ concentration. Colony formation was inhibited even at low Zn2+ concentration (0.131 µmol L-1), which neither retarded growth nor affected photosynthesis. Warming promoted the inhibition effect of Zn2+ on inducible colony formation of S. obliquus. Warming also enhanced Zn2+ toxicity, which caused the growth rate of S. obliquus to be hindered by high Zn2+ concentrations at elevated temperature. Specially, S. obliquus which formed inducible colonies under the condition of Daphnia infochemicals had higher tolerance to Zn2+ toxicity and thus likely exerted protective effects against heavy metals. The results indicated the combined effects of global warming and heavy-metal pollution result in more severe impact on the inducible defense of S. obliquus.

Magnesium depletion suppresses the anti-grazer colony formation in Scenedesmus obliquus.

In aquatic ecosystems, many phytoplankton species have evolved various inducible defense mechanisms against the predation. The expression of these defenses is affected by environmental conditions such as nutrient availability. Here, we investigated the anti-grazer colony formation in Scenedesmus obliquus at different magnesium concentrations (0-7.3 mg L-1 Mg2+) in the presence of zooplankton (Daphnia)-derived infochemicals. Results showed that at adequate Mg2+, S. obliquus formed high proportions of multi-celled (e.g., four- and eight-celled) colonies, resulting in significantly increased number of cells per colony in response to Daphnia filtrate. On the other hand, in Mg2+-deficient treatment, the proportion of multi-celled colonies decreased, together with reduced algal growth rate and photosynthetic efficiency. Finally, the treatment without Mg2+ strongly suppressed the formation of large colony (mainly eight-celled colonies), whereas the algal growth rate was comparable to that in Mg2+ sufficient treatment. Despite the inhibition of colony formation, the time reaching the maximum number of cells per colony was not affected by the Mg2+ concentration, which generally took three days in all groups. Our results indicate that Mg2+ deficient/absent environments significantly reduced anti-grazing colony formation but not the algal growth, suggesting strong dependability of this morphological defensive trait to magnesium fluctuation in S. obliquus.

Chlorophytes prolong mixotrophic Ochromonas eliminating Microcystis: Temperature-dependent effect.

Cyanobacterial blooms, caused by eutrophication and climate warming, exert severely negative effects on aquatic ecosystem. Some species of protozoans can graze on toxic cyanobacteria and degrade microcystins highly efficiently, which shows a promising way to control the harmful algae. However, in the field, many different species of algae coexist with Microcystis and may affect protozoans eliminating Microcystis. Therefore, in this study, we assessed the impacts of chlorophytes, a type of beneficial algae for zooplankton and common competitors of cyanobacteria, on flagellate Ochromonas eliminating toxin-producing Microcystis at different temperatures. Our results showed that Ochromonas still eliminated Microcystis population and degraded the total microcystins with the addition of chlorophytes, although the time of eliminating Microcystis was prolonged and temperature-dependent. Additionally, in the grazing treatments, chlorophytes populations gradually increased with the depletion of Microcystis, whereas Microcystis dominated in the mixed algal cultures without Ochromonas. The findings indicated that although chlorophytes prolong mixotrophic Ochromonas eliminating Microcystis, the flagellate grazing Microcystis helps chlorophytes dominating in the primary producers, which is significant in improving water quality and reducing aquatic ecosystem risks.

Trade-off between reproduction and lifespan of the rotifer Brachionus plicatilis under different food conditions.

Phaeocystis globosa, one of the most typical red tide-forming species, is usually mixed in the food composition of rotifers. To explore how rotifers respond by adjusting life history strategy when feeding on different quality foods, we exposed the rotifer Brachionus plicatilis to cultures with 100% Chlorella, a mixture of 50% P. globosa and 50% Chlorella, or 100% P. globosa. Results showed that rotifers exposed to 100% Chlorella or to mixed diets produced more total offspring and had higher age-specific fecundity than those exposed to 100% P. globosa. Food combination significantly affected the net reproduction rates of rotifers. By contrast, rotifers that fed on 100% P. globosa or on mixed diets had a longer lifespan than those fed on 100% Chlorella. The overall performances (combining reproduction and lifespan together) of rotifers cultured in 100% Chlorella or mixed diets were significantly higher than those cultured in 100% P. globosa. In general, Chlorella favors rotifers reproduction at the cost of shorter lifespan, whereas P. globosa tends to extend the lifespan of rotifers with lower fecundity, indicating that trade-off exists between reproduction and lifespan under different food conditions. The present study also suggests that rotifers may have the potential to control harmful P. globosa.

Microcystis aeruginosa strengthens the advantage of Daphnia similoides in competition with Moina micrura.

Microcystis blooms are generally associated with zooplankton shifts by disturbing interspecific relationships. The influence of Microcystis on competitive dominance by different sized zooplanktons showed species-specific dependence. We evaluated the competitive responses of small Moina micrura and large Daphnia similoides to the presence of Microcystis using mixed diets comprising 0%, 20%, and 35% of toxic M. aeruginosa, and the rest of green alga Chlorella pyrenoidosa. No competitive exclusion occurred for the two species under the tested diet combinations. In the absence of M. aeruginosa, the biomasses of the two cladocerans were decreased by the competition between them. However, the Daphnia was less inhibited with the higher biomass, suggesting the competitive dominance of Daphnia. M. aeruginosa treatment suppressed the population growths of the two cladocerans, with the reduced carrying capacities. Nonetheless, the population inhibition of Daphnia by competition was alleviated by the increased Microcystis proportion in diet. As a result, the competitive advantage of Daphnia became more pronounced, as indicated by the higher Daphnia: Moina biomass ratio with increased Microcystis proportions. These results suggested that M. aeruginosa strengthens the advantage of D. similoides in competition with M. micrura, which contributes to the diversified zooplankton shifts observed in fields during cyanobacteria blooms.