Growth of phylogenetically diverse microalgae under Far-Red light enriched spectra: implication for space missions'...

Photosynthetic oxygenic microorganisms are recognized to be the functional core of Bioregenerative life support systems (BLSS) for sustaining long-term human presence in space. Among these organisms, microalgae are particularly promising due to their elevated growth rates, highly efficient light use, and ability to synthesize a plethora of valuable compounds. Their cultivation could lead to the sustainable production of food enriched in essential nutraceuticals while ensuring oxygen regeneration in confined environments. Understanding their responses to different light spectra is crucial for optimizing their cultivation in close systems and reducing the energetic costs. Considering the growing interest in the potential contribution of Far-Red (FR) light to the photosynthetic process and biomass production, we investigated the acclimations of different microalgal species under light spectra including this low-energy waveband in combination with visible (VIS) light.

Four phylogenetically diverse microalgal species, either capable (Chromera velia and Nannochloropsis gaditana) or incapable (Dixoniella grisea and Chlorella vulgaris) of utilizing solely FR light beside VIS radiation, were cultivated for 10 days under three light regimes: (i) simulated solar light (SOL); (ii) low-VIS, FR-enriched light (FR-e); (iii) FR light peaking at 730 nm (FR). All treatments were characterized by the same total photon flux, but differed in spectral distribution and total available energy. The culture growth during acclimation to different lights was assessed. At the beginning and at the end of experiments, samples were investigated through optical microscopy, spectrophotometry and HPLC techniques. Changes in cell morphology and photosynthetic apparatus in vivo absorption of cultures were analyzed. Photosynthetic pigments were characterized and quantified.

The results showed that microalgal growth under FR-e conditions was consistently higher than expected, being not proportional to the reduced fraction of available VIS photons (37.0% in FR-e vs. 84.3% in SOL). Specifically, growth under FR-e with respect to SOL, reached 61% in C. vulgaris, 82% in D. grisea, 91% in N. gaditana, and 93% in C. velia. Considering that the total energy of the FR-e spectrum is 83.5% of SOL (expressed in W m^-2) this light regime results particularly advantageous for the cultivation of FR-users microalgal species. The assessment of cell features and pigment composition showed similar characteristics of the produced biomass in SOL and FR-e for all tested species. Finally, the experiments also revealed a FR dose dependent photoacclimation in C. velia, whose FR light absorption capacity was enhanced at increasing FR doses.

The results demonstrate that the growth of diverse microalgal species is differently influenced by a low-VIS FR-enriched spectrum. For FR-user species under this light regime the energy costs can be reduced for the efficient production of biomass, while maintaining a comparable quality. The most promising species turned out to be N. gaditana, due to its high growth rate and the capability to use FR light beside the VIS one. These findings are crucial for a sustainable microalgal cultivation in the frame of BLSS.