Responses of the Microalga Chlorophyta sp. to Bacterial Quorum Sensing Molecules (N-Acylhomoserine Lactones): Aromatic Protein-Induced Self-Aggregation

Omics approaches for microalgal applications: Prospects and challenges

In recent impetus of phycological research, microalgae have emerged as a potential candidate for various arena of application-driven research. Omics-based tactics are used for disentangling the regulation and network integration for biosynthesis/degradation of metabolic precursors, intermediates, end products, and identifying the networks that regulate the metabolic flux. Multi-omics coupled with data analytics have facilitated understanding of biological processes and allow ample access to diverse metabolic pathways utilized for genetic manipulations making microalgal factories more efficient. The present review discusses state-of-art "Algomics" and the prospect of microalgae and their role in symbiotic association by using omics approaches including genomics, transcriptomics, proteomics and metabolomics. Microalgal based uni- and multi-omics approaches are critically analyzed in wastewater treatment, metal toxicity and remediation, biofuel production, and therapeutics to provide an imminent outlook for an array of environmentally sustainable and economically viable microalgal applications.

Microalgal interstrains differences in algal-bacterial biofloc formation during liquid digestate treatment

In this study, the effect of microalgal strains on the formation of algal-bacterial biofloc was investigated in liquid digestate pretreated by a sequencing batch reactor (SBR), which loaded much aerobic bacteria from activated sludge. Six microalgal strains resulted in three cases: no-bioflocculation (Scenedesmus obliquus and Botryococcus braunii), optimal-bioflocculation with high flocculation activity and good growth (Chlorella sp. BWY-1, Haematococcus pluvialis and Dictyosphaerium ehnenbergianum) and over-bioflocculation with high flocculation activity and bad growth (Chlorella vulgaris). Chlorella sp. BWY-1 provided a better level of flocculation activity and growth. Polysaccharides and proteins were present in EPS of algal-bacterial biofloc, and their distribution was confirmed by staining with alcian blue and fluorescein isothiocyanate (FITC).

Hormesis effects of phosphorus on the viability of Chlorella regularis cells under nitrogen limitation

BACKGROUND

Phosphorus (P) is an essential element of microalgae, which is either required for anabolism or for energy metabolism. When employing a nitrogen limitation strategy to trigger microalgal intracellular lipid accumulation, P supplementation was always simultaneously applied to compensate for the accompanied growth inhibition.

RESULTS

This study identified that P exerts hormesis effects on microalgae. Slight excess of P (≤ 45 mg L^-1) under nitrogen limitation condition stimulated the cell growth of Chlorella regularis and achieved a 10.2% biomass production increase. This also improved mitochondrial activity by 25.0% compared to control (P = 5.4 mg L^-1). The lipid productivity reached 354.38 mg (L d)^-1, which increased by 39.3% compared to control. Such an improvement was caused by the intracellularly stored polyphosphate energy pool. However, large excess of P (250 mg L^-1) inhibited the cell growth by 38.8% and mitochondrial activity decreased by 71.3%. C. regularis cells showed obvious poisoning status, such as enlarged size, plasmolysis, deformation of cell walls, and disorganization of organelles. This is probably because the over-accumulated P protonated the amide-N and disrupted membrane permeability.

CONCLUSIONS

These results provide new insight into the roles of P in microalgae lipid production: P does not always play a positive role under nitrogen limitation conditions.

Stability and performance of algal-bacterial granular sludge in shaking photo-sequencing batch reactors with special focus on phosphorus accumulation

The granular stability, nutrients removal and phosphorus (P) accumulation of algal-bacterial aerobic granular sludge (AGS) was examined by using shaking photoreactors (at a fixed light/dark cycle of 12 h/12 h). During the 25 days' operation, algal-bacterial AGS possessed good granular integrity (8.4 ± 0.6%), and excellent removals of dissolved organic carbon (94.8 ± 1.6%) and total nitrogen (71.1 ± 3.3%). More extracellular proteins (153.7 ± 2.3 mg/g) were excreted from the granules with a high proteins/polysaccharides ratio (∼7.4) on day 25, especially the tightly bound proteins mainly responsible for granular stability. Decrease in P content, especially non-apatite inorganic P relating to Fe-PO₄ precipitates, was detected in the granules to some extent, although 54.8 ± 17.1% of total P removal was achieved during the light-on cycles. Still, high P bioavailability (92.0%) was kept in the algal-bacterial AGS throughout the test period. Further optimization of light-on/light-off cycle and hydraulic/sludge retention time is demanding for better and stable P accumulation in the algal-bacterial granules with high bioavailability.

Advances in the technologies for studying consortia of bacteria and cyanobacteria/microalgae in wastewaters

The excessive generation and discharge of wastewaters have been serious concerns worldwide in the recent past. From an environmental friendly perspective, bacteria, cyanobacteria and microalgae, and the consortia have been largely considered for biological treatment of wastewaters. For efficient use of bacteria‒cyanobacteria/microalgae consortia in wastewater treatment, detailed knowledge on their structure, behavior and interaction is essential. In this direction, specific analytical tools and techniques play a significant role in studying these consortia. This review presents a critical perspective on physical, biochemical and molecular techniques such as microscopy, flow cytometry with cell sorting, nanoSIMS and omics approaches used for systematic investigations of the structure and function, particularly nutrient removal potential of bacteria‒cyanobacteria/microalgae consortia. In particular, the use of specific molecular techniques of genomics, transcriptomics, proteomics metabolomics and genetic engineering to develop more stable consortia of bacteria and cyanobacteria/microalgae with their improved biotechnological capabilities in wastewater treatment has been highlighted.

Effect of light intensity on the characteristics of algal-bacterial granular sludge and the role of N-acyl-homoserine lactone in the granulation

The effects of light intensity on the development of algal-bacterial granular sludge (ABGS) were investigated over a period of 12 weeks. The ABGS developed at low light intensity (142 ± 10 μmol m^-2·s^-1) exhibited excellent settling ability (SVI₃₀ of 30.9 mL/g), COD and TN removal efficiencies (97.6% and 60.4%, respectively). High light intensity (316 ± 12 μmol m^-2·s^-1) accelerated granular biomass growth (5.3 g/L) and enhanced the TP removal efficiency (83.7%). Extracellular polymeric substance (EPS) analysis revealed that low light intensity induced more large weight distribution protein production (9-12 kDa and 50-150 kDa), predominantly tryptophan and aromatic proteins. Furthermore, N-acyl-homoserine lactones (AHLs) with a side chain ≤ C₁₀ were commonly shared in the ABGS, and the ABGS developed at low light intensity had a higher C₆- and 3OC₈-HSL content, which effectively promoted the biofilm formation. The add-back studies showed that the AHLs facilitated the regulation of EPS synthesis. Statistical analysis indicated that the AHLs content had a close correlation with the EPS production, the 50th percentile of the particle size distribution and microbial community assembly, suggesting that AHLs-mediated quorum sensing have an important ecological role in EPS expression and algal-bacterial granulation. Overall, this study describes the ABGS development at different light intensities and the mechanisms of ABGS formation treating synthetic domestic wastewater.

Hydrodynamic shear force shaped the microbial community and function in the aerobic granular sequencing batch reactors for low carbon to nitrogen (C/N) municipal wastewater treatment

The lab-scale aerobic granules process was applied for low carbon to nitrogen (C/N < 4) wastewater treatment under different hydrodynamic shear forces. Results revealed that aerobic granules exhibited strong adaptability and stability. The aerobic granules might adopt an extracellular polymeric substances (EPS) regulating mechanism to address the changes in operational conditions, especially through growing secretion of fluorescence protein. The hydrodynamic shear force determinedly shaped and regulated the diversity and structure of dominant microbial community, briefly, reduced aeration intensity with increased time led to higher microbial richness, lower diversity and evenness, and shifts of predominant microorganisms. Phylogenetic classification of the key functional groups including bacteria related to carbon and nutrients removal, EPS production and quorum sensing (QS) presented much more differences among the reactors subject to different conditions. Therefore, the present work adds insight into the comprehensive understanding of the effect of aeration induced hydrodynamic shear force on aerobic granules.

Establishment of stable microalgal-bacterial consortium in liquid digestate for nutrient removal and biomass accumulation

In this study, a microalgal-bacterial consortium (MBC) was established in liquid digestate (LD) by optimizing sequencing batch reactor (SBR) operating parameters and microalgae inoculation to address the abovementioned challenges. The bacteria from LD SBR-Activated Sludge System effluent under the optimum conditions of 25 °C, 7.0 g/L MLSS, 5 mg/L DO concentration, and 6 h hydraulic retention time with 0.5 mg/L DW Chlorella sp. BWY-1 could form stable MBCs outdoors in an airlift photoreactor. The stable MBC facilitates the continuous removal of nitrogen and phosphorus, promotes the accumulation of biomass and lipids, and contributes to the improvement of the sedimentation. The results from this study provided a new technique for the purification and utilization of LD, more importantly decreasing the environmental threat caused by improperly processed LD.

Natural sunlight induced rapid formation of water-born algal-bacterial granules in an aerobic bacterial granular photo-sequencing batch reactor

Wastewater treatment by means of algal-bacterial granules has become a hot topic worldwide recently. Rapid granulation of algal-bacterial granules was achieved in an aerobic bacterial granular sequencing batch reactor (SBR) under natural sunlight exposure. Occurrence of abundant filamentous bacteria bridging the water-born algae, and overproduction of extracellular polymeric substances (EPS) (especially polysaccharides (PS), tryptophan & protein-like, and humic acid-like substances) were observed on the first 3 days, while the algae grew into the inner side of the granules and mature granules were obtained on day 7. The growth of the water-born algae slightly decreased the settleability, mean sizes of the granules, but stimulated the bioactivity significantly. Whereas, the biomass retention decreased before day 3, and got stable soon with the maturation period with symbiotic growth of algal-bacterial biomass. Illumina results revealed that the introduction of algae reduced the richness and diversity of bacterial community. Besides, few changes in structure and some compositions shifts in bacterial communities were found, while the predominant algae shifted from Diatomea to green algae Chlorophyceae. The possible mechanism for natural sunlight induced granulation of algal-bacterial granules was thus proposed based on the interactions between algae and bacteria.

Quorum sensing molecules in activated sludge could trigger microalgae lipid synthesis

Cultivating microalgae using wastewater is an economical strategy to produce biofuel; however, microbial contamination has to be controlled strictly. Microalgae lipid accumulation can be triggered by environmental pressures, and here, we studied whether microbial contamination is the pressure for microalgae. We hypothesized this pressure was forced via cell-to-cell communication with quorum sensing molecules (QSMs). In this work, we verified the impacts of QSMs produced by activated sludge (wastewater-born microbial consortiums) on both lipid content and biomass production of the microalgae Chlorophyta sp., since in combination, they determined lipid productivity. With QSMs stress, the lipid content of Chlorophyta sp. increased by ∼84%, while biomass production decreased only slightly. Consistently, enzymes on the fatty acid synthesis pathways were generally up-regulated, while they were slightly down-regulated for DNA replication. In summary, the total lipid production improved by 86%. These results revealed the positive effects of microbial contamination on microalgae biofuel production.

Altered Microbiome Leads to Significant Phenotypic and Transcriptomic Differences in a Lipid Accumulating Chlorophyte

Given the challenges facing the economically favorable production of products from microalgae, understanding factors that might impact productivity rates including growth rates and accumulation of desired products, for example, triacylglycerols (TAG) for biodiesel feedstock, remains critical. Although operational parameters such as media composition and reactor design can clearly effect growth rates, the role of microbe-microbe interactions is just beginning to be elucidated. In this study an oleaginous marine algae Chlorella spp. C596 culture is shown to be better described as a microbial community. Perturbations to this microbial community showed a significant impact on phenotypes including sustained differences in growth rate and TAG accumulation of 2.4 and 2.5 fold, respectively. Characterization of the associated community using Illumina 16S rRNA amplicon and random shotgun transcriptomic analyses showed that the fast growth rate correlated with two specific bacterial species ( Ruegeria and Rhodobacter spp). The transcriptomic response of the Chlorella species revealed that the slower growing algal consortium C596-S1 upregulated genes associated with photosynthesis and resource scavenging and decreased the expression of genes associated with transcription and translation relative to the initial C596-R1. Our studies advance the appreciation of the effects microbiomes can have on algal growth in bioreactors and suggest that symbiotic interactions are involved in a range of critical processes including nitrogen, carbon cycling, and oxidative stress.