Nitrogen-fixing Cyanobacterium Cyanothece Research Articles

Improving feeding profile strategy for hydrogen production by Cyanothece sp. ATCC 51142 using meta-heuristic methods

Hydrogen has received considerable attention as a sustainable energy source to the future and biological hydrogen production has been gaining consideration as a potential method. One microorganism that produces hydrogen is the nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142, which stands out for its rate of hydrogen production. However, there are still some challenges for understanding and improving the hydrogen production process by this cyanobacterium. Thus, this work aimed to perform an in silico study of a feeding profile strategy for hydrogen production by Cyanothece sp. ATCC 51142 in batch/fed-batch process to improve the amount of hydrogen generated during the process. Two meta-heuristic methods were used to find the best feeding profile and the switch time between batch and fed-batch modes. Hydrogen production is similar in the results of both methods, but PSO indicated a simpler feeding policy, leading to 19.5% more hydrogen produced than that reported in the literature.

Rhythmic oscillations in KaiC1 phosphorylation and ATP/ADP ratio in nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142

Cyanobacterial circadian clock composed of the Kai oscillator has been unraveled in the model strain Synechococcus elongatus PCC 7942. Recent studies with nitrogen-fixing Cyanothece sp. ATCC 51142 show rhythmic oscillations in the cellular program even in continuous light albeit with a cycle time of ~11 h. In the present study, we investigate correlation between cellular rhythms, KaiC1 phosphorylation cycle, ATP/ADP ratio, and the redox state of plastoquinone pool in Cyanothece. KaiC1 phosphorylation cycle of Cyanothece was similar to that of Synechococcus under diurnal cycles. However, under continuous light, the cycle time was shorter (11 h), in agreement with physiological and gene expression studies. Interestingly, the ATP/ADP ratio also oscillates with an 11 h period, peaking concomitantly with the respiratory burst. We propose a mathematical model with C/N ratio as a probable signal regulating the clock in continuous light and emphasize the existence of a single timing mechanism regardless of the cycle time.

Demonstration of a two-stage aerobic/anaerobic chemostat for the enhanced production of hydrogen and biomass from unicellular nitrogen-fixing cyanobacterium

Due to incompatible demands between the cyanobacterial growth and its sequential hydrogen (H2) photoproduction mechanism, a two-stage chemostat photobioreactor (PBR) system was employed to cultivate a unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142. Our developed system, consisting of a series of two physically separated PBRs, has been operated non-stop for consecutive 750h (~31days), without any losses in its performance. Based on nutrient kinetics determined in batch PBR, a dilution rate of 0.015h−1 was chosen to replenish the culture with all of essential substrates, while not interrupting the H2 formation reaction. The physiological steady-state condition was achieved on day 12. The dry biomass concentrations, accumulated in the primary growth and the secondary H2-production PBRs, reached the final values of 2 and 1.5gL−1 respectively. By comparing to a single-stage batch system, the chemostat displayed more than 6.4 times higher H2 productivity and 7.3 folds greater biomass yield. The steady-state conversion from glycerol into H2 was ~28.3%. The stationary growth phase was also observed when the nutrient-replete culture eventually became under light-limited condition. Additionally, glycerol was reproducibly demonstrated as an effective anaerobic-inducer onto an air-incubated culture of this Cyanothece strain.

Dimeric chlorite dismutase from the nitrogen-fixing cyanobacterium Cyanothece sp. PCC7425.

SummaryIt is demonstrated that cyanobacteria (both azotrophic and non‐azotrophic) contain heme b oxidoreductases that can convert chlorite to chloride and molecular oxygen (incorrectly denominated chlorite ‘dismutase’, Cld). Beside the water‐splitting manganese complex of photosystem II, this metalloenzyme is the second known enzyme that catalyses the formation of a covalent oxygen–oxygen bond. All cyanobacterial Clds have a truncated N‐terminus and are dimeric (i.e. clade 2) proteins. As model protein, Cld from C yanothece sp. PCC7425 (CCld) was recombinantly produced in E scherichia coli and shown to efficiently degrade chlorite with an activity optimum at pH 5.0 [k cat 1144 ± 23.8 s−1, KM 162 ± 10.0 μM, catalytic efficiency (7.1 ± 0.6) × 106 M−1 s−1]. The resting ferric high‐spin axially symmetric heme enzyme has a standard reduction potential of the Fe(III)/Fe(II) couple of −126 ± 1.9 mV at pH 7.0. Cyanide mediates the formation of a low‐spin complex with k on = (1.6 ± 0.1) × 105 M−1 s−1 and k off = 1.4 ± 2.9 s−1 (KD ∼ 8.6 μM). Both, thermal and chemical unfolding follows a non‐two‐state unfolding pathway with the first transition being related to the release of the prosthetic group. The obtained data are discussed with respect to known structure–function relationships of Clds. We ask for the physiological substrate and putative function of these O2‐producing proteins in (nitrogen‐fixing) cyanobacteria.

Modelling of light and temperature influences on cyanobacterial growth and biohydrogen production

Dynamic simulation is a valuable tool to assist the scale-up and transition of biofuel production from laboratory scale to potential industrial implementation. In the present study two dynamic models are constructed, based on the Aiba equation, the improved Lambert–Beer's law and the Arrhenius equation. The aims are to simulate the effects of incident light intensity, light attenuation and temperature upon the photo-autotrophic growth and the hydrogen production of the nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142. The results are based on experimental data derived from an experimental setup using two different geometries of laboratory scale photobioreactors: tubular and flat-plate. All of the model parameters are determined by an advanced parameter estimation methodology and subsequently verified by sensitivity analysis. The optimal temperature and light intensity facilitating biohydrogen production in the absence of light attenuation have been determined computationally to be 34°C and 247μmolm−2s−1, respectively, whereas for cyanobacterial biomass production they are 37°C and 261μmolm−2s−1, respectively. Biomass concentration higher than 0.8gL−1 is also demonstrated to significantly enhance the light attenuation effect, which in turn inducing photolimitation phenomena. At a higher biomass concentration (3.5gL−1), cyanobacteria are unable to activate photosynthesis to maintain their lives in a photo-autotrophic growth culture, and biohydrogen production is significantly inhibited due to the severe light attenuation.

Modelling light transmission, cyanobacterial growth kinetics and fluid dynamics in a laboratory scale multiphase photo-bioreactor for biological hydrogen production

An integrated reactor model was designed in this research to simulate fluid dynamics, local light intensity and the growth rate of nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142 in a flat-plate photo-bioreactor by CFD technique. Previous research had already given the parameters in different algal growth kinetic equations. In this research, parameters were modified by CFD technique to improve the accuracy of cyanobacterial growth kinetic models. Finally, effects of recycling gas flow rate and geometry of sparger on local light intensity, growth rate, and fluid dynamics were analysed. Results show that recycling gas flow can increase liquid velocity and bubble volume fraction, however it prevents light transmission and the growth of cyanobacterium. Geometry of the sparger can affect liquid movement and distribution of both local light intensity and local growth rate.

Competition and facilitation between the marine nitrogen-fixing cyanobacterium Cyanothece and its associated bacterial community.

N2-fixing cyanobacteria represent a major source of new nitrogen and carbon for marine microbial communities, but little is known about their ecological interactions with associated microbiota. In this study we investigated the interactions between the unicellular N2-fixing cyanobacterium Cyanothece sp. Miami BG043511 and its associated free-living chemotrophic bacteria at different concentrations of nitrate and dissolved organic carbon and different temperatures. High temperature strongly stimulated the growth of Cyanothece, but had less effect on the growth and community composition of the chemotrophic bacteria. Conversely, nitrate and carbon addition did not significantly increase the abundance of Cyanothece, but strongly affected the abundance and species composition of the associated chemotrophic bacteria. In nitrate-free medium the associated bacterial community was co-dominated by the putative diazotroph Mesorhizobium and the putative aerobic anoxygenic phototroph Erythrobacter and after addition of organic carbon also by the Flavobacterium Muricauda. Addition of nitrate shifted the composition toward co-dominance by Erythrobacter and the Gammaproteobacterium Marinobacter. Our results indicate that Cyanothece modified the species composition of its associated bacteria through a combination of competition and facilitation. Furthermore, within the bacterial community, niche differentiation appeared to play an important role, contributing to the coexistence of a variety of different functional groups. An important implication of these findings is that changes in nitrogen and carbon availability due to, e.g., eutrophication and climate change are likely to have a major impact on the species composition of the bacterial community associated with N2-fixing cyanobacteria.

Effects of light and temperature on the photoautotrophic growth and photoinhibition of nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142

The unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. ATCC 51142 is a promising strain with a remarkable capability of producing large quantities of hydrogen, an energy carrier long being promoted as an ideal fuel. Under extreme environmental conditions, significant reduction of cellular photosynthetic capability is commonly observed in algae and cyanobacteria. Even less severe conditions can induce photo-inhibitive growth dynamics, which in turn result in a marked decrease in biomass and gas productivities. In this study a detailed analysis of the effect of two extrinsic parameters, namely light intensity and temperature, on the photoautotrophic growth of Cyanothece was performed in order to reveal critical conditions that would lead to undesired photoinhibition. A high degree of coherence between cyanobacterial growth and nutrient uptake kinetics was observed, as well as a strong dependence on the change of the two parameters. Nitrogen depletion was confirmed as a trigger, which transforms an exponential into a stationary growth phase. A non-linear relationship between the maximum specific growth rate and the irradiance up to 320μEm−2s−1 was identified and found to be dominated by light saturation rather than photoinhibition. The relationship between the specific growth rate and the temperature was found to be linear until a remarkable drop in the final biomass productivity and cyanobacterial photosynthetic capability was observed at 40°C. The cause of this is a high temperature-induced photoinhibition effect.

On the dynamics and constraints of batch culture growth of the cyanobacterium Cyanothece sp. ATCC 51142

The unicellular, nitrogen fixing cyanobacterium Cyanothece sp. ATCC 51142 is of a remarkable potential for production of third-generation biofuels. As the biotechnological potential of Cyanothece 51142 varies with the time of the day, we argue that it will, similarly, depend on the phase of the culture growth. Here, we study the batch culture dynamics to discover the dominant constraints in the individual growth phases and identify potential for inducing or delaying transitions between culture growth phases in Cyanothece 51142. We found that specific growth rate in the exponential phase of the culture is much less dependent on incident irradiance than the photosynthetic activity. We propose that surplus electrons that are released by water splitting are used in futile processes providing photoprotection additional to non-photochemical quenching. We confirm that the transition from exponential to linear phase is caused by a light limitation and the transition from linear to stationary phase by nitrogen limitation. We observe spontaneous diurnal metabolic oscillations in stationary phase culture that are synchronized over the entire culture without an external clue. We tentatively propose that the self-synchronization of the metabolic oscillations is due to a cell-to-cell communication of the cyanobacteria that is necessary for nitrogenase activity in nitrate depleted medium.

Experimental validation of a nonequilibrium model of CO2 fluxes between gas, liquid medium, and algae in a flat-panel photobioreactor

Carbon dioxide (CO₂) availability strongly affects the productivity of algal photobioreactors, where it is dynamically exchanged between different compartments, phases, and chemical forms. To understand the underlying processes, we constructed a nonequilibrium mathematical model of CO₂ dynamics in a flat-panel algal photobioreactor. The model includes mass transfer to the algal suspension from a stream of bubbles of CO₂-enriched air and from the photobioreactor headspace. Also included are the hydration of dissolved CO₂ to bicarbonate ion (HCO₃⁻) as well as uptake and/or cycling of these two chemical forms by the cells. The model was validated in experiments using a laboratory-scale flat-panel photobioreactor that controls light, temperature, and pH and where the concentration of dissolved CO₂, and partial pressure of CO₂ in the photobioreactor exhaust are measured. First, the model prediction was compared with measured CO₂ dynamics that occurred in response to a stepwise change in the CO₂ partial pressure in the gas sparger. Furthermore, the model was used to predict CO₂ dynamics in photobioreactors with unicellular, nitrogen-fixing cyanobacterium Cyanothece sp. The metabolism changes dramatically during a day, and the distribution of CO₂ is expected to exhibit a pronounced diurnal modulation that significantly deviates from chemical equilibrium.

Metabolic Rhythms of the Cyanobacterium Cyanothece sp. ATCC 51142 Correlate with Modeled Dynamics of Circadian Clock

These experiments aim to reveal the dynamic features that occur during the metabolism of the unicellular, nitrogen fixing cyanobacterium Cyanothece sp. when exposed to diverse circadian forcing patterns (LD 16:8, LD 12:12, LD 8:16, LD 6:6). The chlorophyll concentration grew rapidly from subjective morning when first illuminated to around noon, then remained stable from later in the afternoon and throughout the night. The optical density measured at 735 nm was stable during the morning chlorophyll accumulation, then increased in the early afternoon toward a peak, followed at dusk by a rapid decline toward the late night steady state. The authors propose that these dynamics largely reflect accumulation and subsequent consumption of glycogen granules. This hypothesis is consistent with the sharp peak of respiration that coincides with the putative hydrocarbon catabolism. In the long-day regimen (LD 16:8), these events may mark the transition from the aerobic photosynthetic metabolism to microaerobic nitrogen metabolism that occurs at dusk, and thus cannot be triggered by the darkness that comes later. Rather, control is likely to originate in the circadian clock signaling an approaching night. To explore the dynamics of the link between respiration and circadian oscillations, the authors extrapolated an earlier model of the KaiABC oscillator from Synechococcus elongatus to Cyanothece sp. The measured peak of respiratory activity at dusk correlated strongly in its timing and time width with the modeled peak in accumulation of the KaiB(4) complex, which marks the late afternoon phase of the circadian clock. The authors propose a hypothesis that high levels of KaiB(4) (or of its Cyanothece sp. analog) trigger the glycogen catabolism that is reflected in the experiments in the respiratory peak. The degree of the correlation between the modeled KaiB(4) dynamics and the dynamics of experimentally measured peaks of respiratory activity was further tested during the half-circadian regimen (LD 6:6). The model predicted an irregular pattern of the KaiABC oscillator, quite unlike mechanical or electrical clock pacemakers that are strongly damped when driven at double their endogenous frequency. This highly unusual dynamic pattern was confirmed experimentally, supporting strongly the validity of the circadian model and of the proposed direct link to respiration.

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.