Algae biostimulants: A critical look at microalgal biostimulants for sustainable agricultural practices.

For the growing human population to be sustained during present climatic changes, enhanced quality and quantity of crops are essential to enable food security worldwide. The current consensus is that we need to make a transition from a petroleum-based to a bio-based economy via the development of a sustainable circular economy and biorefinery approaches. Both macroalgae (seaweeds) and microalgae have been long considered a rich source of plant biostimulants with an attractive business opportunity in agronomy and agro-industries. To date, macroalgae biostimulants have been well explored. In contrast, microalgal biostimulants whilst known to have positive effects on development, growth and yields of crops, their commercial implementation is constrained by lack of research and cost of production. The present review highlights the current knowledge on potential biostimulatory compounds, key sources and their quantitative information from algae. Specifically, we provide an overview on the prospects of microalgal biostimulants to advance crop production and quality. Key aspects such as specific biostimulant effects caused by extracts of microalgae, feasibility and potential of co-cultures and later co-application with other biostimulants/biofertilizers are highlighted. An overview of the current knowledge, recent advances and achievements on extraction techniques, application type, application timing, current market and regulatory aspects are also discussed. Moreover, aspects involved in circular economy and biorefinery approaches are also covered, such as: integration of waste resources and implementation of high-throughput phenotyping and -omics tools in isolating novel strains, exploring synergistic interactions and illustrating the underlying mode of microalgal biostimulant action. Overall, this review highlights the current and future potential of microalgal biostimulants, algal biochemical components behind these traits and finally bottlenecks and prospects involved in the successful commercialisation of microalgal biostimulants for sustainable agricultural practices.

Mycosporine-like amino acid and aromatic amino acid transcriptome response to UV and far-red light in the cyanobacterium Chlorogloeopsis fritschii PCC 6912.

The "UV sunscreen" compounds, the mycosporine-like amino acids (MAAs) are widely reported in cyanobacteria and are known to be induced under ultra-violet (UV) light. However, the impact of far red (FR) light on MAA biosynthesis has not been studied. We report results from two experiments measuring transcriptional regulation of MAA and aromatic amino acid pathways in the filamentous cyanobacterium Chlorogloeopsis fritschii PCC 6912. The first experiment, comparing UV with white light, shows the expected upregulation of the characteristic MAA mys gene cluster. The second experiment, comparing FR with white light, shows that three genes of the four mys gene cluster encoding up to mycosporine-glycine are also upregulated under FR light. This is a new discovery. We observed corresponding increases in MAAs under FR light using HPLC analysis. The tryptophan pathway was upregulated under UV, with no change under FR. The tyrosine and phenylalanine pathways were unaltered under both conditions. However, nitrate ABC transporter genes were upregulated under UV and FR light indicating increased nitrogen requirement under both light conditions. The discovery that MAAs are upregulated under FR light supports MAAs playing a role in photon dissipation and thermoregulation with a possible role in contributing to Earth surface temperature regulation.

Valorising nutrient-rich digestate: Dilution, settlement and membrane filtration processing for optimisation as a waste-based media for microalgal cultivation.

Digestate produced from the anaerobic digestion of food and farm waste is primarily returned to land as a biofertiliser for crops, with its potential to generate value through alternative processing methods at present under explored. In this work, valorisation of a digestate resulting from the treatment of kitchen and food waste was investigated, using dilution, settlement and membrane processing technology. Processed digestate was subsequently tested as a nutrient source for the cultivation of Chlorella vulgaris, up to pilot-scale (800L). Dilution of digestate down to 2.5% increased settlement rate and induced release of valuable compounds for fertiliser usage such as nitrogen and phosphorus. Settlement, as a partial processing of digestate offered a physical separation of liquid and solid fractions at a low cost. Membrane filtration demonstrated efficient segregation of nutrients, with micro-filtration recovering 92.38% of phosphorus and the combination of micro-filtration, ultra-filtration, and nano-filtration recovering a total of 94.35% of nitrogen from digestate. Nano-filtered and micro-filtered digestates at low concentrations were suitable substrates to support growth of Chlorella vulgaris. At pilot-scale, the microalgae grew successfully for 28 days with a maximum growth rate of 0.62 day-1 and dry weight of 0.86  g⋅L-1. Decline in culture growth beyond 28 days was presumably linked to ammonium and heavy metal accumulation in the cultivation medium. Processed digestate provided a suitable nutrient source for successful microalgal cultivation at pilot-scale, evidencing potential to convert excess nutrients into biomass, generating value from excess digestate and providing additional markets to the anaerobic digestion sector.

Synthesis, Regulation and Degradation of Carotenoids Under Low Level UV-B Radiation in the Filamentous Cyanobacterium Chlorogloeopsis fritschii PCC 6912.

Carotenoids in cyanobacteria play an important role in protecting against and in repairing damage against low level UV-B radiation. Here we use transcriptomics and metabolomic HPLC pigment analysis to compare carotenoid pathway regulation in the filamentous cyanobacterium Chlorogloeopsis fritschii PCC 6912 exposed to white light and to white light supplemented with low level UV-B. Under UV-B changes in carotenoid transcription regulation were found associated with carotenogenesis (carotenoid synthesis), photoprotection and carotenoid cleavage. Transcriptional regulation was reflected in corresponding pigment signatures. All carotenogenesis pathway genes from geranylgeranyl-diphosphate to lycopene were upregulated. There were significant increases in expression of gene homologs (crtW, crtR, cruF, and cruG) associated with routes to ketolation to produce significant increases in echinenone and canthaxanthin concentrations. There were gene homologs for four ß-carotene-ketolases (crtO and crtW) present but only one crtW was upregulated. Putative genes encoding enzymes (CruF, CrtR, and CruG) for the conversion of γ-carotene to myxol 2'-methylpentoside were upregulated. The hydroxylation pathway to nostaxanthin via zeaxanthin and caloxanthin (gene homologs for CrtR and CrtG) were not upregulated, reflected in the unchanged corresponding pigment concentrations in zeaxanthin, caloxanthin and nostaxanthin, Transcripts for the non-photochemical quenching related Orange-Carotenoid-Protein (OCP) and associated Fluoresence-Recovery-Protein (FRP) associated with photoprotection were upregulated, and one carotenoid binding Helical-Carotenoid-Protein (HCP) gene homolog was downregulated. Multiple copies of genes encoding putative apocarotenoid related carotenoid oxygenases responsible for carotenoid cleavage were identified, including an upregulated apo-ß-carotenal-oxygenase gene homologous to a retinal producing enzyme. Our study provides holistic insight into the photoregulatory processes that modulate the synthesis, photoprotection and cleavage of carotenoids in cyanobacterial cells exposed to low level UV-B. This is important to understanding how regulation of metabolism responds to a changing environment and how metabolism can be modulated for biotechnological purposes.

Pink- and orange-pigmented Planctomycetes produce saproxanthin-type carotenoids including a rare C45 carotenoid.

Planctomycetes are ubiquitous and environmentally important Gram-negative aquatic bacteria with key roles in global carbon and nitrogen cycles. Many planctomycetal species have a pink or orange colour and have been suggested to produce carotenoids. Potential applications as food colorants or anti-oxidants have been proposed. Hitherto, the planctomycetal metabolism is largely unexplored and the strain pigmentation has not been explored. For a holistic view of the complex planctomycetal physiology, we analysed carotenoid profiles of the pink-pigmented strain Rhodopirellula rubra LF2T and of the orange strain Rubinisphaera brasiliensis Gr7. During LC-MS/MS analysis of culture extracts, we could identify three saproxanthin-type carotenoids including a rare C45 carotenoid. These compounds, saproxanthin, dehydroflexixanthin and 2'-isopentenyldehydrosaproxanthin, derive from the common carotenoid precursor lycopene and are characterized by related end groups, namely a 3-hydroxylated ß-carotene-like cyclohexene ring as one end group and simple hydration on the other end of the molecule. Based on the observed molecule structure we present putative pathways for their biosynthesis. Results support Planctomycetes as a promising, yet mostly untapped source of carotenoids.

Far-Red Light Acclimation for Improved Mass Cultivation of Cyanobacteria.

Improving mass cultivation of cyanobacteria is a goal for industrial biotechnology. In this study, the mass cultivation of the thermophilic cyanobacterium Chlorogloeopsis fritschii was assessed for biomass production under light-emitting diode white light (LEDWL), far-red light (FRL), and combined white light and far-red light (WLFRL) adaptation. The induction of chl f was confirmed at 24 h after the transfer of culture from LEDWL to FRL. Using combined light (WLFRL), chl f, a, and d, maintained the same level of concentration in comparison to FRL conditions. However, phycocyanin and xanthophylls (echinone, caloxanthin, myxoxanthin, nostoxanthin) concentration increased 2.7-4.7 times compared to LEDWL conditions. The productivity of culture was double under WLFRL compared with LEDWL conditions. No significant changes in lipid, protein, and carbohydrate concentrations were found in the two different light conditions. The results are important for informing on optimum biomass cultivation of this species for biomass production and bioactive product development.

Light Intensity and Nitrogen Concentration Impact on the Biomass and Phycoerythrin Production by Porphyridium purpureum.

Several factors have the potential to influence microalgae growth. In the present study, nitrogen concentration and light intensity were evaluated in order to obtain high biomass production and high phycoerythrin accumulation from Porphyridium purpureum. The range of nitrogen concentrations evaluated in the culture medium was 0.075-0.450 g L-1 and light intensities ranged between 30 and 100 µmol m-2 s-1. Surprisingly, low nitrogen concentration and high light intensity resulted in high biomass yield and phycoerythrin accumulation. Thus, the best biomass productivity (0.386 g L-1 d-1) and biomass yield (5.403 g L-1) were achieved with NaNO3 at 0.075 g L-1 and 100 µmol m-2 s-1. In addition, phycoerythrin production was improved to obtain a concentration of 14.66 mg L-1 (2.71 mg g-1 of phycoerythrin over dry weight). The results of the present study indicate that it is possible to significantly improve biomass and pigment production in Porphyridium purpureum by limiting nitrogen concentration and light intensity.

Intracellular and Extracellular Metabolites from the Cyanobacterium Chlorogloeopsis fritschii, PCC 6912, During 48 Hours of UV-B Exposure.

Cyanobacteria have many defence strategies to overcome harmful ultraviolet (UV) stress including the production of secondary metabolites. Metabolomics can be used to investigate this altered metabolism via targeted and untargeted techniques. In this study we assessed the changes in the intra- and extracellular low molecular weight metabolite levels of Chlorogloeopsis fritschii (C. fritschii) during 48 h of photosynthetically active radiation (PAR) supplemented with UV-B (15 µmol m-2 s-1 of PAR plus 3 µmol m-2 s-1 of UV-B) and intracellular levels during 48 h of PAR only (15 µmol m-2 s-1) with sampling points at 0, 2, 6, 12, 24 and 48 h. Gas chromatography-mass spectrometry (GC-MS) was used as a metabolite profiling tool to investigate the global changes in metabolite levels. The UV-B time series experiment showed an overall significant reduction in intracellular metabolites involved with carbon and nitrogen metabolism such as the amino acids tyrosine and phenylalanine which have a role in secondary metabolite production. Significant accumulation of proline was observed with a potential role in stress mitigation as seen in other photosynthetic organisms. 12 commonly identified metabolites were measured in both UV-B exposed (PAR + UV-B) and PAR only experiments with differences in significance observed. Extracellular metabolites (PAR + UV-B) showed accumulation of sugars as seen in other cyanobacterial species as a stress response to UV-B. In conclusion, a snapshot of the metabolome of C. fritschii was measured. Little work has been undertaken on C. fritschii, a novel candidate for use in industrial biotechnology, with, to our knowledge, no previous literature on combined intra- and extracellular analysis during a UV-B treatment time-series. This study is important to build on experimental data already available for cyanobacteria and other photosynthetic organisms exposed to UV-B.

Modulation of Polar Lipid Profiles in Chlorella sp. in Response to Nutrient Limitation.

We evaluate the effects of nutrient limitation on cellular composition of polar lipid classes/species in Chlorella sp. using modern polar lipidomic profiling methods (liquid chromatography⁻tandem mass spectrometry; LC-MS/MS). Total polar lipid concentration was highest in nutrient-replete (HN) cultures with a significant reduction in monogalactosyldiacylglycerol (MGDG), phosphatidylglycerol (PG), phosphatidylcholine (PC), and phosphatidylethanolamine (PE) class concentrations for nutrient-deplete (LN) cultures. Moreover, reductions in the abundance of MGDG relative to total polar lipids versus an increase in the relative abundance of digalactosyldiacylglycerol (DGDG) were recorded in LN cultures. In HN cultures, polar lipid species composition remained relatively constant throughout culture with high degrees of unsaturation associated with acyl moieties. Conversely, in LN cultures lipid species composition shifted towards greater saturation of acyl moieties. Multivariate analyses revealed that changes in the abundance of a number of species contributed to the dissimilarity between LN and HN cultures but with dominant effects from certain species, e.g., reduction in MGDG 34:7 (18:3/16:4). Results demonstrate that Chlorella sp. significantly alters its polar lipidome in response to nutrient limitation, and this is discussed in terms of physiological significance and polar lipids production for applied microalgal production systems.

Using microalgae in the circular economy to valorise anaerobic digestate: challenges and opportunities.

Managing organic waste streams is a major challenge for the agricultural industry. Anaerobic digestion (AD) of organicwastes is a preferred option in the waste management hierarchy, as this processcangenerate renewableenergy, reduce emissions from wastestorage, andproduce fertiliser material.However, Nitrate Vulnerable Zone legislation and seasonal restrictions can limit the use of digestate on agricultural land. In this paper we demonstrate the potential of cultivating microalgae on digestate as a feedstock, either directlyafter dilution, or indirectlyfromeffluent remaining after biofertiliser extraction. Resultant microalgal biomass can then be used to produce livestock feed, biofuel or for higher value bio-products. The approach could mitigate for possible regional excesses, and substitute conventional high-impactproducts with bio-resources, enhancing sustainability withinacircular economy. Recycling nutrients from digestate with algal technology is at an early stage. We present and discuss challenges and opportunities associated with developing this new technology.

Comparing Nutrient Removal from Membrane Filtered and Unfiltered Domestic Wastewater Using Chlorella vulgaris.

The nutrient removal efficiency of Chlorella vulgaris cultivated in domestic wastewater was investigated, along with the potential to use membrane filtration as a pre-treatment tool during the wastewater treatment process. Chlorella vulgaris was batch cultivated for 12 days in a bubble column system with two different wastewater treatments. Maximum uptake of 94.18% ammonium (NH4-N) and 97.69% ortho-phosphate (PO4-P) occurred in 0.2 µm membrane filtered primary wastewater. Membrane filtration enhanced the nutrient uptake performance of C. vulgaris by removing bacteria, protozoa, colloidal particles and suspended solids, thereby improving light availability for photosynthesis. The results of this study suggest that growing C. vulgaris in nutrient rich membrane filtered wastewater provides an option for domestic wastewater treatment to improve the quality of the final effluent.

Distribution of mycosporine-like amino acids along a surface water meridional transect of the Atlantic.

The composition and abundance of mycosporine-like amino acids (MAAs) were investigated in the surface waters along a 13,000-km meridional transect (52° N to 45° S) in the Atlantic Ocean (Atlantic Meridional Transect programme: Cruise AMT 18: 4/10/2008-10/11/2008). MAAs were ubiquitous along the transect, although the composition of the MAAs was variable. Highest concentrations were in the far south (below 40° S; MAA >1 µg L(-1)) and in north subtropical equatorial region (NER: 0-25° N; MAA up to 0.8 µg L(-1)). Highest MAA relative to chlorophyll-a occurred in the NER (MAA/chl-a ratio between 2 and 5). MAA/chl-a significantly correlated with the preceding month's mean daily UV dose and with UV-B/UV-A. In the far south, high MAA concentrations coincided with high phytoplankton biomass, high nutrients and a deep mixed layer associated with the austral spring. Here, the phytoplankton community was dominated by micro- and nano-eukaryotes. At the NER, the high MAA/chl-a coincided with low nutrient concentrations, a shallow mixed layer depth (20-70 m) and to a lesser extent to a shallow nitracline (40-90 m). Here, the phytoplankton consisted primarily of picophytoplankton (0-0.2 µm), dominated by the pico-cyanobacteria Synechococcus sp. and Prochlorococcus sp. and by the nitrogen fixing filamentous cyanobacterium Trichodesmium. The low nitrate concentrations (<0.1 µmol L(-1)) at the NER suggest that nitrogen fixation was required for MAA production. Specific MAAs could not easily be assigned to particular groups of phytoplankton and we could not rule out the possibility that MAAs were associated with symbiotic cyanobacteria contained within heterotrophic dinoflagellates or diatoms.

The relevance of marine chemical ecology to plankton and ecosystem function: an emerging field.

Marine chemical ecology comprises the study of the production and interaction of bioactive molecules affecting organism behavior and function. Here we focus on bioactive compounds and interactions associated with phytoplankton, particularly bloom-forming diatoms, prymnesiophytes and dinoflagellates. Planktonic bioactive metabolites are structurally and functionally diverse and some may have multiple simultaneous functions including roles in chemical defense (antipredator, allelopathic and antibacterial compounds), and/or cell-to-cell signaling (e.g., polyunsaturated aldehydes (PUAs) of diatoms). Among inducible chemical defenses in response to grazing, there is high species-specific variability in the effects on grazers, ranging from severe physical incapacitation and/or death to no apparent physiological response, depending on predator susceptibility and detoxification capability. Most bioactive compounds are present in very low concentrations, in both the producing organism and the surrounding aqueous medium. Furthermore, bioactivity may be subject to synergistic interactions with other natural and anthropogenic environmental toxicants. Most, if not all phycotoxins are classic secondary metabolites, but many other bioactive metabolites are simple molecules derived from primary metabolism (e.g., PUAs in diatoms, dimethylsulfoniopropionate (DMSP) in prymnesiophytes). Producing cells do not seem to suffer physiological impact due to their synthesis. Functional genome sequence data and gene expression analysis will provide insights into regulatory and metabolic pathways in producer organisms, as well as identification of mechanisms of action in target organisms. Understanding chemical ecological responses to environmental triggers and chemically-mediated species interactions will help define crucial chemical and molecular processes that help maintain biodiversity and ecosystem functionality.