Effect of temperature on growth, photosynthesis and biochemical composition of Nannochloropsis oceanica, grown outdoors in tubular photobioreactors

Photosynthesis and biochemical characterization of the green alga Chlamydopodium fusiforme (Chlorophyta) grown in a thin-layer cascade

Photosynthesis, growth and biochemical composition of the biomass of the freshwater microalga Chlamydopodium fusiforme cultures outdoors in a thin-layer cascade were investigated. Gross oxygen production measured off-line in samples taken from the outdoor cultures was correlated with the electron transport rate estimated from chlorophyll a fluorescence measurements. According to photosynthesis measurements, a mean of 38.9  ±  10.3 mol of photons were required to release one mole of O2, which is 4.86 times higher than the theoretical value (8 photons per 1 O2). In contrast, according to the fluorescence measurements, a mean of 11.7  ±  0.74 mol of photons were required to release 1 mol of O2. These findings indicate that fluorescence-based photosynthesis rates may not be fully replace oxygen measurements to evaluate the performance of an outdoor culture. Daily gross biomass productivity was 0.3 g DW L-1 day-1 consistently for 4 days. Biomass productivity was strongly affected by the suboptimal concentration at which the culture was operated and by the respiration rate, as the substantial volume of culture was kept in the dark (about 45% of the total volume). As the cells were exposed to excessive light, the photosynthetic activity was mainly directed to the synthesis of carbohydrates in the biomass. In the morning, carbohydrate content decreased because of the dark respiration. Per contra, protein content in the biomass was lower at the end of the day and higher in the morning due to carbohydrate consumption by respiration. The data gathered in these trials are important for the future exploitation of Chlamydopodium fusiforme as a potential novel species in the field of microalgae for the production of bio-based compounds.

Optimization of the culture of Chlorella sorokiniana PA.91 by RSM: effect of temperature, light intensity, and MgAC-NPs

The unique physicochemical properties of magnesium amino clay nanoparticles (MgAC-NPs) tends to be beneficial in the application as a co-additive in treating microalgae. Also, MgAC-NPs can create oxidative stress in the environment, concurrently elective control bacteria in mixotrophic culture, and stimulate CO₂ biofixation. The condition of the cultivation of newly isolated strains, Chlorella sorokiniana PA.91, was optimized for the first time for MgAC-NPs at various temperatures and light intensities in the culture medium of municipal wastewater (MWW) by central composite design in the response surface methodology (RSM-CCD). This study examined synthesized MgAC-NP with their FE-SEM, EDX, XRD, and FT-IR characteristics. The synthesized MgAC-NPs were naturally stable, cubic shaped, and within the size range of 30-60 nm. The optimization results show that at culture conditions of 20 °C, 37 μmol m^-2 s^-1, and 0.05 g L^-1, microalga MgAC-NPs have the best growth productivity and biomass performance. Maximum dry biomass weight (55.41%), specific growth rate (30.26%), chlorophyll (81.26%), and carotenoids (35.71%) were achieved under the optimized condition. Experimental results displayed that C.S. PA.91 has a high capacity for lipid extraction (1.36 g L^-1) and significant lipid efficiency (45.1%). Also, in 0.2 and 0.05 g L^-1 of the MgAC-NPs, COD removal efficiency 91.1% and 81.34% from C.S. PA.91 showed, respectively. These results showed the potential of C.S. PA.91-MgAC-NPs for nutrient removal in wastewater treatment plants and their quality as sources of biodiesel.

Growth and fatty acid distribution over lipid classes in Nannochloropsis oceanica acclimated to different temperatures

After light, temperature is the most relevant environmental parameter in outdoors cultivation of microalgae. Suboptimal and supraoptimal temperatures negatively impact growth and photosynthetic performance with a subsequent effect on lipid accumulation. It is generally recognised that lower temperatures trigger an increase in fatty acid desaturation while higher temperatures trigger the opposite reaction. The effect of temperature on lipid classes has been less studied in microalgae and in certain cases, the effect of light cannot be completely excluded. In this research, the effect of temperature on growth, photosynthesis, and lipid class accumulation in Nannochloropsis oceanica was studied at a fixed light gradient with a constant incident light intensity (670 μmol m^-2 s^-1). A turbidostat approach was used to achieve temperature acclimated cultures of Nannochloropsis oceanica. Optimal growth was found at 25-29°C, while growth was completely arrested at temperatures higher than 31°C and lower than 9°C. Acclimation to low temperatures triggered a decrease in absorption cross section and photosynthesis rates with a tipping point at 17°C. Reduced light absorption was correlated with a decrease in content of the plastid lipids monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol. The increase of diacylglyceryltrimethylhomo-serine content at lower temperatures indicated a relevant role of this lipid class in temperature tolerance. Triacylglycerol content increased at 17°C and decreased at 9°C emphasising a metabolic switch in stress response. Total and polar eicosapentaenoic acid content remained constant at 3.5 and 2.4% w/w, despite the fluctuating lipid contents. Results show an extensive mobilisation of eicosapentaenoic acid between polar lipids classes at 9°C to ensure cell survival under critical conditions.

A two-prong mutagenesis and adaptive evolution strategy to enhance the temperature tolerance and productivity of Nannochloropsis oculata

Incorporation of microalgae in biorefineries intended to help society reach carbon neutrality is hindered by algal growth inhibition at high temperatures, necessitating the use of costly and carbon-intensive cooling systems. In the present study, a two-prong strategy of random mutagenesis and adaptive laboratory evolution to generate robust thermotolerant strains of Nannochloropsis oculata, was used. The best mutants demonstrated increased productivity at 35 °C, which was 10 °C higher than the optimal temperature of the wild type. In a 2-L photobioreactor at 35 °C, biomass and lipid productivity were 1.43-fold and 2.24-fold higher, respectively, than wild type at 25 °C. Higher pigment and carbohydrate content contributed to the mutants' rapid growth and enhanced photosynthetic efficiency. Metabolomics and lipidomics showed rewiring of the central carbon metabolism and membrane lipid synthesis in thermotolerant strains to ensure cellular homeostasis without compromising productivity. Tagatose and phosphatidylethanolamine upregulation were identified as future genetic targets for further enhancing lipid production.

Diel biochemical and photosynthetic monitorization of Skeletonema costatum and Phaeodactylum tricornutum grown in outdoor pilot-scale flat panel photobioreactors

Diatoms are currently considered valuable feedstocks for different biotechnological applications. To deepen the knowledge on the production of these microalgae, the diel pattern of batch growth, photosystem II performance, and accumulation of target metabolites of two commercially relevant diatoms, Phaeodactylum tricornutum and Skeletonema costatum, were followed outdoors in 100-L flat panel photobioreactors. S. costatum presented a higher light-to-biomass conversion resulting in higher growth than P. tricornutum. Both fluorescence data and principal component analysis pointed to temperature as a limiting factor for the growth of P. tricornutum. Higher protein and carbohydrate contents were found in P. tricornutum, whereas S. costatum fatty acids were characterized by a higher unsaturation degree. Higher productivities were found at 1 p.m. for protein, lipid, and ash in the case of S. costatum. Overall, S. costatum showed great potential for outdoor cultivation, revealing a broader temperature tolerance and increased biomass productivity than P. tricornutum.

Characterization of Nannochloropsis oceanica Rose Bengal Mutants Sheds Light on Acclimation Mechanisms to High Light When Grown in Low Temperature

A barrier to realizing Nannochloropsis oceanica's potential for omega-3 eicosapentaenoic acid (EPA) production is the disparity between conditions that are optimal for growth and those that are optimal for EPA biomass content. A case in point is temperature: higher content of polyunsaturated fatty acid, and especially EPA, is observed in low-temperature (LT) environments, where growth rates are often inhibited. We hypothesized that mutant strains of N. oceanica resistant to the singlet-oxygen photosensitizer Rose Bengal (RB) would withstand the oxidative stress conditions that prevail in the combined stressful environment of high light (HL; 250 μmol photons m-2 s-1) and LT (18°C). This growth environment caused the wild-type (WT) strain to experience a spike in lipid peroxidation and an inability to proliferate, whereas growth and homeostatic reactive oxygen species levels were observed in the mutant strains. We suggest that the mutant strains' success in this environment can be attributed to their truncated photosystem II antennas and their increased ability to diffuse energy in those antennas as heat (non-photosynthetic quenching). As a result, the mutant strains produced upward of four times more EPA than the WT strain in this HL-LT environment. The major plastidial lipid monogalactosyldiacylglycerol was a likely target for oxidative damage, contributing to the photosynthetic inhibition of the WT strain. A mutation in the NO10G01010.1 gene, causing a subunit of the 2-oxoisovalerate dehydrogenase E1 protein to become non-functional, was determined to be the likely source of tolerance in the RB113 mutant strain.

Effect of nano-TiO2 on humic acid utilization from piggery biogas slurry by microalgae

Resource recovery from piggery biogas slurry has become an inevitable demand for sustainable development of pig industry. Microalgae show great potential in recovering nitrogen and phosphorus from piggery slurry, but struggle to utilize organic pollutants, as most of them are inert components (e.g., humic acids, HAs). In this study, nano-TiO₂ was used to enhance the utilization of HAs by microalgae from piggery biogas slurry. Results showed that the optimal conditions for microalgal growth and HAs removal by the microalgae-TiO₂ coupling system were TiO₂ dosage of 0.30 g/L, microalgal inoculation concentration of 0.40 g/L, light intensity of 360 µmol photon/(m²·s) and temperature of 30 °C, with the microalgal chlorophyll concentration of 6.51 mg/L and HAs removal efficiency of 50.14%. Analysis of HAs composition variations in the piggery biogas slurry indicated that the decrease of HAs was caused by their decomposition into small molecules under the photocatalytic reactions of TiO₂.

Enhancing microalgae growth and product accumulation with carbon source regulation: New perspective for the coordination between photosynthesis and aerobic respiration

The coordination between photosynthesis and aerobic respiration under mixotrophic cultivation can make a difference to the growth and biochemical composition of microalgae. However, the response of carbon metabolism to carbon source composition under mixotrophic microalgae cultivation has not been well studied. In this study, the synergistic effects of inorganic carbon (IC) and organic carbon (OC) supply on the growth and carbon metabolism of Chlorella vulgaris under mixotrophic cultivation were investigated. The increase of the proportion of HCO₃^- had a positive effect on the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), which promoted the biomass production and carbon fixing. The activity of citrate synthase was attenuated with the increase of IC/OC ratio, indicating that the energy needed for the biomass production in groups with high IC/OC ratio was contributed by photoreaction. Biochemical analysis showed that CO₃^2- was more efficient than HCO₃^- for carbohydrate and lipid accumulation of Chlorella vulgaris, and the highest amount of carbohydrate (30.2%) and lipid (35.8%) was recorded with the combined use of CO₃^2- and glucose. The results could provide a new perspective on carbon metabolism and enzyme regulation in mixotrophic microalgae cultivation.

In situ monitoring of chlorophyll a fluorescence in Nannochloropsis oceanica cultures to assess photochemical changes and the onset of lipid accumulation during nitrogen deprivation

In situ chlorophyll a fluorescence measurements were applied to monitor changes in the photochemical variables of Nannochloropsis oceanica cultures under nitrogen-deplete and nitrogen-replete (control) conditions. In addition, growth, lipid, fatty acid, and pigment contents were also followed. In the control culture, growth was promoted along with pigment content, electron transport rate (ETR), and polyunsaturated fatty acids, while total lipid content and fatty acid saturation level diminished. Under nitrogen-deplete conditions, the culture showed a higher de-epoxidation state of the xanthophyll cycle pigments. Fast transients revealed a poor processing efficiency for electron transfer beyond Q_A , which was in line with the low ETR due to nitrogen depletion. Lipid content and the de-epoxidation state were the first biochemical variables triggered by the change in nutrient status, which coincided with a 20% drop in the in situ effective quantum yield of PSII (ΔF'/F_m '), and a raise in the V_j measurements. A good correlation was found between the changes in ΔF'/F_m ' and lipid content (r = -0.96, p < 0.01). The results confirm the reliability and applicability of in situ fluorescence measurements to monitor lipid induction in N. oceanica.

Variables Governing Photosynthesis and Growth in Microalgae Mass Cultures

Since the 1950s, microalgae have been grown commercially in man-made cultivation units and used for biomass production as a source of food and feed supplements, pharmaceuticals, cosmetics and lately biofuels, as well as a means for wastewater treatment and mitigation of atmospheric CO2 build-up. In this work, photosynthesis and growth affecting variables—light intensity, pH, CO2/O2 exchange, nutrient supply, culture turbulence, light/dark cell cycling, biomass density and culture depth (light path)—are reviewed as concerns in microalgae mass cultures. Various photosynthesis monitoring techniques were employed to study photosynthetic performance to optimize the growth of microalgae strains in outdoor cultivation units. The most operative and reliable techniques appeared to be fast-response ones based on chlorophyll fluorescence and oxygen production monitoring, which provide analogous results.

Operation Regimes: A Comparison Based on Nannochloropsis oceanica Biomass and Lipid Productivity

Microalgae are currently considered to be a promising feedstock for biodiesel production. However, significant research efforts are crucial to improve the current biomass and lipid productivities under real outdoor production conditions. In this context, batch, continuous and semi-continuous operation regimes were compared during the Spring/Summer seasons in 2.6 m3 tubular photobioreactors to select the most suitable one for the production of the oleaginous microalga Nannochloropsis oceanica. Results obtained revealed that N. oceanica grown using the semi-continuous and continuous operation regimes enabled a 1.5-fold increase in biomass volumetric productivity compared to that cultivated in batch. The lipid productivity was 1.7-fold higher under semi-continuous cultivation than that under a batch operation regime. On the other hand, the semi-continuous and continuous operation regimes spent nearly the double amount of water compared to that of the batch regime. Interestingly, the biochemical profile of produced biomass using the different operation regimes was not affected regarding the contents of proteins, lipids and fatty acids. Overall, these results show that the semi-continuous operation regime is more suitable for the outdoor production of N. oceanica, significantly improving the biomass and lipid productivities at large-scale, which is a crucial factor for biodiesel production.