Effects of fluctuating temperature and silicate supply on the growth, biochemical composition and lipid accumulation of Nitzschia sp

Copper and zinc interact significantly in their joint toxicity to Chlamydomonas reinhardtii: Insights from physiological and transcriptomic investigations

Copper (Cu) and zinc (Zn) often discharge simultaneously from industrial and agricultural sectors and cause stress to aquatic biota. Although microalgae have been extensively investigated for their responses to Cu or Zn exposure, how they cope with the mixtures of two metals, especially at transcriptomic level, remains largely unknown. In this study, Chlamydomonas reinhardtii was exposed to environmentally relevant concentrations of two metals. It was found that Zn promoted the entry of Cu into the algal cells. With the increase of combined toxicity, extracellular polymeric substances (EPS) and cell wall functional groups immobilized significant amounts of Cu and Zn. Furthermore, C. reinhardtii adjusted resistance strategies internally, including starch consumption and synthesis of chlorophyll and lipids. Upon high level of Cu and Zn coexistence, synergistic effects were observed in lipid peroxidation and catalase (CAT) activity. Under 1.05 mg/L Cu + 0.87 mg/L Zn, 256 differentially expressed genes (DEGs) were mainly involved in oxidative phosphorylation, ribosome, nitrogen metabolism; while 4294 DEGs induced by 4.21 mg/L Cu + 3.48 mg/L Zn were mainly related to photosynthesis, citric acid cycle, etc. Together, this study revealed a more comprehensive understanding of mechanisms of Cu/Zn detoxification in C. reinhardtii, emphasizing critical roles of photosynthetic carbon sequestration and energy metabolism in the metal resistance.

Production of lipids and proteins from marine diatoms under changing pH and silica

Diatom algae are increasingly explored as an alternative sustainable source for functional biomolecules likes fucoxanthin, and eicosapentaenoic acid. But biomolecule quantity and quantity are influenced by growth conditions. So, effect of differential silica concentration (0-120 mg L^-1) and medium pH (5.5-9.5) on growth and cellular biochemical composition of commercially important marine diatom species were studied. Growth rate of Thalassiosira sp., Skeletonema sp., and Chaetoceros sp., was higher with 30 mg L^-1 Si at a pH of 7.5-8.5. Highest carbohydrate (153.71 mg g^-1) and protein (17.34 mg g^-1) content was found in Skeletonema sp. Silica concentration positively influenced chlorophyll and carotenoid content in a dose dependent manner. A medium pH of 8.5 and Si concentration between 60 and 120 mg L^-1 was ideal for lipid production. The optimum concentration of Si and pH for maximum biomolecule production have been reported with further scope of utilizing these conditions in commercial scale systems.

Evaluation of co-culturing a diatom and a coccolithophore using different silicate concentrations

Globally, the demand for sustainable energy production and high-value biological compounds have become intertwined in an attempt to improve the feasibility of sustainable algal cultivation. Marine microalgae, especially diatoms and coccolithophores, represent viable cultures that can produce biofuels and high-value compounds. Growing them in co-culture offers the potential to produce lipids and pigments, while also generating CaCO₃ for C sequestration. The main objective of this work was to investigate competition or co-existence of the diatom Chaetoceros gracilis and the coccolithophore Pleurochrysis Carterae. The focus was on the effects of silicate and co-culturing on the growth rate, productivity, pigment production, and ash production for C. gracilis and P. carterae in laboratory conditions. The results showed that, in monoculture, 2-mM Si enhanced the specific growth rate of C. gracilis, but did not affect P. carterae. Regardless of silicate concentration, C. gracilis was more productive than P. carterae. In co-culture, P. carterae had a slower growth rate, indicating an inhibitory effect of C. gracilis on P. carterae. Neither silicate concentration nor co-culturing had an impact on the contents of pigments fucoxanthin, chlorophyll-a, and chlorophyll-c, which means that pigment productivity was proportional to biomass productivity. Finally, the ash content increased in all cultures with the lower silicate concentration (0.2 mM) in the medium. With one exception, the ash content was dominated by SiO₂ regardless of silicate amount, and CaCO₃ was a major part of the ash only when P. carterae was grown separately with the higher silicate level. These results highlight that co-culturing did not provide an advantage for improving biomass, pigments, or CaCO₃ productivity.

Effects of fluctuating temperature in open raceway ponds on the biomass accumulation and harvest efficiency of Spirulina in large-scale cultivation

It is of great significance to select strains with wide adaptability to temperature range for large-scale commercial cultivation of Spirulina. The aim of this study was to comprehend how the strain H-208 grew and whether this strain had any advantages in temperature adaptation compared with local production strain during the large-scale cultivation in Inner Mongolia. The results showed that the strain H-208 could adapt to the new environmental condition quickly, and the daily average biomass dry weight of strain H-208 was 49% and 52% more than that of production strain M-1 in first cycle (20.24 g/m2/day) and second cycle (16.90 g/m2/day) of acclimation experiment, respectively. The growth rate of strain H-208 was 0.055 and 0.066 g/L/day from July 22 to July 25 and from July 26 to July 29, respectively, while the growth rate of strain M-1 was only 0.036 and 0.032 g/L/day, respectively, during the same cultured days in 605-m2 raceway ponds before high temperature. The harvesting efficiency of H-208 and M-1 was 95.1% and 72.1% before high temperature, and that was 95.3% and 52.5% after being stressed by high temperature, respectively. Meanwhile, it was also observed that the filaments of the two strains contracted and their pitches were smaller than that before high temperature stress, especially the strain M-1. In 20-m2 raceway ponds of recovery experiment after high temperature, the percentage of daily average biomass dry weight of strain H-208 was 68% more than that of strain M-1, which demonstrated that strain H-208 could recover and grow rapidly, and its self-regulation ability was superior to that of strain M-1.

Temperature-Dependent Lipid Accumulation in the Polar Marine Microalga Chlamydomonas malina RCC2488

The exploration of cold-adapted microalgae offers a wide range of biotechnological applications that can be used for human, animal, and environmental benefits in colder climates. Previously, when the polar marine microalga Chlamydomonas malina RCC2488 was cultivated under both nitrogen replete and depleted conditions at 8°C, it accumulated lipids and carbohydrates (up to 32 and 49%, respectively), while protein synthesis decreased (up to 15%). We hypothesized that the cultivation temperature had a more significant impact on lipid accumulation than the nitrogen availability in C. malina. Lipid accumulation was tested at three different temperatures, 4, 8, and 15°C, under nitrogen replete and depleted conditions. At 4°C under the nitrogen replete condition C. malina had the maximal biomass productivity (701.6 mg L-1 day-1). At this condition, protein content was higher than lipids and carbohydrates. The lipid fraction was mainly composed of polyunsaturated fatty acids (PUFA) in the polar lipid portion, achieving the highest PUFA productivity (122.5 mg L-1 day-1). At this temperature, under nitrogen deficiency, the accumulation of carbohydrates and neutral lipids was stimulated. At 8 and 15°C, under both nitrogen replete and depleted conditions, the lipid and carbohydrate content were higher than at 4°C, and the nitrogen stress condition did not affect the algal biochemical composition. These results suggest that C. malina is a polar marine microalga with a favorable growth temperature at 4°C and is stressed at temperatures ≥8°C, which directs the metabolism to the synthesis of lipids and carbohydrates. Nevertheless, C. malina RCC2488 is a microalga suitable for PUFA production at low temperatures with biomass productivities comparable with mesophilic strains.

Screening of High Temperature-Tolerant Oleaginous Diatoms

Screening suitable strains with high temperature adaptability is of great importance for reducing the cost of temperature control in microalgae cultivation, especially in summer. To obtain high temperature-tolerant diatoms, water samples were collected in summer from 7 different regions of China across the Northeast, North and East. A total of 731 water samples was collected and from them 131 diatom strains were isolated and identified based on the 18S rRNA sequences. Forty-nine strains out of the 131 diatoms could survive at 30°C, and 6 strains with relatively high biomass and lipid content at high temperature were selected and were found to be able to grow at 35°C. Cyclotella sp. HB162 had the highest dry biomass of 0.46 g/l and relatively high triacylglycerol (TAG) content of 237.4 mg/g dry biomass. The highest TAG content of 246.4 mg/g dry biomass was obtained in Fistulifera sp. HB236, while Nitzschia palea HB170 had high dry biomass (0.33 g/l) but relatively low TAG content (105.9 mg/g dry biomass). N. palea HB170 and Fistulifera sp. HB236 presented relatively stable growth rates and lipid yields under fluctuating temperatures ranging from 28 to 35°C, while Cyclotella HB162 maintained high lipid yield at temperatures below 25°C. The percentage of saturated fatty acids and monounsaturated fatty acids in all the 6 strains was 84-91% in total lipids and 90-94% in TAGs, which makes them the ideal feedstock for biodiesel.

Effect of culture condition on the growth, biochemical composition and EPA production of alkaliphilic Nitzschia plea isolated in the Southeast of China

To overcome the contamination in open pond, microalgal strain selection should focus on species with tolerability to extreme environments. In this study, a native alkaliphilic algae, diatom Nitzschia plea was obtained in Southeast of China, which could tolerate high concentration of NaHCO₃ (0.15 mol/L) and high pH (> 10). The effects of initial pH, light intensity and temperature on cell growth, biochemical composition and fatty acid profile of N. plea were investigated. Results indicated its specific growth rate could reach 1.2 day^-1, lipid content was in the range 14.6-30.2% of dry weight, eicosapntemacnioc acid (EPA, C20:5) accounted for around 15% of total fatty acids. Alkalic condition benefited for both cell growth and EPA synthesis. Appropriately increasing light intensity and temperature could improve cell growth rate and lipid synthesis, although the proportion of EPA in total fatty acids decreased slightly. The optimal culture condition (pH 9.00, temperature 35.0 °C, light intensity 158.6 µmol/m²s) was suggested for maximum yield of EPA based on the response surface model. The overall biomass productivity and EPA productivity were 0.301 g/L/day and 7.43 mg/L/day, respectively. In conclusion, alkalic environment was helpful for the steady operation of open pond cultivation of N. plea with the characteristics of fast growth rate and high EPA content, which exhibited its commercial value.