Damian Laird

Alexandrium spp. blooms produce a range of toxins, including spirolides, goniodomins, and paralytic shellfish toxins (PSTs). Of these, PSTs are the most impactful due to their high affinity for voltage-gated sodium ion channels in nerve cell membranes. This interaction can cause neurological effects such as paralysis and, in severe cases, may lead to death. Given the implications of Alexandrium blooms on public health, all mitigation, prevention, and treatment strategies aim to reduce their socioeconomic impacts. However, monitoring harmful algal blooms remains difficult due to confounding influences such as pollution, climate change, and the inherent variability of environmental conditions. These factors can complicate early detection and management efforts, especially as the intensity and frequency of blooms continue to rise, further exacerbating their socioeconomic consequences. This review offers insights into several management approaches to prevent and control Alexandrium blooms, focusing on modified nano-clays as a promising emergency mitigation measure for low-density toxic algal blooms, especially in areas predominantly used for recreational fishing. However, it is recommended that treatment be coupled with monitoring to alleviate reliance on treatment alone.

Key Contribution
Prevention and control strategies for Alexandrium blooms should consider the specific use of the affected waterway, the carbon footprint associated with modified clay production, and the broader ecological and environmental impacts of clay application. These factors are essential when evaluating the overall efficiency and sustainability of clay-based treatment methods.

Alexandrium spp. blooms and paralytic shellfish poisoning pose serious economic threats to coastal communities and aquaculture. This study evaluated the removal efficiency of two Alexandrium minutum strains using natural kaolinite clay (KNAC) and kaolinite with polyaluminum chloride (KPAC) at three concentrations (0.1, 0.25, and 0.3 g L−1), two pH levels (7 and 8), and two cell densities (1.0 and 2.0 × 107 cells L−1) in seawater. PAC significantly enhanced removal, achieving up to 100% efficiency within two hours. Zeta potential analysis showed that PAC imparted positive surface charges to the clay, promoting electrostatic interactions with negatively charged algal cells and enhancing flocculation through Van der Waals attractions. In addition, the study conducted a cost estimate analysis and found that treating one hectare at 0.1 g L−1 would cost approximately USD 31.75. The low KPAC application rate also suggests minimal environmental impact on benthic habitats.

Microalgae could serve as an improved source of bioavailable heme b for the treatment of chronic anemia in humans and animals and/or be used as an additive for plant-based protein products to provide a ‘meaty’ taste. Unfortunately, ‘standard’ spectrophotometric assays developed for heme b -rich samples, particularly meat and blood samples, are not viable for microalgae due to lower heme b levels and spectral interference from pigments such as chlorophyll. Removing interferents in heme b extracts from photosynthetic organisms is time-consuming and risks loss of product. Analysis of 6 different microalgal strains using a spectrophotometer and literature best-practice HPLC method revealed that HPLC was consistently more sensitive and had a 1000-fold lower LoD compared to the spectrophotometric method (0.1 pmol cf 230 pmol). The HPLC method also offered the advantage of not having to remove pigment interferences in the extraction phase. The instrumental method comparison highlighted the inefficiencies in the traditional sequential acetone extraction process, prompting trials of several alternate extraction protocols. It was found that a single-step acidic N,N -dimethylformamide (DMF: HCl, 98:2 v/v) extraction improved heme b yield by ~ 50%, reduced extraction time and solvent use by ~ 90%, and extracted heme b remained stable for two weeks at -25 °C, enabling batch analysis. Coupling this extraction procedure with HPLC analysis provides a robust analytical tool for advancing the evaluation of microalgae-derived heme b in medicinal and functional food settings.

Heme is an iron-coordinated tetrapyrrole molecule that plays an indispensable role in various biological processes such as respiration, oxygen metabolism, oxygen transfer, photosynthetic electron transportation and oxidative stress responses. Heme is also the primary source of functional, bioavailable iron in the human body. Iron deficiency results in a range of acute and chronic human health consequences. Iron deficiency anemia affects approximately 25 % of the global human population. New sources of bioavailable heme are required to help combat this ubiquitous mineral deficiency. Microalgae do biosynthesize heme but our knowledge of the regulation of those pathways, what the actual yields of heme may be, and how heme yields may be improved is restricted to a few model species. In this review we explore the potential of microalgae as an alternative source of bioavailable heme, suggest some strategies that may be pursued, and highlight some of the challenges that still need to be overcome. Developing heme enriched microalgae could transform our access to bioavailable iron through direct use as a dietary supplement and/or incorporation into functional foods.

Ruminant farming represents the largest anthropogenic source of methane, a greenhouse gas with ~30 times the global warming potential of carbon dioxide. To reduce enteric methane production in ruminants, macroalgae, such as Asparagopsis (Rhodophyta), have been trialed as feed additives. In vivo trials have reported up to a 98 % reduction in methane from cattle, when Asparagopsis taxiformis is included in their feed at 0.2 % organic matter. These observations have been widely attributed to the high bromoform content of Asparagopsis. However, inclusion of Asparagopsis has also been shown to adversely impact production of volatile fatty acids and feed intake. There are inconsistent results reported in the literature surrounding the effects of bromoform-containing seaweeds on ruminants, with the cause of rumen modification from in vivo trials not fully elucidated. On top of this, there are current difficulties surrounding the cultivation and distribution of Asparagopsis, both presenting barriers to its implementation on large scales. Further, detailed studies are required to implement this technology in a manner that is safe and effective for both animals and humans. This review aims to provide an overview of the current scientific standings of this technology and summarises the areas which require further investigation.

Microalgae can treat waste streams containing elevated levels of organic carbon and nitrogen. This process can be economically attractive if high value products are created simultaneously from the relatively low-cost waste stream. Co-production of two high value microalgal products, phycocyanin and polyhydroxybutyrate (PHB), was investigated using non-axenic Arthrospira platensis MUR126 and supplemental organic carbon (acetate, oxalate, glycerol and combinations). All supplemented cultures had higher biomass yield (g/L) than photoautotrophic control. All cultures produced PHB (3.6–7.8% w/w), except the control and those fed oxalate. Supplemented cultures showed a two to three-fold increase in phycocyanin content over the eight-day cultivation. Results indicate co-production of phycocyanin and PHB is possible in A. platensis, using mixed-waste organic carbon. However, supplementation resulted in growth of extremophile bacteria, particularly in cultures fed glycerol, and this had a negative impact on culture health. Refinement of the carbon dosing rate is required to minimise impacts of native bacterial contamination.

Although Medicago sativa forms highly effective symbioses with the comparatively acid-sensitive genus Ensifer, its introduction into acid soils appears to have selected for symbiotic interactions with acid-tolerant R. favelukesii strains. Rhizobium favelukesii has the unusual ability of being able to nodulate and fix nitrogen, albeit sub-optimally, not only with M. sativa but also with the promiscuous host Phaseolus vulgaris. Here we describe the genome of R. favelukesii OR191 and genomic features important for the symbiotic interaction with both of these hosts. The OR191 draft genome contained acid adaptation loci, including the highly acid-inducible lpiA/acvB operon and olsC, required for production of lysine- and ornithine-containing membrane lipids, respectively. The olsC gene was also present in other acid-tolerant Rhizobium strains but absent from the more acid-sensitive Ensifer microsymbionts. The OR191 symbiotic genes were in general more closely related to those found in Medicago microsymbionts. OR191 contained the nodA, nodEF, nodHPQ, and nodL genes for synthesis of polyunsaturated, sulfated and acetylated Nod factors that are important for symbiosis with Medicago, but contained a truncated nodG, which may decrease nodulation efficiency with M. sativa. OR191 contained an E. meliloti type BacA, which has been shown to specifically protect Ensifer microsymbionts from Medicago nodule-specific cysteine-rich peptides. The nitrogen fixation genes nifQWZS were present in OR191 and P. vulgaris microsymbionts but absent from E. meliloti-Medicago microsymbionts. The ability of OR191 to nodulate and fix nitrogen symbiotically with P. vulgaris indicates that this host has less stringent requirements for nodulation than M. sativa but may need rhizobial strains that possess nifQWZS for N2-fixation to occur. OR191 possessed the exo genes required for the biosynthesis of succinoglycan, which is required for the Ensifer-Medicago symbiosis. However, 1H-NMR spectra revealed that, in the conditions tested, OR191 exopolysaccharide did not contain a succinyl substituent but instead contained a 3-hydroxybutyrate moiety, which may affect its symbiotic performance with Medicago hosts. These findings provide a foundation for the genetic basis of nodulation requirements and symbiotic effectiveness with different hosts.

Solar cultivation of microalgae in photobioreactors is a valuable bioprocess for the sustainable production of commercially useful metabolites. However, the conventional culture temperature control method in solar closed photobioreactors of evaporative cooling is neither economical nor sustainable. In this study, a novel spectrally selective, insulated glazed flat plate (IGP) photobioreactor employing an infrared reflecting system embedded in the illumination surface was used for cultivation of Nannochloropsis sp. The impact of the temperature control technology on protein, lipid, carbohydrate content and fatty acid profile of Nannochloropsis sp. was investigated and compared to closed photobioreactors using passive evaporative cooling (PEC) and an infrared reflecting film (IRF) on the surface as well as an open raceway pond (ORP). Among all cultivation systems tested, the biochemical composition of biomass (mg g−1 organic biomass) showed a general trend of lipid > protein > carbohydrate, with no large variation of each across treatments. However, the areal and volumetric productivities of these constituents were significantly higher in the photobioreactors than in the ORP; results consistent with biomass productivity data. Of the major saturated and monounsaturated fatty acids present, only the proportion of C16:0, which is 24% higher in the photobioreactors than in the ORP, changed significantly among cultivation systems. The highest content of high-value dietary fatty acids, eicosapentaenoic acid (EPA, C20:5n-3; 15.5%) and ϒ-linolenic acid (C18:3n-6; 8%) were found in the ORP but were similar to that produced in the IGP (15.9 and 3.4%, respectively). Among all photobioreactors, the IGP had the least diel temperature changes and an EPA content that was 21% higher than PEC. Photobioreactors constructed with spectrally selective materials effectively allow management of internal reactor temperature with no significant negative impact on biochemical and fatty acid profiles of microalgae.

The need for thermal regulation in microalgal photobioreactors is a significant impediment to their large-scale adoption. The energy costs associated with thermal regulation alone can easily result in a negative energy balance. Self-sustaining photovoltaic powered photobioreactors that do not require cooling systems provide an opportunity to maximize biomass productivity, generate local electricity, reduce thermal regulation requirements, and significantly improve the energy balance of the system. Net energy analysis of a spectrally-selective, insulated-glazed photovoltaic photobioreactor (IGP) with an integrated capability for renewable electricity generation used to cultivate Nannochloropsis sp. without freshwater-based cooling resulted in a net energy ratio of 2.96, a figure comparable to agricultural bio-oil crops such as Jatropha and soybean. Experimental data from pilot-scale operation of this novel photobioreactor producing Nannochloropsis biomass under outdoor conditions was extrapolated to a 1-ha IGP installation. Annual biomass productivity reached 66.0-tons dry weight ha−1, equivalent to overall energy output of 1696.2 GJ ha−1. The integrated semi-transparent photovoltaic panels generated an additional 1126.8 GJ ha−1 yr−1 (313.0 MWh ha−1 yr−1). Energy demands from plant building materials, machinery, fertilizers, plant operations, and biomass harvesting constituted total energy input with a combined value of 707.3 GJ ha−1 yr−1. Comparison with a conventional photobioreactor requiring passive evaporative cooling showed novel photobioreactor had a 73% greater net energy ratio. Nannochloropsis cultivation in IGP system ensured co-production of lipid and protein of 34.7 and 25.7-tons ha−1 yr−1, respectively. These results suggest that this novel photobioreactor could be a viable and sustainable biomass production technology for mass microalgal cultivation.

The operation of solar microalgal photobioreactors requires sufficient cooling and heating to maintain reliable high productivity year-round. These operations are energy-intensive and expensive. Growth characteristics and phycocyanin production of Arthrospira platensis were investigated during the austral winter using a thermally-insulated photobioreactor with photovoltaic panel integration for electricity generation. This was compared with a control photobioreactor under a cycle of heating (13-hour night) and thermostat-regulated cooling, and continuously heated raceway pond. Average temperature in the photovoltaic photobioreactor (21.0 ± 0.03 °C) was similar to that in the heated control. Biomass productivity of Arthrospira in the novel photobioreactor was 67% higher than in the raceway pond but significantly lower than the control. Phycocyanin productivity (16.3 ± 1.43 mgg−1d−1 and purity (1.2 ± 0.03) showed no variation between photobioreactors but was significantly lower in the raceway pond. Electrical energy output of the photovoltaic photobioreactor exceeded mixing energy needs by 75%. These results indicate that the novel photobioreactor offers a reliable, energy-efficient platform for large-scale production of high-value chemicals from microalgae.