Highly saline conditions represent a strong challenge for most microorganisms in freshwater ecosystems. Eukaryotic freshwater green algae from the Chlorophyta clade were investigated for their ability to survive in and adapt to increased salt concentration in the growth medium. Striking differences were detected between the responses of the various algae species to the elevated salt concentrations. The investigated Chlamydomonas reinhardtii cc124 and Coelastrella sp. MACC-549 algae showed a moderate resistance to increased salt concentration, while Chlorella sp. MACC-360 exhibited high salt tolerance, showed unaltered growth characteristics and photosynthetic efficiency compared to the saline-free control conditions even at 600 mM NaCl concentration. Diverse physiological responses to elevated salt concentrations were described for the tested algae including variations in their growth capacity, characteristic morphological changes, alterations in the structure and function of the photosynthetic machinery and differences in the production of reactive oxygen species. Special alterations were identified in the lipid and exopolysaccharide production patterns of the tested algal strains in response to high salinity. As a conclusion Chlorella sp. MACC-360 algae showed outstanding salt tolerance features. Together with the concomitant lipid-producing phenotype under highly saline conditions this unicellular green alga is a promising candidate for biotechnological applications.

The nuclear, chloroplast and mitochondrial genomes of a unicellular green algal species of the Coelastrella genus was sequenced, assembled and annotated. The strain was previously classified as Chlamydomonas sp. MACC-549 based on morphology and partial 18S rDNA analysis. However, the proposed multi-loci phylogenomic approach described in this paper placed this strain within the Coelastrella genus, therefore it was re-named to Coelastrella vacuolata MACC-549. The strain was selected for de novo sequencing based on its potential value in biohydrogen production as revealed in earlier studies. This is the first thorough report and characterization for green algae from the Coelastrella genus. The whole genome annotation of Coelastrella vacuolata MACC-549 (including nuclear, chloroplast and mitochondrial genomes) shed light on interesting metabolic and sexual breeding features of this algae and served as a basis to taxonomically classify this strain.

Algal-bacterial co-cultures represent an alternative way for algal biohydrogen generation. Efficient algal hydrogen production requires anaerobiosis and electrons accessible for the algal FeFe‑hydrogenases. A number of factors strongly influence the development of this optimal environment. Various algal strains were tested for hydrogen evolution with a selected bacterial partner, a fully hydrogenase deficient Escherichia coli. During the hunt for the most efficient algae strains, gas-to-liquid phase ratio, algal optical density and algal cell size were identified as crucial factors influencing algal hydrogen evolution rate, accumulated algal hydrogen yield, carbon dioxide and oxygen levels as well as acetic acid consumption in illuminated algal-bacterial cultures. The highest accumulated hydrogen yields were observed for the different algal partners under similar experimental setup. The combination of a gas-to-liquid phase ratio of 1/1 with an algae cell density of 3.96 ∗ 10⁸ algae cell ml^{− 1} (OD750: 1) resulted in the highest accumulated algal hydrogen yields under continuous illumination of ~ 50 μmol m^{− 2} s^{− 1} light at 25 °C irrespective of the applied algae strain. Accumulated hydrogen yield was also strongly influenced by the algal cell size, smaller cell size correlated with higher hydrogen evolution rate. The highest accumulated algal hydrogen yield (88.98 ± 2.19 ml H2 l^{− 1} d^{− 1}) was obtained with Chlorella sp. MACC 360 -E. coli ΔhypF co-culture.

Plant biostimulatory effects of the green alga Chlorella sp. MACC-360, the Kocuria rhizophila FSP120 bacterial strain, and the combined inter-kingdom co-culture of the alga and bacterium were investigated using Solanum lycopersicum as a model plant grown under controlled greenhouse conditions. The application of algal–bacterial co-cultures using the soil drench method significantly improved plant growth parameters, vegetative biomass yield, fruit yield, and photosynthetic performance of the tomato plants. The combined treatment resulted in a 43.7% increase in mean fruit yield, while individual applications of K. rhizophila FSP120 and Chlorella sp. MACC-360 enhanced yields by 30.85% and 19.44%, respectively. Although total yield increases did not reach statistical significance due to high intra-group variability, the treatment’s efficacy was statistically confirmed through key yield parameters including significantly higher fruit weight and fruit diameter (p < 0.05). The enhanced specific biostimulatory effects of the combined treatment could be at least partly attributed to the increased level of algal extracellular polymeric substances (EPS), which was a specific effect of algal co-cultivation with a Kocuria rhizophila bacterium. Detailed analysis of plant phenotypic alterations, biomass yield, fruit and flowering parameters, as well as microbial community analysis of the rhizosphere, were conducted and compared among the various treatments. Our results indicate that an appropriately chosen combination and application of biostimulatory microbes can significantly enhance crop production, which might contribute to more sustainable agriculture.