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Flocculation is often regarded as a cost-effective and reliable method for microalgal harvesting. However, the traditional method often requires the addition of chemical agents to induce flocculation. This carries certain disadvantages including the chemical contamination of the biomass and the subsequent need to remove the flocculants from the medium. To address these issues, this study aimed to induce flocculation in a naturally non-flocculating strain (Chlorella vulgaris 13-1) without resorting to chemical additives, with the ultimate goal of increasing harvesting efficiency. Scanning electron microscopy showed that Scotelliopsis reticulata UFA-2, a naturally flocculating strain, produces extracellular polymeric substances (EPS) whereas 13-1 does not. As a result, two methods were used to induce flocculation in 13-1: co-cultivation of UFA-2 and 13-1, and insertion of EPS produced by UFA-2 into the growth medium of 13-1. The co-cultivation of 13-1 with UFA-2 significantly increased the flocculation efficiency compared to that of 13-1 alone (30 % higher flocculation efficiency after one hour of settling and 52 % higher after three hours of settling). Alternatively, the insertion of dry tightly-bound (TB) UFA-2 EPS into 13-1 cultures also improved flocculation efficiency (by 19 % compared to the control), while addition of soluble or loosely-bound (LB) EPS was less efficient (less than 1 % and 10 %, respectively). FTIR results showed that the composition of TB-EPS was different to that of LB and soluble EPS. TB-EPS contained higher proportions of proteins and different types of carbohydrates, potentially contributing to its increased efficacy in inducing flocculation in microalgae.

Metacaspases are cysteine proteases present in plants, fungi and protists. While the association of metacaspases with cell death is studied in a range of organisms, their native substrates are largely unknown. Here, we explored the in vivo proteolytic landscape of the two metacaspases, CrMCA-I and CrMCA-II, present in the green freshwater alga Chlamydomonas reinhardtii, using mass spectrometry-based degradomics approach, during control conditions and salt stress. Comparison between the cleavage events of CrMCA-I and CrMCA-II in metacaspase mutants revealed unique cleavage preferences and substrate specificity. Degradome analysis demonstrated the relevance of the predicted metacaspase substrates to the physiology of C. reinhardtii cells and its adaptation during salt stress. Functional enrichment analysis indicated an involvement of CrMCA-I in the catabolism of carboxylic acids, while CrMCA-II plays an important role in photosynthesis and translation. Altogether, our findings suggest distinct cellular functions of the two metacaspases in C. reinhardtii during salt stress response.

With climate change, droughts are expected to be more frequent and severe, severely impacting plant biomass and quality. Here, we show that overexpressing the Arabidopsis gene AtFtsHi3 (FtsHi3OE) enhances drought-tolerant phenotypes without compromising plant growth. AtFtsHi3 encodes a chloroplast envelope pseudo-protease; knock-down mutants (ftshi3-1) are found to be drought tolerant but exhibit stunted growth. Altered AtFtsHi3 expression therefore leads to drought tolerance, while only diminished expression of this gene leads to growth retardation. To understand the underlying mechanisms of the enhanced drought tolerance, we compared the proteomes of ftshi3-1 and pFtsHi3-FtsHi3OE (pFtsHi3-OE) to wild-type plants under well-watered and drought conditions. Drought-related processes like osmotic stress, water transport, and abscisic acid response were enriched in pFtsHi3-OE and ftshi3-1 mutants following their enhanced drought response compared to wild-type. The knock-down mutant ftshi3-1 showed an increased abundance of HSP90, HSP93, and TIC110 proteins, hinting at a potential downstream role of AtFtsHi3 in chloroplast pre-protein import. Mathematical modeling was performed to understand how variation in the transcript abundance of AtFtsHi3 can, on the one hand, lead to drought tolerance in both overexpression and knock-down lines, yet, on the other hand, affect plant growth so differently. The results led us to hypothesize that AtFtsHi3 may form complexes with at least two other protease subunits, either as homo- or heteromeric structures. Enriched amounts of AtFtsH7/9, AtFtsH11, AtFtsH12, and AtFtsHi4 in ftshi3-1 suggest a possible compensation mechanism for these proteases in the hexamer.

Anaerobic digestion is a promising method for organic waste treatment. While the obtained digestate can function as fertilizer, the liquid fraction produced is rather problematic to discharge due to its high nitrogen and chemical oxygen demand contents. Microalgae have great potential in sustainable nutrient removal from wastewater. This study aimed at evaluating native Swedish microalgae cultivation (batch operation mode, 25°C and continuous light of 80 μmol m−2 s−1) on anaerobic digestion effluent of pulp and paper sludge (PPS) or chicken manure (CKM) to remove ammonium and volatile fatty acids (VFAs). While algal strains, Chlorella vulgaris, Chlorococcum sp., Coelastrella sp., Scotiellopsis reticulata and Desmodesmus sp., could assimilate VFAs as carbon source, acetic acid was the most preferred. Higher algal biomass and cell densities were achieved using PPS compared to CKM. In PPS, Coelastrella sp. and Chlorella vulgaris reached the highest cell densities after 15 days, about 79 × 106 and 43 × 106 cells mL−1, respectively. Although in PPS, ammonium was completely assimilated (195 mg L−1), this was only 46% (172 mg L−1) in CKM. Coelastrella sp. produced the highest biomass concentration independently of the medium (1.84 g L−1 in PPS and 1.99 g L−1 in CKM). This strain is a promising candidate for nutrient removal and biomass production in the aforementioned media, followed by Chlorella vulgaris and Chlorococcum sp. They have great potential to reduce the environmental impact of industrial anaerobic digestion effluents in Nordic countries.

The treatment of organic waste using anaerobic digestion is a promising and well-matured organic waste management method. However, the effluent from anaerobic digestion has a significant discharge risk due to its high ammonium content. Microalgae could be a valuable solution to remove this nitrogen. This work aimed at evaluating the growth of three Nordic microalgae strains (Chlorella vulgaris, Chlorococcum sp. and Coelastrella sp.) in different concentrations of effluent from anaerobic digestion of municipal sewage sludge. None of the strains was able to grow in effluent diluted two times (X2) or three times (X3) due to the high ammonium content (600 and 400 mg L−1, respectively). While Chlorococcum sp. showed a lag phase of 7 and 11-days in 5 times (X5) and 7 times (X7) diluted effluent, respectively, this strain demonstrated 53 % and 86 % total ammonia nitrogen (TAN) removal efficiency after 15 days; in X10 its TAN removal was 100 %. Without any lag phase Coelastrella sp. showed the same TAN removal efficiencies in X5 and X7 as Chlorococcum sp. However, C. vulgaris had the highest TAN removal in X5 (90%) and X7 (90%). Furthermore, this strain showed the highest amount of biomass dry weight production in all media (1.1 g L−1 in X5). Therefore, C. vulgaris and Chlorococcum sp. are promising candidates for nitrogen removal and sustainable algae biomass production, resulting in mitigating the environmental issues of anaerobic digestion effluents in Nordic countries through the conversion of waste streams into resources.

Cold stress imposes a great physiological impact on most photosynthetic organisms. The acclimation to cold is very poorly studied in microalgae, even though understanding the molecular mechanisms underlying their cold tolerance has implications for biotechnological applications, such as the development of cold-tolerant strains for industrial purposes. Scenedesmus sp. B2-2 is a Nordic strain of green freshwater microalgae that thrives in cold conditions. Here, we analyzed transcriptomic changes of B2-2 when exposed to the cold (5°C) and compared it to a control grown at 25°C. The aim was to understand more about the mechanisms underlying B2-2's adaptation to low temperatures. We studied differentially expressed genes (DEGs) related to lipid synthesis, carbon metabolism and photosynthesis. Scenedesmus sp. B2-2 produced more lipids when exposed to cold conditions and did not reduce its carbon metabolism or photosynthetic processes. 24 putative cold-responsive genes were found to be non-responsive in Scenedesmus sp. B2-2 when grown at 5°C. These genes could serve as targets for genetic engineering to enhance cold tolerance in algal strains used in biotechnology. We also studied B2-2´s cell wall changes in response to cold by measuring cell wall thickness at 1, 4, 12, 24, 48, 72, 120 and 240 hours of cold exposure and correlating the results to the transcriptomic data. The results show that the cell wall thickens with increased duration of cold exposure. Several glycosyltransferases were found to be significantly up-regulated throughout cold exposure and may play a role in cell wall thickening.

Biomass from four different Nordic microalgal species, grown in BG-11 medium or synthetic wastewater (SWW), was explored as inexpensive carbohydrate-rich feedstock for polyhydroxybutyrate (PHB) production via microbial fermentation. Thermochemical pre-treatment (acid treatment followed by autoclavation) with 2% hydrochloric acid or 1% sulphuric acid (v/v) was used to maximize sugar yield prior to fermentation. Pre-treatment resulted in ∼5-fold higher sugar yield compared to the control. The sugar-rich hydrolysate was used as carbon source for the PHB-producing extremophilic bacterium Halomonas halophila. Maximal PHB production was achieved with hydrolysate of Chlorococcum sp. (MC-1) grown on BG-11 medium (0.27 ± 0.05 g PHB/ g DW), followed by hydrolysate derived from Desmodesmus sp. (RUC-2) grown on SWW (0.24 ± 0.05 g PHB/ g DW). Nordic microalgal biomass grown on wastewater therefore can be used as cheap feedstock for sustainable bioplastic production. This research highlights the potential of Nordic microalgae to develop a biobased economy.

In recent years, there has been an increasing concern related to the contamination of aqueous ecosystems by heavy metals, highlighting the need to improve the current techniques for remediation. This work intends to address the problem of removing heavy metals from waterbodies by combining two complementary methodologies: adsorption to a copolymer synthesized by inverse vulcanization of sulfur and vegetable oils and phytoremediation by the microalga Chlorella sorokiniana to enhance the metal adsorption. After studying the tolerance and growth of Chlorella sorokiniana in the presence of the copolymer, the adsorption of highly concentrated Cd2+ (50 mg L−1) by the copolymer and microalgae on their own and the combined immobilized system (AlgaPol) was compared. Additionally, adsorption studies have been performed on mixtures of the heavy metals Cd2+ and Cu2+ at a concentration of 8 mg L−1 each. AlgaPol biofilm is able to remove these metals from the growth medium by more than 90%. The excellent metal adsorption capacity of this biofilm can be kinetically described by a pseudo-second-order model.

Nordic microalgae are a group of photosynthetic organisms acclimated to growth at low temperature and in varying light conditions; the subarctic climate offers bright days with moderate temperatures during summer and cold and dark winter months. The robustness to these natural stress conditions makes the species interesting for large-scale cultivation in harsh environments and for the production of high-value compounds. The aim of this study was to explore the ability of nineteen species of Nordic microalgae to produce different bioactive compounds, such as carotenoids or polyphenols. The results showed that some of these strains are able to produce high amounts of carotenoids (over 12 mg·g-1 dry weight) and phenolic compounds (over 20 mg GAE·g-1 dry weight). Based on these profiles, six species were selected for cultivation under high light and cold stress (500 μmol·m-2·s-1 and 10 ˚C). The strains Chlorococcum sp. (MC1) and Scenedesmus sp. (B2–2) exhibited similar values of biomass productivity under standard or stress conditions, but produced higher concentrations of carotenoids (an increase of 40% and 25%, respectively), phenolic compounds (an increase of 40% and 30%, respectively), and showed higher antioxidant capacity (an increase of 15% and 20%, respectively) during stress. The results highlight the ability of these Nordic microalgae as outstanding producers of bioactive compounds, justifying their cultivation at large scale in Nordic environments.