Diversity and ecological function of urease-producing bacteria in the cultivation environment of Gracilariopsis lemaneiformis.

Urease-producing bacteria (UPB) provide inorganic nitrogen for primary producers by hydrolyzing urea, and play an important role in marine nitrogen cycle. However, there is still an incomplete understanding of UPB and their ecological functions in the cultivation environment of the red macroalgae Gracilariopsis lemaneiformis. This study comprehensively analyzed the diversity of culturable UPB and explored their effects on urea uptake by G. lemaneiformis. A total of 34 isolates belonging to four main bacterial phyla i.e. (Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria) were identified through 16S rRNA sequencing and were screened for UPB by urea agar chromogenic medium assay and ureC gene cloning. Our data revealed that only 8 strains contained urease. All of these UPB exhibited different urease activities, which were determined by the Berthelot reaction colorimetry assay. Additionally, the UPB strain (G13) isolated from G. lemaneiformis with higher urease activity was selected for co-culture with G. lemaneiformis to explore its role in promoting or inhibiting nitrogen uptake by macroalgae. The results showed a significant increase in urea consumption in the culture medium and the total cellular nitrogen in G. lemaneiformis in the UPB-co culture group compared to the sterile group. This suggests that the selected UPB strain positively influences nitrogen uptake by G. lemaneiformis. Similarly, isotopic assays revealed that the δ15N content of G. lemaneiformis was significantly higher in the UPB-co culture than in the control group, where δ15N-urea was the only nitrogen source in the culture medium. This indicates that the UPB helped G. lemaneiformis to absorb more nitrogen from urea. Moreover, the highest content of δ15N was found in G. lemaneiformis with epiphytic bacteria compared to sterilized (i.e. control), showing that epiphytic bacteria, along with UPB, have a compound effect in helping G. lemaneiformis absorb more nitrogen from urea. Taken together, these results provide unique insight into the ecological role of UPB and suggest that urease from macroalgae environment-associated bacteria might be an important player in marine nitrogen cycling.

Enhanced Carotenoid Production in Chlamydomonas reinhardtii by Overexpression of Endogenousand Exogenous Beta-Carotene Ketolase (BKT) Genes.

Chlamydomonas reinhardtii is a unicellular green alga that can grow heterotrophically by using acetate as a carbon source. Carotenoids are natural pigments with biological activity and color, which have functions such as antioxidant, anti-inflammatory, vision protection, etc., and have high commercial value and prospects. We transformed Chlamydomonas reinhardtii with the BKT genes from Phaffia rhodozyma (PrBKT) and Chlamydomonas reinhardtii (CrBKT) via plasmid vector, and screened out the stable transformed algal strains C18 and P1. Under the condition that the cell density of growth was not affected, the total carotenoid content of C18 and P1 was 2.13-fold and 2.20-fold higher than that of the WT, respectively. CrBKT increased the levels of ß-carotene and astaxanthin by 1.84-fold and 1.21-fold, respectively, while PrBKT increased them by 1.11-fold and 1.27-fold, respectively. Transcriptome and metabolome analysis of C18 and P1 showed that the overexpression of CrBKT only up-regulated the transcription level of BKT and LCYE (the gene of lycopene e-cyclase). However, in P1, overexpression of PrBKT also led to the up-regulation of ZDS (the gene of ζ-carotene desaturase) and CHYB (the gene of ß-carotene hydroxylase). Metabolome results showed that the relative content of canthaxanthin, an intermediate metabolite of astaxanthin synthesis in C18 and P1, decreased. The overall results indicate that there is a structural difference between CrBKT and PrBKT, and overexpression of PrBKT in Chlamydomonas reinhardtii seems to cause more genes in carotenoid pathway metabolism to be up-regulated.

Industry chain and challenges of microalgal food industry-a review.

Currently, the whole world is facing hunger due to the increase in the global population and the rising level of food consumption. Unfortunately, the impact of environmental, climate, and political issues on agriculture has resulted in limited global food resources. Thus, it is important to develop new food sources that are environmentally friendly and not subject to climate or space limitations. Microalgae represent a potential source of nutrients and bioactive components for a wide range of high-value products. Advances in cultivation and genetic engineering techniques provide prospective approaches to widen their application for food. However, there are currently problems in the microalgae food industry in terms of assessing nutritional value, selecting processes for microalgae culture, obtaining suitable commercial strains of microalgae, etc. Additionally, the limitations of real data of market opportunities for microalgae make it difficult to assess their actual potential and to develop a better industrial chain. This review addresses the current status of the microalgae food industry, the process of commercializing microalgae food and breeding methods. Current research progress in addressing the limitations of microalgae industrialization and future prospects for developing microalgae food products are discussed.

Transcriptome analysis reveals the molecular mechanism of differences in growth between photoautotrophy and heterotrophy in Chlamydomonas reinhardtii.

Background: The green alga Chlamydomonas reinhardtii can grow photoautotrophically utilizing light and CO2, and heterotrophically utilizing acetate. The physiological and biochemical responses of autotrophy and heterotrophy are different in C. reinhardtii. However, there is no complete understanding of the molecular physiology between autotrophy and heterotrophy. Therefore, we performed biochemical, molecular and transcriptome analysis of C. reinhardtii between autotrophy and heterotrophy. Results: The cell growth characterization demonstrated that heterotrophic cell had enhanced growth rates, and autotrophic cell accumulated more chlorophyll. The transcriptome data showed that a total of 2,970 differentially expressed genes (DEGs) were identified from photoautotrophy 12h (P12h) to heterotrophy 12h (H12h). The DEGs were involved in photosynthesis, the tricarboxylic acid cycle (TCA), pyruvate and oxidative phosphorylation metabolisms. Moreover, the results of qRT-PCR revealed that the relative expression levels of malate dehydrogenase (MDH), succinate dehydrogenase (SDH), ATP synthase (ATPase), and starch synthase (SSS) were increased significantly from P12h and H12h. The protein activity of NAD-malate dehydrogenase (NAD-MDH) and succinate dehydrogenase (SDH) were significantly higher in the H12h group. Conclusion: The above results indicated that the high growth rate observed in heterotrophic cell may be the effects of environmental or genetic regulation of photosynthesis. Therefore, the identification of novel candidate genes in heterotrophy will contribute to the development of microalga strains with higher growth capacity and better performance for biomass production.

Unraveling the Diverse Profile of N-Acyl Homoserine Lactone Signals and Their Role in the Regulation of Biofilm Formation in Porphyra haitanensis-Associated Pseudoalteromonas galatheae.

N-acyl homoserine lactones (AHLs) are small, diffusible chemical signal molecules that serve as social interaction tools for bacteria, enabling them to synchronize their collective actions in a density-dependent manner through quorum sensing (QS). The QS activity from epiphytic bacteria of the red macroalgae Porphyra haitanensis, along with its involvement in biofilm formation and regulation, remains unexplored in prior scientific inquiries. Therefore, this study explores the AHL signal molecules produced by epiphytic bacteria. The bacterium isolated from the surface of P. haitanensis was identified as Pseudoalteromonas galatheae by 16s rRNA gene sequencing and screened for AHLs using two AHL reporter strains, Agrobacterium tumefaciens A136 and Chromobacterium violaceum CV026. The crystal violet assay was used for the biofilm-forming phenotype. The inferences revealed that P. galatheae produces four different types of AHL molecules, i.e., C4-HSL, C8-HSL, C18-HSL, and 3-oxo-C16-HSL, and it was observed that its biofilm formation phenotype is regulated by QS molecules. This is the first study providing insights into the QS activity, diverse AHL profile, and regulatory mechanisms that govern the biofilm formation phenotype of P. galatheae. These findings offer valuable insights for future investigations exploring the role of AHL producing epiphytes and biofilms in the life cycle of P. haitanensis.

Environmental factors shape the epiphytic bacterial communities of Gracilariopsis lemaneiformis.

Macroalgae host various symbionts on their surface, which play a critical role in their growth and development processes. However, there is still incomplete understanding of this epiphytic bacteria-host algae interactions. This study comprehensively analysed variation of the epiphytic bacterial communities (EBC) composition of red macroalga Gracilariopsis lemaneiformis at different geographic locations and environmental factors (i.e., nitrogen and phosphorus), which shape the EBC composition of G. lemaneiformis. The composition and structure of EBC were characterized using high throughput sequencing of the V3-V4 hypervariable region of the 16S rRNA gene. The results revealed that epiphytic bacteria varied significantly among three different geographic locations in China, i.e., Nan'ao Island (NA), Lianjiang County (LJ), and Nanri Island (NR). Redundancy analysis (RDA) showed that the relative abundance of Bacteroidetes, Firmicutes, Verrucomicrobia, and Epsilonbacteraeota at NR were strongly positively correlated with total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NO3-N), and dissolved inorganic nitrogen (DIN), but negatively correlated with nitrite nitrogen (NO2-N). The relative abundance of Cyanobacteria at NA and LJ were strongly positively correlated with NO2-N, but negatively correlated with TN, TP, NO3-N, and DIN. Besides, the Mantel test results indicated that the EBC composition was significantly correlated with these environmental factors, which was also confirmed by Spearman correlation analysis. Thus, environmental factors such as NO3-N and DIN play a key role in the community composition of epiphytic bacteria on G. lemaneiformis. This study provides important baseline knowledge on the community composition of epiphytic bacteria on G. lemaneiformis and shows correlation between different epiphytic bacteria and their surrounding environmental factors.

Proteome responses of Gracilaria lemaneiformis exposed to lead stress.

Proteome response of plants is an important process that enables them to cope with environmental stress including metal stress. In this study, the proteome of Gracilaria lemaneiformis exposed to lead was investigated. Two-dimensional gel electrophoresis analysis revealed 123 protein spots, among which 14 proteins were significantly differentially expressed and identified using MALDI-TOF MS. Two of the up-regulated proteins were identified and predicted to be involved in photosynthesis and signal transduction, while eleven down-regulated proteins were functionally grouped into five classes including photosynthesis, energy metabolism, protein metabolism, carbohydrate transport and metabolism, and antioxidation proteins. There was also an up-regulation in superoxide dismutase, peroxidase, glutathione s-transferase, and heat-shock protein 70 upon Pb exposure. Proteomic studies provide a better picture of protein networks and metabolic pathways primarily involved in intracellular detoxification and defense mechanisms.