Delayed mixis in rotifers: an adaptive response to the effects of density-dependent sex on population growth

In most cyclically parthenogenetic life cycles, sex is neededto produce resting stages. In several species of cyclicallyparthenogenetic rotifers, some generations of clones are notresponsive to a density-dependent signal that triggers sexualfemale production. These unresponsive rotifers hatch from restingeggs and typically pass 8–12 generations of female parthenogenesisbefore becoming receptive to the mixis signal. We addressedthe selection for mixis delay using a simulation model. A delayof sexual reproduction could increase population growth throughparthenogenesis and thus the number of resting eggs ultimatelyproduced. In a monomorphic population without mixis delay, wedetermined the optimal ratio of mictic to amictic females (mixisratio) to be 45%, and the optimal population density thresholdfor induction of mictic females (mixis threshold) to be 82 rotifersL^–1. This mixis pattern, however, was not an evolutionarilystable strategy. A mixis ratio of 14% and threshold of 70 rotifersL^–1 proved to be resistant to invasion by other mixispatterns. When we gave this phenotype a mixis delay of 8–10days, it could invade a population with the same mixis pattern,but lacking a mixis delay. The advantage of delaying mixis wasrelatively small, suggesting that a polymorphism is possible.     

Low crowding threshold for induction of sexual reproduction and diapause in a Patagonian rotifer

1. This study investigates the basis and ecological significance of the extremely high propensity for mixis (sexual reproduction) observed in laboratory populations of Brachionus calyciflorus from a temporary pond in Patagonia. 2. Experiments with stem females hatched from resting eggs showed that these females were exclusively amictic but produced mictic daughters even at very low population densities. In six experiments, newly hatched stem females were cultured individually in different volumes (1.5, 12, 40 and 150 mL). The percentage of mictic daughters (mixis ratio) was high in the smaller volumes (e.g. 44–83% in 1.5 mL) and lower in the largest volume (6–21% in 150 mL). A regression analysis combining the data from these experiments showed a highly significant decrease in mixis ratio with volume and indicated that the lowest population density at which mixis still occurs (mixis threshold) would be 3.4 females L^?1 (95% CL 2.9–4.0 females L^?1). This value is considerably lower than mixis thresholds for other rotifers (25–250 females L^?1 for many species and 9000–477 000 females L^?1 for some Spanish Brachionus plicatilis). 3. In three additional experiments, stem females and their amictic daughters were cultured individually in 150 mL. The percentage of mictic daughters produced by these two generations of females was not significantly different, showing that the mixis response to crowding is not inhibited in the stem‐female generation. 4. Laboratory experiments showed that two common predators of the Patagonian B. calyciflorus (the calanoid copepod Parabroteas sarsi and the backswimmer Notonecta vereertbruggheni) each ate 60–70 B. calyciflorus predator^?1 per day and cleared all rotifers from c. 250 mL per day. Thus, a very low mixis threshold and high maximal mixis ratio may ensure production of some resting eggs soon after colonisation of the pond and before complete removal from the plankton. 5. Two laboratory experiments showed that resting eggs of the Patagonian B. calyciflorus hatched at variable rates (28 and 81%) after a brief diapause when kept in the conditions under which they were produced and oviposited (20–21 °C; L: D 16: 8). Early hatching of resting eggs from pond sediment may allow multiple periods of colonisation and resting egg production in a season. This may offset the fitness cost of limited population growth through female parthenogenesis in the face of unpredictable and abrupt risk of extinction because of predators.     

Maternal Effect by Stem Females in <Emphasis Type="Italic">Brachionus plicatilis</Emphasis>: Effect of Starvation on Mixis Induction in Offspring

We examined whether starvation during the initial period of life in stem females affected reproductive characteristics of the offspring. Starvation treatment had different effects on rotifers hatched from resting eggs and those hatched from amictic eggs. When stem females experienced starvation after hatching, this induced a higher percentage of mixis in their offspring. When the same starvation treatment was applied to rotifers hatched from amictic eggs, there was no effect on the induction of mixis. It is probable that stem females hatched from resting eggs have specific features that are vulnerable to unfavorable environmental conditions, and that these features can be inherited by their offspring through the maternal cytoplasm.     

Genetic variability and transgenerational regulation of investment in sex in the monogonont rotifer Brachionus plicatilis

In cyclical parthenogens such as aphids, cladocerans and rotifers, the coupling between sexual reproduction and the production of resting stages (diapausing eggs) imposes strong constraints on the timing of sex. Whereas induction of sex is generally triggered by environmental cues, the response to such cues may vary across individuals according to genetic and nongenetic factors. In this study, we explored genetic and epigenetic causes of variation for the propensity for sex using a collection of strains from a Spanish population of monogonont rotifers (Brachionus plicatilis) in which variation for the threshold population density at which sex is induced (mixis threshold) had been documented previously. Our results show significant variation for the mixis threshold among 20 clones maintained under controlled conditions for 15 asexual generations. The effect of the number of clonal generations since hatching of the diapausing egg on the mixis ratio (proportion of sexual offspring produced) was tested on 4 clones with contrasted mixis thresholds. The results show a negative correlation between the mixis threshold and mixis ratio, as well as a significant effect of the number of clonal generations since fertilization, sex being repressed during the first few generations after hatching of the diapausing egg.     

Compensatory population growth in <Emphasis Type="Italic">Daphniopsis tibetana</Emphasis> Sars (Crustacea: Cladocera) following starvation

We speculated that Daphniopsis tibetana may be adapted to short-term lack of food through a mechanism of population growth compensation. D. tibetana were collected from Lake Namuka Co, Tibet, and acclimated to diluted seawater in 2007. All larvae used in experiments were synchronized newborn (<24-h old) and of an average body length of 790 (770–810) μm. We evaluated the effect of continuous starvation for 1, 3, 5, 7, 9, and 11 days and intermittent periods of starvation of 0.5, 1, 1.5, and 2 days on the compensatory population growth effect in D. tibetana. The duration of starvation significantly influenced population growth, body length, and heart rate. After 1 and 3 days of continuous starvation and 0.5 days intermittent starvation, the age of first reproduction was 16.3 days and the reproduction frequency was 0.8 (10 days^?1), which were similar to the control group. In these two starved groups, lifetime fecundity values were 426.3 and 412.3, average life spans were 54.5 and 54.3 days, and intrinsic rates of increase (r _m) were 0.136 and 0.133 days^?1, respectively, which is significantly higher than the other starved groups and not different from the control group (P > 0.05). In the group with 7, 9, and 11 days of continuously starved groups, body lengths were significantly lower (P < 0.05) than those of controls. With increasing starvation time (1, 3, 5, and 7 days), heart rate progressively decreased (184, 178, 172, and 166 min^?1). Body length, measured at the end of the experiment, declined with increasing duration of intermittent starvation. After 2 days of intermittent starvation, final heart rate sharply decreased to 150 min^?1, which was highly significantly different from the other treatment groups. The results of this study conclude that D. tibetana is capable of complete compensatory population growth after short-term starvation.     

Isolation of mixis-related genes from the rotifer <Emphasis Type="Italic">Brachionus plicatilis</Emphasis> using subtractive hybridization

The monogonont rotifer Brachionus plicatilis produces resting eggs through sexual reproduction (mixis), which is affected by external and internal factors. We collected resting eggs from rotifers cultured at 15 and 25°C and hatched them with and without 14-day dormancy in the dark. Stem females hatched from both conditions were further cultured at 15, 20 and 25°C. We obtained two phenotypes, one with high mixis (more than 50%), which was hatched from resting eggs formed at 15°C without dormancy, and another in which sexual reproduction occurred at 25°C and resting eggs were formed at 15°C with a 14-day dormancy. In the latter phenotype, mictic females appeared at 15°C, but not at 25°C. Using subtractive hybridization, we isolated one gene from the latter phenotype of females that had no significant similarity to known genes in BLAST searches. We propose that this gene is unique to rotifer mictic reproduction. Ongoing characterization of this gene attempts understand its role in mixis.     

Sources contribution for benthic invertebrates: an inter-lake comparison in a flood plain system

Dwarfism in males is a common phenomenon in planktonic monogonont rotifers. Considering the small body size of males, some studies suggested that the development time of male rotifers is faster than that of females. However, empirical studies associated with such a speculation are scarce. In this study, we used two common rotifer species, Brachionus calyciflorus and Brachionus plicatilis (each with two geographical populations), to test the hypothesis that the development time of dwarf males is significantly shorter than that of females. Results showed that male eggs (embryos) were deposited much faster than female eggs (embryos) in both Brachionus species. However, the embryonic development time of these small male eggs (embryos) was longer than that of large female eggs (embryos). As a result, males needed significantly longer total development time than females. The total development time of males was 2–3 h longer than that of females in both Brachionus species. Male-producing and female-producing females in the two Brachionus species did not show any difference in development time. Because sexual reproduction begins at high population densities in both species, postponement of development in males will be advantageous by decreasing mating costs in male rotifers.     

Analysis of proteins in conditioned medium that trigger monogonont rotifer mictic reproduction

The initiation, rate, duration, and termination of reproduction in the rotifer life cycle are tightly regulated, but the molecular mechanisms are poorly understood. Environmental signals are sensed and transduced to chemical signals within rotifers to integrate asexual and sexual reproduction. Crowding, diet, and photoperiod are environmental signals used by various rotifer species to initiate sexual reproduction, but most advances in the last 10 years have been in discovering how rotifer crowding is a case of quorum sensing. Female brachionids release a protein into their medium that acts as a pheromone and accumulates to trigger mictic reproduction. Candidates for this mixis-inducing protein (MIP) have been isolated from rotifer-conditioned medium and characterized biochemically. A N-terminal partial amino acid sequence of a particularly promising 39 kD protein was used to prepare a polyclonal antibody. This antibody immunoprecipitated a 22 kD protein from rotifer-conditioned medium and inhibited mixis induction. Future advances will benefit from the production of transgenic rotifers to confirm the mixis-inducing action of this putative MIP gene.     

Life history response and age-specific tolerance to starvation in Brachionus plicatilis O.F. Müller (Rotifera)

To examine the life history response and age-specific tolerance to starvation in the rotifer Brachionus plicatilis O.F. Müller, we carried out two series of individual culture experiments. In the first experiment, rotifers were fed until each of the ages of 1-4 days, and were then starved during the rest of their lifetimes. The control group was fed throughout their lifespans. Rotifers stopped active reproduction just after the onset of food deprivation, and showed shorter subsequent survival times when they were starved at older ages. The finding that the larger the number of offspring produced before food deprivation, the shorter the subsequent lifetime under starvation, appeared to reflect a trade-off with the cost of reproduction. In the second experiment, newborns were starved until each of the ages of 1-5 days, and were fed thereafter. The lifespans of the rotifers starved up to the age of 3 days were not statistically different from those that were not starved. Although the starved rotifers began to reproduce once fed again, their lifetime fecundity decreased significantly from that of the non-starved group. Based on these results, it was suggested that the reproductive suppression caused by starvation would cause rotifers to have a longer lifespan to allow for future reproduction.     

Exploiting Phosphate-Starved cells of <Emphasis Type="Italic">Scenedesmus</Emphasis> sp. for the Treatment of Raw Sewage

Phosphate depletion is one of the favorable ways to enhance the sewage water treatment with the algae, however, detailed information is essential with respect to internal phosphate concentration and physiology of the algae. The growth rate of the phosphate-starved Scenedesmus cells was reduced drastically after 48 h. Indicating cells entered in the stationary phase of the growth cycle. Fourier Transform Infrared analysis of phosphate-starved Scenedesmus cells showed the reduction in internal phosphate concentration and an increase in carbohydrate/phosphate and carbohydrate/lipid ratio. The phosphate-starved Scenedesmus cells, with an initial cell density of, 1 × 10⁶ cells mL^?1 shows 87% phosphate and 100 % nitrogen removal in 24 h. The normal Scenedesmus cells need approximately 48 h to trim down the nutrients from wastewater up to this extent. Other microalgae, Ankistrodesmus, growth pattern was not affected due to phosphate starvation. The cells of Ankistrodesmus was able to reduce 71% phosphate and 73% nitrogen within 24 h, with an initial cell density of, 1 × 10⁶ cells mL^?1.     

Assessment of Genetic Stability in Three Generations of In Vitro Propagated <Emphasis Type="Italic">Jatropha curcas</Emphasis> L. Plantlets Using ISSR Markers

Tissue culture has been widely employed in Jatropha curcas L. for the clonal multiplication of superior genotypes. However, the evaluation of genetic stability is necessary to detect somaclonal variants. In this context, the present aim was to evaluate the genetic stability of J. curcas plantlets, obtained via indirect organogenesis, by means of ISSR markers. To supply the explant sources for in vitro propagation, the first generation of plants was produced from in vitro germination of J. curcas seeds. Fragments of cotyledonary leaves were inoculated into medium supplemented with 1.5 mg L^?1 BAP and 0.05 mg L^?1 of IBA for induction of callogenesis. The resulting calli were transferred to bud induction medium. Subsequently, the buds were cultured in medium for elongation, giving rise to the second generation of plants. These plants provided new buds, which were excised and subcultured in elongation medium, yielding a third generation of plants. To evaluate genetic stability in three plant generations, twelve ISSR primers were used, resulting in 124 bands showing 41.93 % of polymorphism. Increase was observed in the level of somaclonal variation (SV) over the generations. The present study reports, for the first time, the analysis of genetic stability in J. curcas plantlets regenerated via indirect organogenesis by means of ISSR markers. The results suggest that the indirect route is associated to higher levels of genetic instability, which also increased with successive subcultures. The ISSR markers were efficient in detecting SV, and the generated genetic variability may be useful for breeding programs.     

Comparative analysis of phenolic profiles of ovipositional fluid of <Emphasis Type="Italic">Rhinusa pilosa</Emphasis> (Mecinini,Curculionidae) and its host plant <Emphasis Type="Italic">Linaria vulgaris</Emphasis> (Plantaginaceae)

Rhinusa pilosa (Gyllenhal) is a highly specific weevil that induces stem galls on the common toadflax Linaria vulgaris Mill. females oviposit the eggs near the apex of a growing shoot. The act of oviposition is accompanied by secretion of an ovipositional fluid, which is considered to be cecidogen, directly involved in gall induction. The remains of cecidogenic fluid were collected from the surface of the oviposition point on the stem. We performed a comparative analysis of the phenolics extracted from cecidogen, the stem and galls of L. vulgaris and adult and larva of R. pilosa by HPLC-DAD. One compound with A _max at 273, 332 nm (R _t 30.65 min) was exclusively found in the methanol extract of cecidogen. To further characterize the cecidogen and stem phenolic profiles, we used UHPLC coupled with an OrbiTrap mass analyzer. Among 49 phenolic compounds extracted from both the ovipositional fluid and the plant, protocatechuic acid and two phenolic glycosides were exclusively found in cecidogen: diosmetin-O-acetylrutinoside and an unidentified compound. The unknown compound produced an MS² base peak at 387 and 327 and 267 m/z base peaks at MS³ and MS⁴ fragmentation, respectively, and had the molecular formula C₃₂H₃₁O₁₈. The plausible role of phenolic compounds in the induction of gall formation on L. vulgaris is discussed.     

Productivity and biochemical composition of <Emphasis Type="Italic">Tetradesmus obliquus</Emphasis> and <Emphasis Type="Italic">Phaeodactylum tricornutum</Emphasis>: effects of different cultivation approaches

The present work evaluated biomass productivity, carbon dioxide fixation rate, and biochemical composition of two microalgal species, Phaeodactylum tricornutum (Bacillariophyta) and Tetradesmus obliquus (Chlorophyta), cultivated indoors in high-technology photobioreactors (HT-PBR) and outdoors both in pilot ponds and low-technology photobioreactors in a greenhouse in southern Italy. Microalgae were grown in standard media, under nitrogen starvation, and in two liquid digestates obtained from anaerobic digestion of agro-zootechnical and vegetable biomass. P. tricornutum, cultivated in semi-continuous mode in indoor HT-PBRs with standard medium, showed a biomass productivity of 21.0?±?2.3 g m^?2 d^?1. Applying nitrogen starvation, the lipid productivity increased from 2.3 up to 4.5?±?0.5 g m^?2 d^?1, with a 24 % decrease of biomass productivity. For T. obliquus, a biomass productivity of 9.1?±?0.9 g m^?2 d^?1 in indoor HT-PBR was obtained using standard medium. Applying liquid digestates as fertilizers in open ponds, T. obliquus gave a biomass productivity (10.8?±?2.0 g m^?2 d^?1) not statistically different from complete medium such as P. tricornutum (6.5?±?2.2 g m^?2 d^?1). The biochemical data showed that the fatty acid composition of the microalgal biomass was affected by the different cultivation conditions for both microalgae. In conclusion, it was found that the microalgal productivity in standard medium was about doubled in HT-PBR compared to open ponds for P. tricornutum and was about 20 % higher for T. obliquus.     

Hope springs eternal in the starfish gonad: preserved potential for sexual reproduction in a single-clone population of a fissiparous starfish

Among echinoderms, asexual reproduction by fission occurs in few species. This strategy is considered a temporary response to stressful conditions and usually alternates with sexual reproduction events; thus, monoclonal populations are extremely rare. The occurrence of a single-clone population of the starfish Coscinasterias tenuispina at Llançà (NW Mediterranean) allowed us to study intra-clonal variation of the reproductive cycle during a two-year study. The few developed gonads (all male) were found in winter months, coinciding with the minimum photoperiod (ρ = ?0.82; P < 0.001) and lowest temperatures (ρ = ?0.75; P < 0.001), only in best-fed individuals, indicating that food availability influences individual ability for gonad development. Fissiparity happened throughout all the sampled period, but its rate increased with warm temperatures (ρ = 0.68; P < 0.0001). In contrast to what has been reported in other species, no correlation between fission rates and population density was found. The population was maintained over time by asexual reproduction and remained monoclonal. Although sexual reproduction has probably not occurred in this all-male population for a long time, the ability to yearly produce mature gonads is retained by some individuals, indicating that potential to reproduce sexually may be preserved, even in the case of strictly asexual populations.     

Isolation and characterization of <Emphasis Type="Italic">Paenibacillus polymyxa</Emphasis> LY214, a camptothecin-producing endophytic bacterium from <Emphasis Type="Italic">Camptotheca acuminata</Emphasis>

Camptothecin (CPT) is mainly produced and extracted from Camptotheca acuminata and Nothapodytes foetida for pharmaceutical use, i.e., the starting material for chemical conversion to the clinical CPT-type drugs. As the third largest plant anticancer drug, the heavy demand on CPT from global market leads to many research efforts to identify new sources for CPT production. Herein we report the isolation and characterization of a CPT-producing endophytic bacterium Paenibacillus polymyxa LY214 from Camptotheca acuminata. A 10.7 μg l^?1 of CPT was presented in the fermentation broth of P. polymyxa LY214. Its CPT production decreased sharply when the strain of the 2nd generation of P. polymyxa LY214 was cultured and fermented. However, the CPT production remained relatively constant from 2.8 μg l^?1 of the 2nd generation to 0.8 μg l^?1 of the 8th generation of P. polymyxa LY214 under optimized fermentation conditions. A 15- to 30-fold increase of CPT yield was observed when the optimized fermentation conditions, together with the addition of putative biosynthetic precursors of CPT and adsorbent resin XAD16, were applied to ferment the strains of the 7th and 8th generation of P. polymyxa LY214. Bioinformatics analysis of the relative species of P. polymyxa LY214 indicates its potential to produce CPT, which will be helpful to decipher the mysteries of CPT biosynthesis.     

A salty landscape of fear: responses of fish and zooplankton to freshwater salinization and predatory stress

Predator–prey relationships are altered by anthropogenic contaminants. Road salt is a widespread contaminant among freshwater ecosystems, yet a relatively understudied subject in community ecology. Unknown is whether road salt salinization interacts with predatory stress to influence the growth, behavior, or reproduction of freshwater organisms. Using rainbow trout (Oncorhynchus mykiss) and zooplankton (Daphnia pulex), we exposed them to variable levels of road salt (NaCl) crossed with the presence or absence of alarm cues or kairomones. Alarm cue reduced trout activity and aggression and increased shoaling behavior. Road salt reduced trout growth in the high compared to moderate salt concentration, but neither concentration was different from the control. There was no interaction between alarm cues and salt for trout. Road salt and predatory stress had an additive effect on Daphnia abundance. Predatory stress decreased Daphnia abundance by 11%. Compared to the control, salt decreased Daphnia abundance by 40% in 860 mg Cl^?/L and 79% in 1300 mg Cl^?/L, and by the final day abundance was reduced by 85% in 1300 mg Cl^?/L. Road salt and predatory stress had an interactive effect on Daphnia reproduction. Predatory stress in control water and moderate salt levels (230 mg Cl^?/L) increased sexual reproduction of Daphnia, but these responses disappeared at high salt concentrations. Thus, road salt could limit reproductive adaptations to natural and anthropogenic stressors in Daphnia. Our results indicate road salt salinization could alter zooplankton population dynamics directly and by interacting with predatory stress, which might affect energy flow through freshwater food webs.     

Reproductive biology and management of two commercially important groupers in the SW Atlantic

The reproductive biology of Epinephelus morio (red grouper) and Mycteroperca bonaci (black grouper) were evaluated based on 533 specimens collected from artisanal fisheries landings in the Abrolhos Bank, Brazil, between May 2005 and September 2012. Sex ratio for the black grouper was 1:14 (n = 155 females and 11 males; 26.1–147 cm TL) and 1:10 for the red grouper (n = 334 females and 33 males; 15.0–96.0 cm TL). For both species, highest values of the gonadosomatic index (GSI) for females were recorded between July and October, indicating spawning during the austral winter. The length at first maturity (L₅₀) for females was estimated at 62.0 and 47.0 cm TL for the black and red grouper, respectively. Batch fecundity based on TL and TW ranged from 2 to 15.4^?10⁶ and 1.5 to 13.7^?10⁶ for the black and red grouper, respectively. Interviews with experienced fishers revealed that spawning seasons of both groupers are largely unrecognized. Results demonstrate a positive relationship between GSI peaks, lower temperatures and stronger winds. The information provided herein may help decision-making regarding fisheries management and conservation for E. morio and M. bonaci at various levels of governance in the Abrolhos Bank, the region with the largest and richest coralline reefs in the South Atlantic.     

Efficient genetic transformation and regeneration system from hairy root of <Emphasis Type="Italic">Origanum vulgare</Emphasis>

Origanum vulgare L is commonly known as a wild marjoram and winter sweet which has been used in the traditional medicine due to its therapeutic effects as stimulant, anticancer, antioxidant, antibacterial, anti-inflammatory and many other diseases. A reliable gene transfer system via Agrobacterium rhizogenes and plant regeneration via hairy roots was established in O. vulgare for the first time. The frequency of induced hairy roots was different by modification of the co-cultivation medium elements after infection by Agrobacterium rhizogenes strains K599 and ATCC15834. High transformation frequency (91.3 %) was achieved by co-cultivation of explants with A. rhizogenes on modified (MS) medium. The frequency of calli induction with an 81.5 % was achieved from hairy roots on MS medium with 0.25 mg/L^?1 2,4-D. For shoot induction, initiated calli was transferred into a medium containing various concentrations of BA (0.1, 0.25, 0.5, 0.75 and 1 mg/L^?1). The frequency of shoot generation (85.18 %) was achieved in medium fortified with 0.25 mg/L^?1 of BA. Shoots were placed on MS medium with 0.25 mg/l IBA for root induction. Roots appeared and induction rate was achieved after 15 days.     

Life History of <Emphasis Type="Italic">Cochliomyia macellaria</Emphasis> (Fabricius, 1775) (Diptera,Calliphoridae), a Blowfly of Medical and Forensic Importance

The life history traits of blow fly Cochliomyia macellaria (Fabricius, 1775) was studied under semi-controlled laboratory conditions at 29.14°C temperature, 72.53% relative humidity, and 12-h photoperiod. The raw data were analyzed based on the age-stage, two-sex life table, considering the development rates among individuals of both sexes. Cochliomyia macellaria survival rate was 0.43 (♂) and 0.40 (♀), while life expectancy was 17.9 (♂) and 20.9 (♀) days, for adult males and females, respectively. The total fecundity was 681.15 eggs/female, with an average of 3.65 batches/female and 199 eggs/batch. The intrinsic rate of increase (r) was 0.327 days^?1, the finite rate of population increase (λ) was 3.35 days^?1, the mean generation time (T) was 17.15 days, and the net reproduction rate (R ₀ ) was 272.46 offspring/individual. The population parameters found here corroborates that C. macellaria population act as a r selected species under laboratory conditions. Additionally, development data and accumulated degree days (ADD) for each stage of C. macellaria are provided and its implications for the forensic use are discussed.     

Investigation of lipid production by nitrogen-starved <Emphasis Type="Italic">Parachlorella kessleri</Emphasis> under continuous illumination and day/night cycles for biodiesel application

This study aims to investigate the triacylglycerol (TAG) productivity of Parachlorella kessleri grown under continuous illumination and to investigate its metabolism in simulated day/night cycles in order to estimate the feasibility of a large-scale production in outdoor solar photobioreactors. The strain was chosen for its ability to accumulate large amounts of triacylglycerol during nitrogen starvation. Several protocols of nitrogen starvation were tested in continuous illumination as well as in simulated day/night cycles. Sudden and progressive nitrogen starvation conditions have enhanced the TAG concentration and productivity of P. kessleri reaching up to 48 dry wt% and 4.4 × 10^?3 kg m^?2 day^?1, respectively. Microalgal cell metabolism was significantly affected by the day/night illumination cycles. The energy-rich compounds (TAGs and carbohydrates) were accumulated by P. kessleri during the photoperiods and partly consumed during the dark to sustain the microalgae vitality. This TAG oxidation ultimately led to a 26% decrease in TAG productivity in cultures exposed to day/night cycles compared to ones exposed to continuous illumination of equal 24-h average photon flux density. The results can dictate the optimal time for harvesting cells for recovering the largest amount of TAGs.