Light-limited Chemostat Research Articles

Fast-growing phototrophic microorganisms and the productivity of phototrophic cultures.

Fast-growing cyanobacterial and microalgal strains are considered to be a promising resource to overcome current productivity barriers of phototrophic cultivation. The purpose of this communication, however, is to argue that a high maximal growth rate itself is not a sufficient or necessary property for high phototrophic productivity. Rather, the light-limited specific growth rate of a phototrophic microorganism is a product of several factors, including the rate of light absorption, the photosynthetic efficiency, and the maximal biomass yield per mol photons. It is suggested that, in addition to the maximal growth rate, reports on fast-growing strains should also assess photosynthetic efficiency and maximal biomass yield as predictors of culture productivity. The arguments within the communication are underpinned by a theoretical analysis of a light-limited chemostat, compared to its heterotrophic counterpart. It is shown that for the light-limited chemostat maximal productivity occurs at low dilution rates.

Optimal proteome allocation strategies for phototrophic growth in a light-limited chemostat

BackgroundCyanobacteria and other phototrophic microorganisms allow to couple the light-driven assimilation of atmospheric text {CO}_{2} directly to the synthesis of carbon-based products, and are therefore attractive platforms for microbial cell factories. While most current engineering efforts are performed using small-scale laboratory cultivation, the economic viability of phototrophic cultivation also crucially depends on photobioreactor design and culture parameters, such as the maximal areal and volumetric productivities. Based on recent insights into the cyanobacterial cell physiology and the resulting computational models of cyanobacterial growth, the aim of this study is to investigate the limits of cyanobacterial productivity in continuous culture with light as the limiting nutrient.ResultsWe integrate a coarse-grained model of cyanobacterial growth into a light-limited chemostat and its heterogeneous light gradient induced by self-shading of cells. We show that phototrophic growth in the light-limited chemostat can be described using the concept of an average light intensity. Different from previous models based on phenomenological growth equations, our model provides a mechanistic link between intracellular protein allocation, population growth and the resulting reactor productivity. Our computational framework thereby provides a novel approach to investigate and predict the maximal productivity of phototrophic cultivation, and identifies optimal proteome allocation strategies for developing maximally productive strains.ConclusionsOur results have implications for efficient phototrophic cultivation and the design of maximally productive phototrophic cell factories. The model predicts that the use of dense cultures in well-mixed photobioreactors with short light-paths acts as an effective light dilution mechanism and alleviates the detrimental effects of photoinhibition even under very high light intensities. We recover the well-known trade-offs between a reduced light-harvesting apparatus and increased population density. Our results are discussed in the context of recent experimental efforts to increase the yield of phototrophic cultivation.

Quantifying the potential of microalgae to remove nutrients from wastewater

The main resources limiting microalgae growth are typically phosphorus, nitrogen, and light. Based on the theory of the light limited chemostat, the variable cell quota approach, and photoacclimation models, we build a mathematical model for describing microalgae growth under limitation by these resources. The model is calibrated with a data set from the literature. Then, by numerical simulations, we find that under constant operation of the culture and constant environmental conditions (illumination, temperature, pH, etc.), solutions of the model approach towards either a positive or an extinction steady state. Based on the positive steady state, and in the context of wastewater treatment, we evaluate the capacity of microalgae to remove contaminants. We showed that the impacts of depth, incident light intensity, and dilution rate (or hydraulic retention time) have a crucial role on the optimization of the nutrient removal efficiency

Continuous Microalgae Culture: Operation of Light-Limited Chemostats

Background: Continuous culture is a method with significant benefits that is underused in the area of microalgae. Despite this, with the spread of these cultures into many different areas, such as biofuel production, pigment production for the food and pharmaceutics industries, effluent treatment, biosorption of heavy metals, it is necessary and imperative to make advancements in the technology in terms of knowledge and use. Therefore, it is necessary to identify its fundamental basis and to fully understand its operation in order to be able to successfully maximise important parameters such as biomass or metabolite productivity. Methods: This article presents the fundamental concepts and applies them clearly and simply in continuous culture photobioreactors operated as light-limited chemostats, using a Scenedesmus obliquus culture. Results: This article presents the basis and applications of a simple light-limited chemostat method, which is the most commonly-used type of operation for achieving high levels of biomass productivity. The usefulness of operation curves: microalgae concentration(X) - dilution rate (D) and volumetric productivity of cells (Qx) - D and how to build them are also presented, along with the operational and environmental design parameters that affect them. Conclusion: The construction of operation curves for the microalga in question must be carried out in a laboratory with day-night cycles of temperature and irradiance to obtain critical D values and the optimal D at which to successfully operate the outdoor culture. Keywords: Continuous culture, microalgae, Scenedesmus obliquus, light limited chemostat, operation curves, photobioreactors.

Driving Species Competition in a Light-limited Chemostat

In this paper, we tackle the problem of microalgae selection in a continuous photobioreactor where microalgae growth is limited by light. We propose a closed-loop control for selecting, for a given range of light intensity, the strain with the maximum growth rate from the microalgae population. In particular, we are interested in strains with high growth rate for high light intensity, i.e., strains with high resistance to photoinhibition. Firstly, we recall the framework of the light-limited chemostat. Then, we propose a nonlinear adaptive control which regulates the light intensity at the bottom of the photobioreactor in monoculture. This light is of particular interest as it defines the winner of the competition in a multispecies culture operated in open-loop mode. Finally, we show that the proposed controller allows the selection of a strain of interest in the case of a culture with η species.

Competition and facilitation between unicellular nitrogen‐fixing cyanobacteria and non—nitrogen‐fixing phytoplankton species

Recent discoveries show that small unicellular nitrogen‐fixing cyanobacteria are more widespread than previously thought and can make major contributions to the nitrogen budget of the oceans. We combined theory and experiments to investigate competition for nitrogen and light between these small unicellular diazotrophs and other phytoplankton species. We developed a competition model that incorporates several physiological processes, including the light dependence of nitrogen fixation, the switch between nitrate assimilation and nitrogen fixation, and the release of fixed nitrogen. Model predictions were tested in nitrogen‐limited and lightlimited chemostat experiments using the unicellular nitrogen‐fixing cyanobacterium Cyanothece sp. Miami BG 043511, the picocyanobacterium Synechococcus bacillaris CCMP 1333, and the small green alga Chlorella cf sp. CCMP 1227. Parameter values of the species were estimated by calibration of the model in monoculture experiments. The model predictions were subsequently tested in a series of competition experiments at different nitrate levels. The model predictions were generally in good agreement with observed population dynamics. As predicted, in experiments with high nitrate input concentrations, the species with lowest critical light intensity (S. bacillaris) competitively excluded the other species. At low nitrate input concentration, nitrogen release by Cyanothece enabled stable coexistence of Cyanothece and S. bacillaris. More specifically, model simulations predicted that fixed nitrogen release by Cyanothece enabled S. bacillaris to become four times more abundant in the species mixture than it would have been in monoculture. This intricate interplay between competition and facilitation is likely to be a major determinant of the relative abundances of unicellular nitrogen‐fixing cyanobacteria and non—nitrogen—fixing phytoplankton species in the oligotrophic ocean.

Competition for Light between Toxic and Nontoxic Strains of the Harmful Cyanobacterium Microcystis

The cyanobacterium Microcystis can produce microcystins, a family of toxins that are of major concern in water management. In several lakes, the average microcystin content per cell gradually declines from high levels at the onset of Microcystis blooms to low levels at the height of the bloom. Such seasonal dynamics might result from a succession of toxic to nontoxic strains. To investigate this hypothesis, we ran competition experiments with two toxic and two nontoxic Microcystis strains using light-limited chemostats. The population dynamics of these closely related strains were monitored by means of characteristic changes in light absorbance spectra and by PCR amplification of the rRNA internal transcribed spacer region in combination with denaturing gradient gel electrophoresis, which allowed identification and semiquantification of the competing strains. In all experiments, the toxic strains lost competition for light from nontoxic strains. As a consequence, the total microcystin concentrations in the competition experiments gradually declined. We did not find evidence for allelopathic interactions, as nontoxic strains became dominant even when toxic strains were given a major initial advantage. These findings show that, in our experiments, nontoxic strains of Microcystis were better competitors for light than toxic strains. The generality of this finding deserves further investigation with other Microcystis strains. The competitive replacement of toxic by nontoxic strains offers a plausible explanation for the gradual decrease in average toxicity per cell during the development of dense Microcystis blooms.

COMPETITION FOR NUTRIENTS AND LIGHT: STABLE COEXISTENCE, ALTERNATIVE STABLE STATES, OR COMPETITIVE EXCLUSION?

Competition theory has put forward three contrasting hypotheses: Compe- tition for nutrients and light may lead to (i) stable coexistence of species, (ii) alternative stable states, or (iii) competitive exclusion. This paper presents a detailed investigation of competition among phytoplankton species to test these three different hypotheses. First, we developed a competition model combining competition for nutrients and light. Next, we ran monoculture experiments in phosphorus-limited and light-limited chemostats to estimate the model parameters for five freshwater phytoplankton species. Finally, we tested the model predictions in competition experiments, using phosphorus levels ranging from oligotrophic to eutrophic conditions. The population dynamics in the competition experi- ments were all in agreement with the model predictions. This demonstrates that competition for nutrients and light can be accurately predicted over a wide range of productivities. The experiments revealed that the intensity of competition remained constant or even decreased with increasing nutrient supply. Contrary to expectation, there were no trade-offs between competitive abilities for phosphorus and light. Species that were strong competitors for phosphorus were strong competitors for light as well. Hence, we found neither stable coexistence nor alternative stable states. All competition experiments led to competitive exclusion. Furthermore, the physiological traits of the species indicated that, if one would find trade-offs in competitive abilities, competition for phosphorus and light would lead to alternative stable states rather than to stable coexistence. These results suggest that stable coexistence mediated by competition for phosphorus and light is rare, and hence an unlikely explanation for the high biodiversity commonly found in phytoplankton communities.

Principles of the light-limited chemostat: theory and ecological applications.

Light is the energy source that drives nearly all ecosystems on planet Earth. Yet, light limitation is still poorly understood. In this paper, we present an overview of the principles of the light-limited chemostat. The theory for light-limited chemostats differs considerably from the standard theory for substrate-limited chemostats. In particular, photons cannot be mixed by vigorous stirring, so that phototrophic organisms experience the light-limited chemostat as a heterogeneous environment. Similar to substrate-limited chemostats, however, light-limited chemostats do reach a steady state. This allows the study of phototrophic microorganisms under well-controlled light conditions, at a constant specific growth rate, for a prolonged time. The theory of the light-limited chemostat is illustrated with several examples from laboratory experiments, and a variety of ecological applications are discussed.

Growth yield determination in a chemostat culture of the marine microalgaIsochrysis galbana

The growth yield of the PUFA-producing marine microalgaIsochrysis galbana ALII-4 grown in a light limited chemostat, was measured under a wide variety of conditions of incident irradiance (I O ) and dilution rates (D). The experiments were conducted under laboratory conditions at 20 °C under continuous light. D ranged from 0.0024 to 0.0410 h−1 at three intensities of Io (820, 1620 and 3270 µmol photon m−2 s−1) close to those found in outdoor cultures. A maximum efficiency Ψ max = 0.616 g mol photon−1 was obtained at I O = 820 µmol photon m−2 s−1 and D = 0.030 h−1 and the maximum capacity of the biomass to metabolize the light harvested was found to be 13.1 µmol photon g−1 s−1. Above this value, a significant drop in the system efficiency was observed. A new approach based in the averaged irradiance is used to assess the photon flux absorbed by the biomass.

A model for light‐limited continuous cultures: Growth, shading, and maintenance

Light-limited growth in continuous cultures of phototrophic organisms is modeled. It is assumed that light energy up-take rate depends hyperbolically on light intensity and that the maintenance costs are proportional to biomass. Modeling the light distribution caused by shading within the vessel is necessary to explain the existence of steady state in light-limited chemostats. The model fits well to experimental data from literature on light-limited chemostats and turbidostats. Attention is given to the implications of the model for the estimation of the specific maintenance rate constant in light-limited continuous cultures.

Effect of temperature, light intensity and dilution rate on the cellular composition of red algaPorphyridium cruentum in light-limited chemostat cultures

The fatty acid, polysaccharide and pigment contents ofPorphyridium cruentum at different growth temperatures, light intensities and dilution rates were studied in light-limited chemostat. The highest biomass yield was obtained at 25°C over the range of dilution rate studied (0.079 to 0.30 day−1). The production of fatty acids increased with increasing dilution rates. At low dilution rates, the contents of fatty acids at 20°C, 25°C and 30°C were comparable. Temperature did not affect the content of linolenic acid (18∶2) though fatty acids with less than two double bonds predominated at higher temperatures, those with more than two double bonds accumulated at lower temperatures. The yields of arachidonic acid (20∶4) increased under light of higher intensity. The maximum production of extracellular and cellular polysaccharides occured at low dilution rates under high light at 25°C. The amounts of chlorophylla, carotenoids and phycoerythrin all increased in the same proportions with dilution rate, and were highest at 25°C.

Buoyant density changes in the cyanobacterium Microcystis aeruginosa due to changes in the cellular carbohydrate content

The cyanobacterium Microcystis aeruginosa was grown in light-limited chemostat cultures with various light—dark rhythms providing a total periodicity length of 24 h. The buoyant density of the cells changed in parallel with the carbohydrate content. Short incubation experiments with different light intensities, and experiments with the inhibitors iodoacetic acid and arsenate, showed that the buoyant density changes were due to variations in the cellular carbohydrate content. It seems likely that the low dark-growth yield on carbohydrate, which had been stored during the light period, served to facilitate buoyancy changes.

Buoyant density changes in the cyanobacteriumMicrocystis aeruginosadue to changes in the cellular carbohydrate content

The cyanobacterium Microcystis aeruginosa was grown in light-limited chemostat cultures with various light—dark rhythms providing a total periodicity length of 24 h. The buoyant density of the cells changed in parallel with the carbohydrate content. Short incubation experiments with different light intensities, and experiments with the inhibitors iodoacetic acid and arsenate, showed that the buoyant density changes were due to variations in the cellular carbohydrate content. It seems likely that the low dark-growth yield on carbohydrate, which had been stored during the light period, served to facilitate buoyancy changes.

Light and nutrient status of algal cells

Chemostats were used to construct nutrient (vitamin B12) and energy budgets for the flagellateMonochrysis(Pavlova lutheri(Droop) Green) under varying conditions of nutrient and light limitation and luxury, the aim being to describe light limitation and luxury in terms analogous to those of the Cell Quota nutrient model. Measurements were made of steady-state dilution rate, biomass, cell quota, cell chlorophylla, cell energy content, dark respiration and photosynthetic efficiency, under low irradiance (<13 W per m2).Cell energy content was 0·54 joules per million cells and was not influenced by growth rate or nutrient/light status. Cell chlorophyllawas 25% less in the nutrient-limited cultures and did not show a clear trend with growth rate except in cultures receiving less than 4·3 W per m2. The effective specific extinction coefficient was 0·12 cm2perμg chlorophyll. The photosynthesis/irradiance relation was linear. The mean slope (= efficiency) in samples from the light-limited chemostats was 0·225 and was independent of dilution rate. Samples from nutrient-limited chemostats showed a much lower slope, which decreased with decreasing dilution rate. These findings were confirmed by the chemostat kinetic data. Dark respiration varied with dilution rate in a hyperbolic manner and was greater in the nutrient- than the light-limited chemostat.Light level had no effect on the subsistence quota when the nutrient was limiting, whereas under light-limiting conditions the subsistence quotas were enlarged in inverse proportion to the incident irradiance, thus showing a threshold rather than multiplicative interaction between light and the nutrient.Energy absorption was very little reduced on nutrient limitation, the great reduction in overall efficiency being associated mainly with the reduction in photosynthetic efficiency.The reciprocal of photosynthetic efficiency was shown to be analogous to a standardized nutrient cell quota, and an analogous coefficient of luxury could be defined as the ratio of the light-limited to nutrient-limited photosynthetic efficiency at the same growth rate. Thus, light limitation and luxury could be described in terms compatible with the nutrient model without having to introduce new concepts, and a practical criterion between light- and nutrient-limitation could be provided.

Effects of a light-dark cycle on the growth ofScenedesmus protuberans Fritsch in light-limited continuous cultures

Growth ofScenedesmus protuberans Fritsch was studied in light-limited chemostat type continuous cultures, with a light-dark cycle of 16, respectively 8 hours.

The control of carbon dioxide assimilation and ribulose 1,5-diphosphate carboxylase activity in Anacystis nidulans grown in a light-limited chemostat.

The blue-green alga, Anacystis nidulans, was grown in a light-limited chemostat at specific growth rates ranging from 0.02 h-1 to 0.10h-1. The rate of carbon dioxide assimilation, measured under optimal experimental conditions, increased 3 to 5 fold with increasing growth rate over this range. Ribulose 1,5-diphosphate carboxylase (RuDPCase) activity per organism increased four fold over the same growth rate range. RuDPCase specific activity in terms of total protein remained constant at all growth rates. The total protein content per organism and the percentage of protein in the dry weight increased in faster growing organisms. The dry weight per organism also increased as growth rate increased. These results are discussed to show that RuDPCase activity is not controlled by transcriptional mechanisms but that its activity per cell is regulated by gene dosage.