Synechocystis sp. PCC 6803, a model organism used for bioenergy and bioplastic production, was grown in continuous culture to assess its most important bioenergetic parameters. Biomass yield on light energy of 1.237 g mol photons−1 and maintenance energy requirement of 0.00312 mol photons g−1 h−1 were calculated. This corresponded to a light conversion efficiency of 12.5 %, based on the model of Pirt which was about 35 % lower than the theoretical one based on the stoichiometric equation for the formation of biomass on carbon dioxide, water, and nitrate. The maximum F v/F m ratio recorded in the Synechocystis cultures was 0.57; it progressively declined to 0.45 as the dilution rate increased. An over-reduction of reaction centers at a high dilution rate was also recorded, together with an increased VJ phase for the chlorophyll fluorescence transient. In contrast, the chlorophyll optical cross section increased by about 40 % at the fastest dilution rate, and compensated for the decline in F v/F m, thus resulting in a constant total photosynthesis rate (photosynthesis plus respiration). Chlorophyll content was maximum at the lowest dilution rate and was 48 % lower at the highest one, while phycocyanin, and total carotenoids decreased by about 42 % and 37 %, respectively. Carotenoid analysis revealed increased echinenone, zeaxanthin, and myxoxanthophyll contents as the dilution rate increased (40.6, 63.8 and 35.5 %, respectively, at the fastest dilution rate). A biochemical analysis of the biomass harvested at each different dilution rates showed no changes in the lipid content (averaging 11.2 ± 0.6 % of the dry weight), while the protein content decreased as the dilution rate increased, ranging between 60.7 ± 1.1 and 72.6 ± 0.6 %. Amino acids pattern did not vary with the dilution rate. Carbohydrate content ranged from 9.4 to 16.2 % with a mean value of 11.2 ± 1.4 %. The present work provides useful information on the threshold of light conversion efficiency in Synechocystis, as well as basic bioenergetic parameters that will be helpful for future studies related to its genetic transformation and metabolic network reconstruction.

Microalgae cultures were used for a WW treatment to remediate nutrients while producing biomass and recycling water. In these trials, raceway ponds (RWPs; 1 and 0.5 ha) were located next to a municipal (WW) treatment plant in Mérida, Spain. The ponds were used for continuous, all-year-round microalgae production using WW as a source of nutrients. Neither CO₂ nor air was supplied to cultures. The objective was to validate photosynthesis monitoring techniques in large-scale bioreactors. Various in-situ/ex-situ methods based on chlorophyll fluorescence and oxygen evolution measurements were used to follow culture performance. Photosynthesis variables gathered with these techniques were compared to the physiological behavior and growth of cultures. Good photosynthetic activity was indicated by the build-up of dissolved oxygen concentration up to 380% saturation, high photochemical yield (Fv/Fm = 0.62–0.71), and relative electron transport rate rETR between 200 and 450 μmol e⁻ m⁻² s⁻¹ at midday, which resulted in biomass productivity of about 15–25 g DW m⁻² day⁻¹. The variables represent reliable markers reflecting the physiological status of microalgae cultures. Using waste nutrients, the biomass production cost can be significantly decreased for abundant biomass production in large-scale bioreactors, which can be exploited for agricultural purposes.

Arthrospira platensis and Arthrospira maxima are prokaryotic microalgae commercially marketed as spirulina. The pigments extracted from these algae are widely used for cosmetic and nutraceutical applications. This work aimed to evaluate the influence of three light-emitting lamps (white, orange and blue) on the growth and biomass composition of two strains of A. platensis (M2 and M2M) and one of A. maxima. The obtained data show strain- and light-dependent responses of the microalgae. In addition, white and orange lights led to a similar overall effect by increasing the levels of chlorophyll a and carotenoids. However, exposure to orange light resulted in the highest dry weight (5973.3 mg L⁻¹ in M2M), whereas white light stimulated an increase in the carbohydrate fraction (up to 42.36 g 100 g⁻¹ in A. maxima). Conversely, blue light led to a constant increase in the concentration of phycocyanin (14 g 100 g⁻¹ in A. maxima) and a higher content of proteins in all strains. These results provide important environmental information for modulating the growth of different spirulina strains, which can be used to address the synthesis of biochemical compounds of strategic importance for the development of new nutraceutical foods.

The microalga Scenedesmus sp. (Chlorophyceae) was cultured in a raceway pond (RWP) placed in a greenhouse. The objective of this case study was to monitor the photosynthesis performance and selected physicochemical variables (irradiance, temperature, dissolved oxygen concentration) of microalgae cultures in situ at various depths of RWP. The data of actual photochemical yield Y(II), the electron transport rate monitored by in vivo chlorophyll fluorescence and photosynthetic oxygen production both in situ and ex situ revealed that (i) even in diluted cultures (0.6 g DW L⁻¹), the active photic layer in the culture was only about 1 cm, indicating that most of the culture was “photosynthetically” inactive; (ii) the mechanism of non-photochemical quenching may not be fast enough to respond once the cells move from the surface to the deeper layers; and (iii) even when cells were exposed to a high dissolved oxygen concentration of about 200% sat and higher, the cultures retained a relatively high Y(II) > 0.35 when monitored in situ. The presented work can be used as exemplary data to optimize the growth regime of microalgae cultures in large-scale RWPs by understanding the interplay between photosynthetic activity, culture depth and cell concentration.

Urban environments are contaminated by a multitude of air pollutants. Tropospheric ozone (O₃), nitrogen dioxide (NO₂) as well as coarse particulate matter (PM₁₀) and fine particulate matter (PM_2.5) are the most dangerous for human health. However, urban greenery, in particular trees, offer a variety of ecosystem services, including the ability to improve air quality. We planted 170 young trees in the city of Florence using five species with proven capabilities to remove air pollutants, and open-field research was conducted to assess their pollution removal potential. Multi-sensor monitoring devices were used to monitor air pollutant concentrations and meteorological parameters from the first three years after planting. The devices were installed inside/outside the plantation and above/below the canopies. The experiment showed that the selection of suitable species effectively led to an improvement in air quality, with a reduction in monitored air pollutants below the canopy. In detail, a reduction in O₃ and NO₂ was detected for the second (2023) and third (2024) growing seasons, while a reduction in PM₁₀ was only observed in 2024. The highest average reduction percentage was found for O₃ (−9.1%) and PM₁₀ (−24.5%) during 2023 and 2024, respectively. These findings highlight that nature-based solutions are really effective in air pollution mitigation, suggesting their implementation to further expand urban reforestation programmes and preserve human health.

Light and salt stress affect the growth of plants and microorganisms, causing photo-oxidative stress. The cyanobacterium Synechocystis is notable for its adaptability to and sustainability in seawater. In this study, the synergistic effects of different light intensities and salt concentrations on the growth and biomass composition of Synechocystis were examined. Cultures were grown in BG11 medium (control) and with 20 and 40 g L⁻¹ marine salts (obtained from a commercial sea water preparation) at 100, 200, and 400 μmoles photons m⁻² s⁻¹ (LL, ML, and HL, respectively) to assess the interactive effects of salinity stress and increasing light intensity. The effect of salinity stress was most pronounced under LL and ML, where the highest accumulation of all major carotenoids was observed; under HL, the contents of most carotenoids significantly increased mainly at the highest salt concentration but to a lesser extent). Under LL and ML echinenone reached the highest values (2.71-fold and 3.75-fold higher than in the control, respectively), whereas β-carotene showed the highest increase at LL, reaching concentrations three times those of the control. At HL myxoxanthophyll exhibited the highest increase with marine salt (1.9-fold higher than in the control). The results show that Synechocystis could grow at all light intensities and marine salt concentrations via increased synthesis of carotenoids in response to physiological stress.