Combined effects of four Microcystis aeruginosa strains and Scenedesmus obliquus concentrations on population dynamics and resting egg formation of two Daphnia species

We examined the hypothesis that toxic effects of Microcystis aeruginosa to population dynamics, ephippial production, and resting egg formation of Daphnia were restricted by food quality levels. Four unicellular, toxic M. aeruginosa strains with contrasting concentration of microcystin and two Daphnia species were used in this study. The number of offspring at first reproduction, population densities, and maximum population growth rate of two Daphnia species were lower at the highest toxic M. aeruginosa 7820 treatment compared with other treatments under two concentrations of Scenedesmus obliquus. The maximum number of offspring at first reproduction of two Daphnia species appeared in the higher toxic M. aeruginosa 526 strain at the high concentration of S. obliquus. The effects of M. aeruginosa strains, S. obliquus, and their combination on the number of offspring at first reproduction and the maximum population growth rate of two Daphnia species were significant (P < 0.001). Two Daphnia species could not reproduce in the highest toxic M7820 treatment under two concentrations of S. obliquus. They had lower population size and maximum population growth rate in the higher toxic M526 treatment at the low concentration of S. obliquus, but they were higher at the high concentration of S. obliquus. This result suggests that high S. obliquus concentration could relieve the toxicity of M. aeruginosa to Daphnia, and Daphnia could utilize the lower toxic Microcystis as food. The cumulative ephippia numbers of two Daphnia species were more at the high concentration of S. obliquus than those at the low concentration. The percentage of ephippia containing no resting eggs of two Daphnia species was evidently higher at the low concentration of S. obliquus but was lower at the high concentration of S. obliquus. Our results indicated that the cumulative ephippia numbers of Daphnia were population density dependent at the high-level food, and the productions of ephippia in Daphnia were significantly controlled by microcystin concentration at the low-level food.     

PSII-efficiency,polysaccharide production,and phenotypic plasticity of Scenedesmus obliquus in response to changes in metabolic carbon flux

To investigate the response of Scenedesmus obliquus to changes in metabolic carbon flux, S. obliquus was cultured in medium with different concentrations of glyoxylate over 9 days. Results showed that growth rates were not affected in the lower concentration glyoxylate (0.25 and 0.5 mM). However, growth rate of S. obliquus was inhibited in the higher concentration glyoxylate (0.85 and 1.25 mM) during the early phase before recovering at higher densities. Changes in growth rates in different glyoxylate concentrations were in line with changes in F_v/F_m and Φ_PSII. Colony formation was observed in S. obliquus in the four glyoxylate treatments. As a consequence, the mean number of cells per particle of S. obliquus in the glyoxylate treatments were significantly higher than those in the control. The total polysaccharide content of S. obliquus cells increased with increased glyoxylate concentrations. The increased glyoxylate-stimulated polysaccharide levels were directly correlated with colony size of S. obliquus.     

Does microcystin disrupt the induced effect of Daphnia kairomone on colony formation in Scenedesmus?

In natural aquatic system, Scenedesmus and Microcystis species usually coexist. Microcystins are released into water after lysis of Microcystis cells during the collapse of heavy blooms. The released toxins can then come into contact with a wide range of aquatic organisms. In this study, we used filtered Daphnia test water containing kairomone from Daphnia magna to stimulate the inducible colony formation in Scenedesmus obliquus under microcystin-contaminated system, to examine how microcystin affects the induced effect of Daphnia kairomone on colony formation in S. obliquus. The results showed neither microcystin nor Daphnia kairomone affected the growth of S. obliquus. Microcystin neither promoted nor impaired the overall Daphnia-induced colony formation in S. obliquus, except reducing the proportion of eight-celled colonies on day 2, indicating that the effect of microcystin was just short-term and in general did not disrupt grazer-induced colony formation of S. obliquus.     

Allelopathic effects of the submerged macrophyte <Emphasis Type="Italic">Potamogeton malaianus</Emphasis> on <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis>

Allelopathic effects of the submerged macrophyte Potamogeton malaianus on Scenedesmus obliquus were assessed using a two-phase approach under controlled laboratory conditions. In the co-culture experiment (phase І), the growth of S. obliquus at two different initial cell densities was significantly inhibited by P. malaianus. Moreover, the growth inhibition was dependent on the biomass density of P. malaianus. Antioxidant enzymes (SOD, CAT and POD), MDA, APA, total soluble protein, protein electrophoretic pattern and morphology of S. obliquus were determined after the co-culture experiment was terminated. The activities of SOD, CAT, POD and APA at the low initial cell density were stimulated, the contents of MDA and total soluble protein were increased, and some special protein bands disappeared in P. malaianus treatments. The macrophyte had no effect on the activities of SOD and APA at the high initial cell density, but significantly influenced other physiological parameters of S. obliquus with the increase of biomass density. The morphology of S. obliquus showed no difference in the macrophyte treatments and the controls, and the cultures were dominated by 4-celled coenobia. The results indicated P. malaianus had significant allelopathic effects on the growth and physiological processes of S. obliquus. Moreover, the allelopathic effects depended on initial algal cell density, biomass density of the macrophyte, and their interaction. In the experiment of P. malaianus culture filtrates (phase II), filtrates from combined culture of plant and S. obliquus at the low initial cell density exhibited no apparent growth inhibitory effect on S. obliquus. The result showed that initial addition of growth-inhibiting plant filtrates had no allelopathic effect on S. obliquus. We concluded that the allelopathic effects on S. obliquus were found in the presence of P. malaianus, but not in P. malaianus filtrates. However, the absence of allelopathic effect on S. obliquus might be due to the very low concentrations of allelochemicals in the filtrates. Handling editor: S. M. Thomas     

Influence of <Emphasis Type="Italic">Daphnia</Emphasis> infochemicals on functional traits of <Emphasis Type="Italic">Microcystis</Emphasis> strains (Cyanobacteria)

We conducted a laboratory experiment to investigate the influence of Daphnia infochemicals on growth rate, microcystin production, colony formation and cell size of eight Microcystis strains isolated from two lakes. The strains were characterized genetically by their 16S-23S rDNA ITS sequence. The experiment was composed of four treatments: (1) a control using filtered WC medium, (2) addition of Scenedesmus obliquus culture medium filtrate, (3) addition of Daphnia magna culture medium filtrate and (4) addition of sodium octyl sulphate, a commercially available Daphnia infochemical. Our results showed that sympatric strains differed strongly for the measured functional traits, while no correlations between traits were found. Between-strain differences in growth rate, microcystin production, colony formation and cell size were generally larger than the differences in phenotypes observed between treatments. Despite this, several strains reacted to the infochemicals by changing functional trait values. Daphnia culture medium filtrate and, to a lesser extent, sodium octyl sulphate had a negative influence on the growth rate of half of the strains and stimulated microcystin production in one strain, but the latter effect was not Daphnia-specific as Scenedesmus culture medium filtrate had the same effect. Daphnia culture medium filtrate also induced colony formation in one strain. Our data suggest that Daphnia infochemicals generally have a weak influence on growth rate, microcystin production and colony formation of Microcystis strains as compared to the inter-strain variability, while existing inducible effects are highly strain-specific.     

Food quality of mixed bacteria–algae diets for Daphnia magna

Bacteria can comprise a large fraction of seston in aquatic ecosystems and can therefore significantly contribute to diets of filter-feeding zooplankton. To assess the effect of three heterotrophic bacteria (Flavobacterium sp., Pseudomonas sp. and Escherichia coli) on survival, growth and egg production of juvenile Daphnia magna during six-day growth experiments, five ratios of bacteria–Scenedesmus obliquus mixtures were fed. Potential growth-limiting effects mediated by essential biochemicals were assessed upon supplementation of pure bacterial diets with a sterol (cholesterol) or a polyunsaturated fatty acid (EPA). Pure bacterial diets always had detrimental effects on Daphnia. However, cholesterol supplementation of Flavobacterium sp. enhanced growth rates of Daphnia. Diets containing Pseudomonas impaired Daphnia growth even at low dietary proportions (20%), indicating their toxicity. In contrast, Daphnia grew at relative high dietary proportions of Flavobacterium sp. and E. coli (80–50%). In fact, diets containing small proportions of these heterotrophic bacteria (Flavobacterium ≤50%, E. coli 20%) even significantly increased Daphnia growth rates compared to pure algal diets, indicating a nutritional upgrading by these bacteria. Our results suggest that the relative contribution of bacteria and phytoplankton to total dietary carbon as well as their phylogenetic composition strongly influence Daphnia fitness and potentially other filter-feeding zooplankton under field conditions.     

Potential grazing intensity directly determines the extent of grazer-induced colony formation in Scenedesmus obliquus

The genus Scenedesmus can form large colonies to provide protection against predation by herbivores. Scenedesmus has been investigated to elucidate the varying extents of colony formation in response to predation. However, limited information is available regarding the factors directly affecting the extent of colony formation responses. This study investigated the colony formation responses of Scenedesmus obliquus to infochemicals derived from Daphnia of different ages with the same biomass or feeding intensity. Results showed that water containing Daphnia infochemicals can induce the colony formation in S. obliquus regardless of age, biomass, or feeding intensity of Daphnia. The infochemicals derived from 1-day-old Daphnia caused a higher number of cells per particle than those from 5-day-old Daphnia when the grazers were of the same biomass; the infochemicals from 1-day-old Daphnia also maintained the algal colonial morph longer than those from 3-day-old Daphnia. The number of cells per particle did not differ among the algal populations treated with infochemicals from Daphnia of different ages when the feeding intensity of grazers was held constant. Therefore, potential grazing intensity directly determines the extent of the grazer-induced colony formation in S. obliquus.     

FO-spectra of chlorophyll fluorescence for the determination of zooplankton grazing

In the PHYTO-PAM phytoplankton analyzer the minimal fluorescence of dark-adapted samples (F₀) was assessed, which gives direct information on the chlorophyll-a content. Clearance rates (CR) of Daphnia and Brachionus were calculated from a decrease in chlorophyll-a concentration using the PHYTO-PAM fluorometer for non-sacrificial sampling of chlorophyll-a. Clearance rates of Daphnia were measured and compared with those based on the cell-counts method using an electronic particle counter (Coulter counter). Chlorophyll fluorescence-based CR for Daphnia magna were very strongly correlated with Coulter-based CR, signifying the potential suitability of the PHYTO-PAM in grazing experiments. A procedure for determination of rotifer clearance rates was developed and the effects of rotifer density, duration of the grazing period, and food concentration on CR were investigated. Between 10 and 30 rotifers in 2.5 ml food suspension (i.e. 4–12 rotifers per ml) appeared optimal for calculating CR. The application of the deconvolution of F₀-spectra in food selectivity experiments was evaluated using various mixtures of the green alga Scenedesmus obliquus and the cyanobacterium Microcystis aeruginosa fed to Brachionus. CR for Brachionus on M. aeruginosawere lower than on S. obliquusbut this was not caused by toxicity, because no mortality was observed. The higher CR on Scenedesmus than on Microcystis in the mixtures suggested selectivity. The importance of digital suppression of background fluorescence is highlighted in additional experiments with Daphnia feeding on mixtures of Microcystis and Scenedesmus, or on Microcystis alone. Without background correction of filtered samples, negative clearance rates were obtained for the `blue' Microcystis signal. Soluble fluorescing compounds of cyanobacterial origin, phycocyanin, were released from the Daphniaand contributed 40% to the overall-fluorescence. Deconvolution of F₀-spectra for the determination of chlorophyll-a using the PHYTO-PAM appears to be a suitable tool for determination of rotifer CR even at very low food concentrations. A drawback of the method is that rather high rotifer densities are required. The required grazing period, however, is shorter than for cell-count methods, the method is sensitive, clearance rates can be measured at low food concentrations (< 0.1 mg C l^–1) and information on selective feeding can be obtained.     

Crowding‐induced changes in growth,reproduction and morphology of Daphnia

• 1 Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life‐history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow‐through system in the presence of abundant food.
• 2 Water from Daphnia at 85 L^‐1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small‐bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species.
• 3 Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D. lumholtzi and D. ambigua.
• 4 Chemicals released by crowded conspecifics may provide an additional, density‐dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.

     

Food quantity shapes life history and survival strategies in <Emphasis Type="Italic">Daphnia magna</Emphasis> (Cladocera)

Life history and demographic responses to different food regimes were investigated in three clones of Daphnia magna. Green alga Scenedesmus obliquus served as food source and was given in five amounts, which covered the range from near threshold to excess food concentrations, from 0.05 through 0.15, 0.5, 1.5 to 4.5 mg C l⁻¹. The animals were kept under given food regime from first instar until their natural death. Food regime had a significant effect on Daphnia survival, which was shortest under the highest food regime. However, the response in expected reproduction of an individual differed between clones, with one benefiting more from higher food supply than the others. Life history trade-offs were seen as shortened lifespan versus higher investments in early reproduction.     

Assimilation of E. coli cells by Daphnia magna on the whole organism level

Consumption of E. coli cells by Daphnia magna was studied. It was found that this organism not only ingested E. coli cells but digested them as demonstrated by the release of ¹⁴CO₂ originating from E. coli grown on ¹⁴C-glucose, and by the transfer of the radioactive label from parental Daphnia to their progenies. In addition the effect of antibiotics on the consumption of E. coli cells by Daphnia magna was studied. In long incubation times, antibiotics inhibited bacterial uptake by Daphnia. The microflora isolated from Daphnia was found to be capable of causing leakage of enzymes out of E. coli cells thus playing at least a partial role in the digestion of E. coli cells by Daphnia.     

Effects of Juvenile Host Density and Food Availability on Adult Immune Response,Parasite Resistance and Virulence in a Daphnia-Parasite System

Host density can increase infection rates and reduce host fitness as increasing population density enhances the risk of becoming infected either through increased encounter rate or because host condition may decline. Conceivably, potential hosts could take high host density as a cue to up-regulate their defence systems. However, as host density usually covaries with food availability, it is difficult to examine the importance of host density in isolation. Thus, we performed two full-factorial experiments that varied juvenile densities of Daphnia magna (a freshwater crustacean) and food availability independently. We also included a simulated high-density treatment, where juvenile experimental animals were kept in filtered media that previously maintained Daphnia at high-density. Upon reaching adulthood, we exposed the Daphnia to their sterilizing bacterial parasite, Pasteuria ramosa, and examined how the juvenile treatments influenced the likelihood and severity of infection (Experiment I) and host immune investment (Experiment II). Neither juvenile density nor food treatments affected the likelihood of infection; however, well-fed hosts that were well-fed as juveniles produced more offspring prior to sterilization than their less well-fed counterparts. By contrast, parasite growth was independent of host juvenile resources or host density. Parasite-exposed hosts had a greater number of circulating haemocytes than controls (i.e., there was a cellular immune response), but the magnitude of immune response was not mediated by food availability or host density. These results suggest that density dependent effects on disease arise primarily through correlated changes in food availability: low food could limit parasitism and potentially curtail epidemics by reducing both the host’s and parasite’s reproduction as both depend on the same food.     

Effects of predator density and duration of predator occupancy on crustacean abundance and diversity in experimental pools

Keystone predators, by reducing the abundance of competitively superior prey, may have indirect positive effects on weak competitors, possibly increasing their abundance or preventing local competitive exclusion. By analogy to the Intermediate Disturbance Hypothesis, we would expect species diversity to peak at intermediate predator densities. In a replicated artificial pool experiment, we examined the relationships between density of the backswimmer Notonecta maculata (0, 1, 2, and 4 per 30 l pool) and invertebrate taxon diversity over an 11-week period of predator occupancy. Diversity reached high levels at high predator density sooner than at intermediate density. At the end of the experiment, taxon diversity was greatest at densities of 2 and 4 Notonecta per pool. While the overall predator density–diversity curve was in line with the intermediate disturbance hypothesis, the reduction in diversity from intermediate to high predator density was not statistically significant. Density of the preferred prey Daphnia magna decreased with Notonecta density, while densities of the smaller cladocerans Moina brachiata and Ceriodaphnia spp. increased. Suppression of Daphnia at high Notonecta densities may partially explain the increase in Moina and Ceriodaphnia densities. However, most of the relationship between Notonecta and the smaller cladocerans appears to be independent of Daphnia, suggesting complex interactions within the community. Our results suggest that keystone predation plays a strong role in structuring this community. Although diversity did not decrease significantly at the highest predator density as predicted, such a decrease may be more likely for pools with longer durations of predator occupancy or with higher predator densities.     

The Good,the Bad and the Plenty: Interactive Effects of Food Quality and Quantity on the Growth of Different Daphnia Species

Effects of food quality and quantity on consumers are neither independent nor interchangeable. Although consumer growth and reproduction show strong variation in relation to both food quality and quantity, the effects of food quality or food quantity have usually been studied in isolation. In two experiments, we studied the growth and reproduction in three filter-feeding freshwater zooplankton species, i.e. Daphnia galeata x hyalina, D. pulicaria and D. magna, on their algal food (Scenedesmus obliquus), varying in carbon to phosphorus (C∶P) ratios and quantities (concentrations). In the first experiment, we found a strong positive effect of the phosphorus content of food on growth of Daphnia, both in their early and late juvenile development. Variation in the relationship between the P-content of animals and their growth rate reflected interspecific differences in nutrient requirements. Although growth rates typically decreased as development neared maturation, this did not affect these species-specific couplings between growth rate and Daphnia P-content. In the second experiment, we examined the effects of food quality on Daphnia growth at different levels of food quantity. With the same decrease in P-content of food, species with higher estimated P-content at zero growth showed a larger increase in threshold food concentrations (i.e. food concentration sufficient to meet metabolic requirements but not growth). These results suggest that physiological processes such as maintenance and growth may in combination explain effects of food quality and quantity on consumers. Our study shows that differences in response to variation in food quality and quantity exist between species. As a consequence, species-specific effects of food quality on consumer growth will also determine how species deal with varying food levels, which has implications for resource-consumer interactions.     

Daphnia predation on the amphibian chytrid fungus and its impacts on disease risk in tadpoles

Direct predation upon parasites has the potential to reduce infection in host populations. For example, the fungal parasite of amphibians, Batrachochytrium dendrobatidis (Bd), is commonly transmitted through a free‐swimming zoospore stage that may be vulnerable to predation. Potential predators of Bd include freshwater zooplankton that graze on organisms in the water column. We tested the ability of two species of freshwater crustacean (Daphnia magna and D. dentifera) to consume Bd and to reduce Bd density in water and infection in tadpoles. In a series of laboratory experiments, we allowed Daphnia to graze in water containing Bd while manipulating Daphnia densities, Daphnia species identity, grazing periods and concentrations of suspended algae (Ankistrodesmus falcatus). We then exposed tadpoles to the grazed water. We found that high densities of D. magna reduced the amount of Bd detected in water, leading to a reduction in the proportion of tadpoles that became infected. Daphnia dentifera, a smaller species of Daphnia, also reduced Bd in water samples, but did not have an effect on tadpole infection. We also found that algae affected Bd in complex ways. When Daphnia were absent, less Bd was detected in water and tadpole samples when concentrations of algae were higher, indicating a direct negative effect of algae on Bd. When Daphnia were present, however, the amount of Bd detected in water samples showed the opposite trend, with less Bd when densities of algae were lower. Our results indicate that Daphnia can reduce Bd levels in water and infection in tadpoles, but these effects vary with species, algal concentration, and Daphnia density. Therefore, the ability of predators to consume parasites and reduce infection is likely to vary depending on ecological context.     

CRISPR/Cas-Mediated Targeted Mutagenesis in Daphnia magna

The water flea Daphnia magna has been used as an animal model in ecology, evolution, and environmental sciences. Thanks to the recent progress in Daphnia genomics, genetic information such as the draft genome sequence and expressed sequence tags (ESTs) is now available. To investigate the relationship between phenotypes and the available genetic information about Daphnia, some gene manipulation methods have been developed. However, a technique to induce targeted mutagenesis into Daphnia genome remains elusive. To overcome this problem, we focused on an emerging genome editing technique mediated by the clustered regularly interspaced short palindromic repeats/CRISPR-associated (CRISPR/Cas) system to introduce genomic mutations. In this study, we targeted a functionally conserved regulator of eye development, the eyeless gene in D. magna. When we injected Cas9 mRNAs and eyeless-targeting guide RNAs into eggs, 18–47% of the survived juveniles exhibited abnormal eye morphology. After maturation, up to 8.2% of the adults produced progenies with deformed eyes, which carried mutations in the eyeless loci. These results showed that CRISPR/Cas system could introduce heritable mutations into the endogenous eyeless gene in D. magna. This is the first report of a targeted gene knockout technique in Daphnia and will be useful in uncovering Daphnia gene functions.     

Impacts of declining water levels on the quantity of fish habitat in coastal wetlands of eastern Georgian Bay,Lake Huron

The aim of this study was to examine whether the presence of non-toxic filamentous cyanobacteria can cause egg abortion in Daphnia. To test this hypothesis, we fed females of Daphnia pulicaria with the green alga Scenedesmus obliquus from birth until eggs were laid in the brood chamber. Gravid Daphnia females were then exposed to media containing either S. obliquus or filaments of a non-toxic strain of the cyanobacterium Cylindrospermopsis raciborskii. The females were then observed throughout the whole period of egg development and all events of egg abortion were recorded. Experiments were conducted at three temperatures: 16, 20, and 24°C. In the presence of cyanobacteria, the proportion of aborted eggs ranged from 2.6 to 12.9%, compared with 0–1.8% in the green algae treatment. The number of aborted eggs, but not the number of egg-aborting females, tended to increase at higher temperature. Our data indicate that interference by non-toxic cyanobacterial filaments may affect the reproductive success of Daphnia, not only through suppression of their fecundity, but also via an increase in the number of aborted eggs. Therefore, the reproductive rate of Daphnia based on the fecundity of females may be overestimated in situations where filamentous cyanobacteria are present.     

Food quality influences habitat selection in Daphnia

1. The vertical distribution of Daphnia in stratified lakes strongly depends on the depth profiles of temperature and food resources. However, ecological requirements for these factors are slightly different for juvenile and adult Daphnia. 2. Here, I investigated whether food quality influences the habitat selection of Daphnia pulicaria at night and whether the habitat selection of juvenile and adult D. pulicaria is different. Daphnia were allowed to choose their optimal habitat in large, stratified water columns (plankton towers, Plön) that held either the green alga Scenedesmus obliquus (high quality) in the cold hypolimnion (Hypo‐treatment) or S. obliquus in the warm epi‐ and cold hypolimnion (SCEN‐treatment) or the non‐toxic cyanobacterium Synechococcus elongatus (low quality) in the warm epilimnion and S. obliquus in the cold hypolimnion (SYN treatment). 3. When food (S. obliquus) was present only in the hypolimnion (Hypo‐treatment), juveniles and adults distributed similarly in the water column and spent most of their time in the interface between the warm and the food rich layer. 4. When food was present in the epilimnion and hypolimnion (SCEN‐ and SYN‐treatments), juvenile and adult D. pulicaria moved into the warm and now also food‐rich epilimnion, however, the magnitude of this shift depended on the food type and age class of Daphnia. Adult and juvenile D. pulicaria spent most of their time in the epilimnion when food there was of a high quality (S. obliquus; SCEN‐treatment). However, compared to the juveniles, adult Daphnia spent significantly more time in the colder hypolimnion when epilimnetic food was of a low quality (S. elongatus; SYN‐treament). 5. Therefore, habitat selection of adult D. pulicaria was affected by food quality whereas the habitat selection of juveniles was not. 6. Additional growth and reproduction experiments show that the food quality is likely to be responsible for the different habitat selection of juveniles and adults in the SYN‐treatment. 7. In conclusion, my experiments show that D. pulicaria behaviourally reacts to the quality of its food source.     

Low temperature and <Emphasis Type="Italic">Daphnia</Emphasis>-associated infochemicals promote colony formation of <Emphasis Type="Italic">Scenedesmus obliquus</Emphasis> and its harvesting

Objective

To explore the combined effects of temperature and Daphnia-associated infochemicals on colony formation of Scenedesmus obliquus to faciliate harvesting the algal biomass.

Results

A three-parameter modified Gaussian model fitted the changes of the number of cells per particle in S. obliquus induced by Daphnia culture filtrate well under any temperature. Decreases in temperature enhanced the induced–colony formation of Scenedesmus. The maximum colony size at 15–25 °C was significantly larger than those at 30–35 °C. An additional 1 or 2 days at low temperature was needed to reach the maximum colony size, which indicates the best harvest time for algal biomass.

Conclusion

Induced-colony formation of Scenedesmus by Daphnia culture filtrate at 15–25 °C is recommended to settle algal cells. This condition facilitates harvesting the biomass.