Algal growth enhancement by bacteria: Is consumption of photosynthetic oxygen involved?

Microalgal co-cultivation -recent methods, trends in omic-studies, applications, and future challenges

The burgeoning human population has resulted in an augmented demand for raw materials and energy sources, which in turn has led to a deleterious environmental impact marked by elevated greenhouse gas (GHG) emissions, acidification of water bodies, and escalating global temperatures. Therefore, it is imperative that modern society develop sustainable technologies to avert future environmental degradation and generate alternative bioproduct-producing technologies. A promising approach to tackling this challenge involves utilizing natural microbial consortia or designing synthetic communities of microorganisms as a foundation to develop diverse and sustainable applications for bioproduct production, wastewater treatment, GHG emission reduction, energy crisis alleviation, and soil fertility enhancement. Microalgae, which are photosynthetic microorganisms that inhabit aquatic environments and exhibit a high capacity for CO2 fixation, are particularly appealing in this context. They can convert light energy and atmospheric CO2 or industrial flue gases into valuable biomass and organic chemicals, thereby contributing to GHG emission reduction. To date, most microalgae cultivation studies have focused on monoculture systems. However, maintaining a microalgae monoculture system can be challenging due to contamination by other microorganisms (e.g., yeasts, fungi, bacteria, and other microalgae species), which can lead to low productivity, culture collapse, and low-quality biomass. Co-culture systems, which produce robust microorganism consortia or communities, present a compelling strategy for addressing contamination problems. In recent years, research and development of innovative co-cultivation techniques have substantially increased. Nevertheless, many microalgae co-culturing technologies remain in the developmental phase and have yet to be scaled and commercialized. Accordingly, this review presents a thorough literature review of research conducted in the last few decades, exploring the advantages and disadvantages of microalgae co-cultivation systems that involve microalgae-bacteria, microalgae-fungi, and microalgae-microalgae/algae systems. The manuscript also addresses diverse uses of co-culture systems, and growing methods, and includes one of the most exciting research areas in co-culturing systems, which are omic studies that elucidate different interaction mechanisms among microbial communities. Finally, the manuscript discusses the economic viability, future challenges, and prospects of microalgal co-cultivation methods.

Co-culturing Chlorella vulgaris and Cystobasidium oligophagum JRC1 in the microbial fuel cell cathode for lipid biosynthesis

This study investigated the effect of co-culturing the photobiont and mycobiont in the microbial fuel cell (MFC) cathode on biomass production, lipid generation, and power output. Chlorella vulgaris provides oxygen and nutrients for the yeast Cystobasidium oligophagum JRC1, while the latter offers CO2 and quench oxygen for higher algal growth. The MFC with co-culture enhanced the lipid output of biomass by 28.33%, and the total yield and productivity were 1.47 ± 0.18 g/l and 0.123 g/l/day, respectively. Moreover, with co-culture, the open circuit voltage of 685 ± 11 mV was two times higher than algae alone. The specific growth rate (day-1) at the cathode was 0.367 ± 0.04 in co-culture and 0.288 ± 0.05 with C. vulgaris only. The power density of the system was 5.37 ± 0.21 mW/m2 with 75.88 ± 1.89% of COD removal. The co-culture thus proved beneficial at the MFC cathode in terms of total energy output as 11.5 ± 0.035 kWh/m3, which was 1.4-fold higher than algae alone.

Not dead yet: Diatom resting spores can survive in nature for several millennia

PREMISE

Understanding the adaptive capacities of species over long timescales lies in examining the revived recent and millennia-old resting spores buried in sediments. We show for the first time the revival, viability, and germination rate of resting spores of the diatom Chaetoceros deposited in sub-seafloor sediments from three ages (recent: 0 to 80 years; ancient: ~1250 (Medieval Climate Anomaly) and ~6600 (Holocene Thermal Maximum) calendar year before present.

METHODS

Recent and ancient Chaetoceros spores were revived to examine their viability and germination rate. Light and scanning electron microscopy and Sanger sequencing was done to identify the species.

RESULTS

We show that ~6600 cal. year BP old Chaetoceros resting spores are still viable and that the vegetative reproduction in recent and ancient resting spores varies. The time taken to germinate is three hours to 2 to 3 days in both recent and ancient spores, but the germination rate of the spores decreased with increasing age. The germination rate of the recent spores was ~41% while that of the ancient spores were ~31% and ~12% for the ~1250 and ~6600 cal. year BP old resting spores, respectively. Based on the morphology of the germinated vegetative cells we identified the species as Chaetoceros muelleri var. subsalsum. Sanger sequences of nuclear and chloroplast markers identified the species as Chaetoceros muelleri.

CONCLUSIONS

We identify a unique model system, Chaetoceros muelleri var. subsalsum and show that recent and ancient resting spores of the species buried in sediments in the Baltic Sea can be revived and used for long-term evolutionary studies.

Advances in the use of microalgal-bacterial consortia for wastewater treatment: Community structures, interactions, economic resource reclamation, and study techniques

The rise in living standards has generated a demand for higher aquatic environmental quality. The microalgal community and the surrounding organic molecules, environmental factors, and microorganisms, such as bacteria, are together defined as the phycosphere. The bacteria in the phycosphere can form consortia with microalgae through various forms of interaction. The study of the species in these consortia and their relative proportions is of great significance in determining the species and strains of stable algae that can be used in sewage treatment. This article summarizes the following topics: the interactions between microalgae and bacteria that are required to establish consortia; how symbiosis between algae and bacteria is established; microalgal competition with bacteria through inhibition and anti-inhibition strategies; the influence of environmental factors on microalgal-bacterial aggregates, such as illumination conditions, pH, dissolved oxygen, temperature, and nutrient levels; the application of algal-bacterial aggregates to enhance biomass production and nutrient reuse; and techniques for studying the community structure and interactions of algal-bacterial consortia, such as microscopy, flow cytometry, and omics. PRACTITIONER POINTS: Community structures in microalgal-bacterial consortia in wastewater treatment. Interactions between algae and bacteria in wastewater treatment. Effects of ecological factors on the algal-bacterial community in wastewater treatment. Economically recycling resources from algal-bacterial consortia based on wastewater. Technologies for studying microalgal-bacterial consortia in wastewater treatment.

Bio-purification of sugar industry wastewater and production of high-value industrial products with a zero-waste concept

In recent years, biorefinery approach with a zero-waste concept has gained a lot research impetus to boost the environment and bioeconomy in a sustainable manner. The wastewater from sugar industries contains miscellaneous compounds and need to be treated chemically or biologically before being discharged into water bodies. Efficient utilization of wastewater produced by sugar industries is a key point to improve its economy. Thus, interest in the sugar industry wastes has grown in both fundamental and applied research fields, over the years. Although, traditional methods being used to process such wastewaters are effective yet are tedious, laborious and time intensive. Considering the diverse nature of wastewaters from various sugar-manufacturing processes, the development of robust, cost-competitive, sustainable and clean technologies has become a challenging task. Under the recent scenario of cleaner production and consumption, the biorefinery and/or close-loop concept, though using different technologies and multi-step processes, namely, bio-reduction, bio-accumulation or biosorption using a variety of microbial strains, has stepped-up as the method of choice for a sustainable exploitation of a wide range of organic waste matter along with the production of high-value products of industrial interests. This review comprehensively describes the use of various microbial strains employed for eliminating the environmental pollutants from sugar industry wastewater. Moreover, the main research gaps are also critically discussed along with the prospects for the efficient purification of sugar industry wastewaters with the concomitant production of high-value products using a biorefinery approach. In this review, we emphasized that the biotransformation/biopurification of sugar industry waste into an array of value-added compounds such as succinic acid, L-arabinose, solvents, and xylitol is a need of hour and is futuristic approach toward achieving cleaner production and consumption.

Characterization of Chlorella vulgaris (Trebouxiophyceae) associated microbial communities1

Microalgae exhibit extensive potential for counteracting imminent challenges in the nutraceutical, pharmaceutical, and biomaterial sectors, but lack economic viability. Biotechnological systems for contamination control could advance the economic viability of microalgal feedstock, but the selection of suitable strains that sustainably promote microalgal productivity remains challenging. In this study, total diversity in phototrophic Chlorella vulgaris cultures was assessed by amplicon sequencing comparing cultures subjected to five different cultivation conditions. Overall, 12 eukaryotic and 53 prokaryotic taxa were identified; Alphaproteobacteria (36.7%) dominated the prokaryotic and C. vulgaris (97.2%) the eukaryotic community. Despite altering cultivation conditions, 2 eukaryotic and 40 prokaryotic taxa remained stably associated with C. vulgaris; diversity between systems did not significantly differ (P > 0.05). Among those, 20 cultivable taxa were isolated and identified by 16S rDNA sequencing. Subsequently, controlled co-cultures were investigated showing stable associations of C. vulgaris with Sphingopyxis sp. and Pseudomonas sp.. Out-competition of C. vulgaris due to ammonium or phosphate limitation was not observed, despite significantly elevated growth of Sphingopyxis sp. and Tistrella sp.. (P < 0.05). Nevertheless, C. vulgaris growth was impaired by Tistrella sp.. Hence, the study provides a selection of stable indigenous prokaryotes and eukaryotes for artificially tailoring microbial biocenoses. Following a bottom-up approach, it provides a base for controlled co-cultures and thus the establishment of even more complex biocenoses using interkingdom assemblages. Such assemblages can benefit from functional richness for improved nutrient utilization, as well as bacterial load control, which can enhance microalgal feedstock production through improved culture stability and productivity.

Attachment between heterotrophic bacteria and microalgae influences symbiotic microscale interactions

The surface and surroundings of microalgal cells (phycosphere) are critical interaction zones but have been difficult to functionally interrogate due to methodological limitations. We examined effects of phycosphere-associated bacteria for two biofuel-relevant microalgal species (Phaeodactylum tricornutum and Nannochloropsis salina) using stable isotope tracing and high spatial resolution mass spectrometry imaging (NanoSIMS) to quantify elemental exchanges at the single-cell level. Each algal species responded differently to bacterial attachment. In P. tricornutum, a high percentage of cells had attached bacteria (92%-98%, up to eight bacteria per alga) and fixed 64% more carbon with attached bacteria compared to axenic cells. In contrast, N. salina cells were less commonly associated with bacteria (42%-63%), harboured fewer bacteria per alga, and fixed 10% more carbon without attached bacteria compared to axenic cells. An uncultivated bacterium related to Haliscomenobacter sp. was identified as an effective mutualist; it increased carbon fixation when attached to P. tricornutum and incorporated 71% more algal-fixed carbon relative to other bacteria. Our results illustrate how phylogenetic identity and physical location of bacteria and algae facilitate diverse metabolic responses. Phycosphere-mediated, mutualistic chemical exchanges between autotrophs and heterotrophs may be a fruitful means to increase microalgal productivity for applied engineering efforts.

Transcriptional Activities of the Microbial Consortium Living with the Marine Nitrogen-Fixing Cyanobacterium Trichodesmium Reveal Potential Roles in Community-Level Nitrogen Cycling

Trichodesmium is a globally distributed cyanobacterium whose nitrogen-fixing capability fuels primary production in warm oligotrophic oceans. Like many photoautotrophs, Trichodesmium serves as a host to various other microorganisms, yet little is known about how this associated community modulates fluxes of environmentally relevant chemical species into and out of the supraorganismal structure. Here, we utilized metatranscriptomics to examine gene expression activities of microbial communities associated with Trichodesmium erythraeum (strain IMS101) using laboratory-maintained enrichment cultures that have previously been shown to harbor microbial communities similar to those of natural populations. In enrichments maintained under two distinct CO₂ concentrations for ∼8 years, the community transcriptional profiles were found to be specific to the treatment, demonstrating a restructuring of overall gene expression had occurred. Some of this restructuring involved significant increases in community respiration-related transcripts under elevated CO₂, potentially facilitating the corresponding measured increases in host nitrogen fixation rates. Particularly of note, in both treatments, community transcripts involved in the reduction of nitrate, nitrite, and nitrous oxide were detected, suggesting the associated organisms may play a role in colony-level nitrogen cycling. Lastly, a taxon-specific analysis revealed distinct ecological niches of consistently cooccurring major taxa that may enable, or even encourage, the stable cohabitation of a diverse community within Trichodesmium consortia.IMPORTANCETrichodesmium is a genus of globally distributed, nitrogen-fixing marine cyanobacteria. As a source of new nitrogen in otherwise nitrogen-deficient systems, these organisms help fuel carbon fixation carried out by other more abundant photoautotrophs and thereby have significant roles in global nitrogen and carbon cycling. Members of the Trichodesmium genus tend to form large macroscopic colonies that appear to perpetually host an association of diverse interacting microbes distinct from the surrounding seawater, potentially making the entire assemblage a unique miniature ecosystem. Since its first successful cultivation in the early 1990s, there have been questions about the potential interdependencies between Trichodesmium and its associated microbial community and whether the host's seemingly enigmatic nitrogen fixation schema somehow involved or benefited from its epibionts. Here, we revisit these old questions with new technology and investigate gene expression activities of microbial communities living in association with Trichodesmium.

Metagenomic analysis of the complex microbial consortium associated with cultures of the oil-rich alga Botryococcus braunii

Microalgae are widely viewed as a promising and sustainable source of renewable chemicals and biofuels. Botryococcus braunii synthesizes and secretes significant amounts of long-chain (C30 -C40 ) hydrocarbons that can be subsequently converted into gasoline, diesel, and aviation fuel. B. braunii cultures are not axenic and the effects of co-cultured microorganisms on B. braunii growth and hydrocarbon yield are important, but sometimes contradictory. To understand the composition of the B. braunii microbial consortium, we used high throughput Illumina sequencing of metagenomic DNA to profile the microbiota within a well established, stable B. braunii culture and characterized the demographic changes in the microcosm following modification to the culture conditions. DNA sequences attributed to B. braunii were present in equal quantities in all treatments, whereas sequences assigned to the associated microbial community were dramatically altered. Bacterial species least affected by treatments, and more robustly associated with the algal cells, included members of Rhizobiales, comprising Bradyrhizobium and Methylobacterium, and representatives of Dyadobacter, Achromobacter and Asticcacaulis. The presence of bacterial species identified by metagenomics was confirmed by additional 16S rDNA analysis of bacterial isolates. Our study demonstrates the advantages of high throughput sequencing and robust metagenomic analyses to define microcosms and further our understanding of microbial ecology.

Bioprospecting of microalgae for integrated biomass production and phytoremediation of unsterilized wastewater and anaerobic digestion centrate

Eighteen microalgae, including two local isolates, were evaluated for their ability to grow and remove nutrients from unsterilized primary or secondary wastewater effluents as well as wastewater supplemented with nutrient-rich anaerobic digester centrate (ADC). Most of the tested species except several phylogenetically clustered Chlorella sorokiniana including local isolates and Scenedesmus strains were unable to grow efficiently. This may reflect the presence of certain genetic traits important for robust growth in the unsterilized wastewater. The maximum algal-specific growth rates and biomass density obtained in these bacterial-contaminated cultures were in the range of 0.8-1 day(-1) and 250-350 mg L(-1), respectively. ADC supplementation was especially helpful to biologically treated secondary effluent with its lower initial macronutrient and micronutrient content. As a result of algal growth, total nitrogen and orthophosphate levels were reduced by as much as 90 and 70 %, respectively. Biological assimilation was estimated to be the main mechanism of nitrogen removal in primary and secondary effluents with ammonia volatilization and bacterial nitrification-denitrification contributing for cultures supplemented with ADC. Assimilation by algae served as the principal mechanism of orthophosphate remediation in secondary wastewater cultures, while chemical precipitation appeared also to be important for orthophosphate removal in primary wastewater. Overall, cultivation of microalgae in primary and primary + 5 % ADC may be more favorable from an economical and sustainability perspective due to elimination of the costly and energy-intensive biological treatment step. These findings demonstrate that unsterilized wastewater and ADC can serve as critical nutrient sources for biomass generation and that robust microalgae can be potent players in wastewater phytoremediation.