Post-harvest algae biomass is prone to degradation, resulting in mass loss and compositional changes, and preservation is vital to economic viability of algal products. Effective storage solutions are needed to mitigate for seasonal productivity variations (long-term storage) and to keep post-harvest biomass stable until processing. Ensiling has emerged as a long-term storage solution capable of preserving biomass up to 6 months with little loss without the energy demands of drying. Organic acids produced during ensiling lower biomass pH and prevent growth of degradative bacteria such as Clostridia. However, losses are front-loaded with a majority occurring within the first week before organic acids can accumulate. Currently, there is no information on the stability of algae biomass within the first 24 hr post-harvest or methods available to ensure stability during this period. Freshly harvested Tetradesmus obliquus UTEX 393 biomass was stored in three conditions: ambient atmosphere, anaerobic atmosphere without treatment, and anaerobic atmosphere with citric acid amendment. Citric acid treatment limited mass loss to 1% after 28 days, while untreated biomass experienced 4% mass loss after just 4 hr and 18% mass loss after 4 weeks. The carbohydrate fraction was most affected, with minimal changes to the elemental composition of biomass across treatments. Bacilli bacteria, including lactic acid bacteria, increased in abundance under all storage conditions. Untreated biomass showed a rise in Clostridia, but none were found in citric acid-treated biomass. After 28 days, organic acid composition differed significantly among treatments, with succinic acid being accumulated to 30% of dry cellular weight in citric acid treated UTEX 393 biomass. Citric acid treatment effectively mitigates biomass loss and, surprisingly, promotes substantial production of succinic acid. The unexpected autofermentation of UTEX 393 biomass to a versatile intermediate chemical such as succinic acid at high titers with minimal energy input could contribute to the economic viability of algae cultivation for fuels and chemicals. One-Sentence Summary  Tetradesmus obliquus biomass is susceptible to degradation immediately after harvest; citric acid treatment preserves biomass while stimulating succinic acid accumulation.

Abstract Crop loss due to infection by pests and pathogens is a major barrier to the large-scale production of algal biofuels. Test systems have seen loss of green algae crops due to infection by the fungus-like Amoeboaphelidium occidentale FD01. While current antifungal compounds are effective in inhibiting the infection, their application raises the overall cost of the crop and lowers its economic viability as a biofuel source. Here we show that co-culturing environmentally harvested bacteria alongside algae crops can drastically lower the rate of infection in two different green algae species of interest for biofuel production. These bacteria-algae consortia increase the mean time to crop failure (MTTF) by up to 350% when tested under environmentally relevant conditions. While there was an increase in diversity over time, there was no statistically significant correlation between an increase in diversity and a longer MTTF. Community composition analysis reveals similarities between the bacterial genera growing alongside both green algae species even as bacterial harvest locations differed, although there was not a single dominant genus responsible for the increase in crop protection. These results show a promising new method of anti-fungal crop protection that can be applied to algal biofuels with no increase in fuel cost. Highlights Bacteria-algal cocultures protect against fungal pests without impact to productivity Bacterial community composition is variable over time even as protection persists Bacterial consortia can increase mean time to failure by 350%

A system was developed and demonstrated that uses solid sorbents to capture CO2 passively from ambient air and to subsequently release CO2 when immersed in an alkaline medium. The system was used to cultivate the cyanobacterium Synechocystis sp. PCC 6803 at the flask (50 mL) and bench (12 L) scales. A small pilot-scale system installed in a 4.2-m2 outdoor raceway pond (840 L) was evaluated for over 300 days of outdoor wet/dry cycling and in cultivation trials over four seasons. Sorbent CO2-release capacity and kinetics were reduced due to competitive binding of nitrate and biofouling, and sorbent performance could be partially restored using a wash protocol. Over time, the sorbent beads showed significant reduction in the force needed to induce mechanical failure (i.e., fracture) and loss of quaternary ammonium functional groups necessary for CO2 capture, which may have been due to cumulative UV-induced damage to the polymer. Under the assumption that sorbent performance can be retained during cultivation, preliminary techno-economic analyses showed the potential for the viability of a small biorefinery producing 7.6 million gasoline gallon equivalents of biofuel per year ($3.62/GGE) by offsetting the cultivation costs by first extracting bulk protein as a supplement ($6 kg-1) and phycocyanin as a natural food and beverage dye ($50 kg-1).

Autonomous, high-frequency monitoring of outdoor algal ponds is needed to quantify biomass productivity and detect culture decline in environments prone to contamination, grazers, and variable operating conditions. We report successes and lessons learned in translating a laboratory spectroradiometric monitoring approach to a multi-year autonomous field deployment at the Arizona Center for Algae Technology and Innovation (AzCATI). The system measures spectrally resolved pond reflectance by ratioing upwelling radiance from each raceway to simultaneous downwelling sky irradiance using fiber-coupled spectrometers. A physics-based reflectance model (ASHARP) is fit to each spectrum pair to estimate optical parameters, including a biomass-proxy coefficient (C-a) which enables near-real-time tracking of biomass accumulation and culture state at 2-5 min intervals. From May 2022 through September 2025 the platform operated continuously while scaling from two to six raceway ponds. Several strains of algae were monitored successfully, including the high productivity Tetraselmis striata and Picochlorum celeri. Transitioning data acquisition from a Windows laptop to a Raspberry Pi improved uptime from 57% (2022) to similar to 89% (2024-2025) and enabled routine real-time analysis. Further, we converted relative biomass estimates to absolute ash-free dry weight (AFDW) using experimentally-derived calibrations, providing field-relevant biomass predictions with conservative confidence bounds. These results demonstrate the feasibility of long-term, autonomous optical monitoring for well-mixed open-raceway algal cultivation and provide practical guidance for reliable field operation and scaling.

Abstract For sustainable algal biomass cultivation, we need substantial improvement in annualized productivity by reducing the frequency of crop failure and improved growth in open raceway pond systems. In this study, high-performing strains were identified and optimized for biomass productivity. We utilized next-generation sequencing methods to quantify the ecological features of open raceway systems cultivated at in Arizona. We utilized data from several months of cultivation runs to construct a rich time-series of the ecology dynamics using amplicon sequencing and used custom anomaly detection, “PondSentry”, for the early prediction of pond crashes. PondSentry uses tensor decomposition of higher-order joint moments to detect incipient anomalies in multivariate data and displays significant improvements from standard knowledge-based anomaly detection methods. The PondSentry strategy identifies signs of deteriorating pond health at an average of three days before an actual crash event, with rank order of the ecological features plausible for crop failures driven by organisms such as Amoeboaphelidium occidentale FD01. These findings are independently confirmed with PCR and microscopy studies at an Arizona cultivation site. PondSentry’s time-series-based anomaly detection of crashes provides a suitable monitoring strategy for eukaryotic crash agents in unialgal culture. The early warnings can be used to time interventions or harvests to prevent biomass loss. The PondSentry strategy strengthens the role of data science and data-driven methods in algal cultivation and can increase the feasibility of algal-biomass based products.

Two direct air capture (DAC) systems were designed and demonstrated to passively capture CO2 from ambient air and use moisture to release the CO2 into an alkaline medium. A bench-scale system delivering ∼1 g CO2 d-1 was demonstrated in a laminar flow hood, and a small pilot-scale system that could deliver ∼100 g CO2 d-1 was operated outdoors in a 4.2 m2 raceway pond. Novel elongated mesh-tube packets containing anion-exchange resin (AER) beads were found to reduce drying and CO2 loading time 4.3-fold compared to larger mesh bags. Technoeconomic analysis (TEA) estimates the cost of capturing CO2 into an alkaline solution, suitable for cultivating photosynthetic microorganisms, to be $229 per tonne for a practical scenario based on current results and $72 per tonne for an aspirational scenario considering improvements to sorbent capacity, hydrophobicity, and sorbent lifetime. TEA further estimates an additional $110 per tonne to extract CO2 from solution, purify it, and compress it to 15 MPa, suitable for sequestration. Moisture-driven processes have the potential to use up to 87% less energy than thermal and/or vacuum swing DAC by using energy from water evaporation.

Sudden pond crashes remain a major barrier to reliable algal biomass production, and the biological processes driving these events are not well understood. Here we report the first documented crash of Picochlorum celeri, a species otherwise recognized for exceptional resilience in outdoor cultivation. By tracking temporal microbiome dynamics during this biomass collapse in raceway ponds, we identified distinct community trends before, during, and after the crash. Metagenome-assembled genome (MAG)-level analysis revealed clear clustering by biomass, indicating that microbial assemblages remained stable over days to weeks and shifted prior to collapse. Specific taxa showed contrasting associations with Picochlorum biomass, with Psychromarinibacter positively correlated and an Azotimanducaceae-affiliated pseudomonad negatively correlated. These results indicate that microbial community change was detectable prior to measurable biomass decline. Complementing these biological signatures, high-frequency spectroradiometric reflectance measurements detected physiological deviations 24–36 h before biomass loss, including suppressed diel growth and reduced accuracy of reflectance-based biomass estimates during early crash stages. These findings link microbial dynamics to operational outcomes and highlight opportunities for biotic countermeasures, including bacteriophages, protective bacteriomes, and small-molecule inhibitors. Together, this work demonstrates that even resilient hosts such as P. celeri are vulnerable to microbial disruption and establishes a path from early detection to targeted intervention strategies, while outlining a conceptual framework for integrating microbiome and optical indicators into dynamic reliability models that enhance the scalability of algal cultivation.

The development of large-scale microalgae growth for biofuel production is currently limited by the cost of biomass production. However, new approaches to infrastructure and cultivation practices are bringing the field closer to realization. Macronutrients in the cultivation media contribute significant costs, especially since their concentrations have not been optimized for specific strains and conditions. Environmental photobioreactors (ePBRs) were used to simulate cultivation under outdoor conditions, during which the nitrogen and phosphorus levels in the media were varied. The growth of two potential biofuel production strains, Picochlorum celeri and Tetraselmis striata, with varying nutrient inputs during summer and winter scripts, respectively, was studied. This study demonstrated that nitrogen and phosphorus in f/2 media could be reduced by more than 60% from the standard formulation, while maintaining growth rates in a semi-continuous harvesting approach. Experiments comparing the standard and reduced nutrient input concentrations were also conducted for both species in 820 L outdoor raceway ponds, in Mesa, AZ. P. celeri grown in these ponds in October had a growth rate of 10.6 +/- 0.7 g/m2/day and 10.6 +/- 0.3 g/m2/day for the standard and low-nutrient P. celeri ponds, respectively. T. striata grown in April-May had a growth rate of 16.6 +/- 1.4 g/m2/day for the standard nutrient input ponds and 17.4 +/- 1.1 g/m2/day for the low-nutrient input ponds, and in October 14.5 +/- 0.6 g/m2/day for standard nutrient ponds and 14.4 +/- 0.6 g/m2/day for low-nutrient ponds. These outdoor data therefore confirmed the indoor ePBR data. Techno-economic analysis shows that, if high growth rates can be attained at lower nutrient concentrations, a reduction of at least 60% in nutrient costs can be achieved. Such results highlight the importance of managing macronutrient media inputs, as these have a considerable contribution to biomass production costs in large-scale facilities. The analysis also points to the importance of maintaining high spent medium recycling rates in an industrial deployment, so as to minimize the losses of nitrogen and phosphorus compounds.

As active ingredients of sunscreens, UV-filters are regulated as over-the-counter drugs in the United States. Additional regulations are being considered and implemented as their ecological impacts are increasingly scrutinized. This creates significant commercial challenges and opportunities for eco-friendly alternatives. Cyanobacteria, that synthesize UV-absorbing mycosporine-like amino acids (MAAs), offer a biologically and ecologically safer solution compared to traditional chemical UV-filters. This work outlines a pilot-scale framework for biomanufacturing MAAs, concurrent with value-added exopolysaccharide (EPS) and pigments, through outdoor, scaled-up cultivation (∼800 L) of a stable cyanobacterial consortium. The consortium was comprised of nitrogen-fixing Cyanothece sp. HCC 1134, Picosynechococcus sp. PCC 7002, and Synechococcus bacillaris CCMP 1333. Sunlight exposure during cultivation nearly doubled intracellular MAA production, while growth-phase timing and nutrient availability predominantly influenced EPS synthesis. Spectrophotometric and LC-MS analyses revealed a differential distribution of MAAs. MAAs predominantly absorbing with local maxima in the UV-A range (e.g., shinorine, porphyra-334, asterina-330, palythine threonine) were present at higher intracellular concentrations, whereas extracellularly MAAs with local absorbance maxima in the UV-B range were predominant. Alongside their broad-spectrum UV absorption characteristics, the temperature and photostability were evaluated in comparison with existing chemical UV filters, highlighting their potential as promising candidates for sunscreen formulations.

Algal biomass is a promising sustainable feedstock for low-carbon fuels, chemicals, and proteins, but large-scale deployment remains economically challenging due to high cultivation costs. This study evaluates the economic viability of algal biomass production using batch and semi-continuous cultivation in open raceway ponds. A modular engineering process model was developed to quantify mass and energy flows across key processes, including seed production, biomass cultivation, and dewatering. Two seed train configurations were evaluated: high-density systems using photobioreactors and low-density systems relying on covered and lined ponds. This process model informed a techno-economic analysis to determine the minimum biomass selling price (MBSP) across four cultivation and seed train scenarios. Results indicate that semi-continuous cultivation achieves a lower MBSP ($1130-$1200 per metric tonne) than batch cultivation ($1380-$2040 per metric tonne), primarily due to reduced seed train costs. Seed production costs ranged from $40 to $350 per metric tonne in low-density configurations and $105 to $940 per metric tonne in high-density configurations, making batch systems with high-density seed trains economically unviable. Economic viability in semi-continuous cultivation was found to be sensitive to culture stability, with cost-parity reached if mean-time-to-failure decreases to 26 days. Additionally, batch cultivation can match the cost levels of semi-continuous systems at productivities exceeding 28 g m- 2 d- 1. These findings provide key insights into optimizing algal biomass production costs by balancing seed train configuration, cultivation strategy, and productivity thresholds, offering a pathway to more costcompetitive algae-based bioproducts.

Microalgae are a promising resource for production of industrially relevant compounds, including renewable fuels and nutraceuticals. However, little is known about the presence and impacts of viruses in outdoor cultivation ponds. We present sequences of bacteriophage and potential megavirus genomes from metagenomic sequencing of microalgae ponds in Mesa, Arizona, USA.

While photosynthetic algae-based systems have shown promise for reducing the carbon footprint associated with biofuel and biochemical production due higher yields than terrestrial crops, there are challenges associated with CO2 delivery and utilization resulting from the chemical and physical environment experienced. Point-source CO2 delivery is a critical component of intensive algal cultivation, but a significant fraction of the CO2 sparged into the aqueous environment is lost. In this context, we review the theoretical considerations for deconvoluting carbon transfer efficiency (CTE) and carbon utilization efficiency (CUE), specifically in microalgal cultivation in response to changes in media formulation and alkalinity. We introduce an empirical and operational approach to increase the efficiency of CO2 transfer and ultimately prime algal cultures for photosynthetic carbon assimilation. We define operational boundaries for improving CUE under a neutral pH regime, with conditions that maintain high algal biomass productivity. Our work supports both the implementation of strategies for increasing CUE as well as provides a framework for monitoring inorganic and organic carbon balances in controlled aqueous systems. The integration of water chemistry in media formulation with dissolved inorganic carbon (DIC) and alkalinity are primary drivers of the inorganic carbon flux from a concentrated CO2 source towards an accessible carbon source for microalgae. We outline a systematic approach by leveraging control over carbon delivery, operational pH in the neutral pH regime, and alkalinity to match available DIC of the media with the demands of the algae to help optimize CTE and CUE. This control increases the feasibility of large-scale biotic CO2 capture in aqueous systems.

A system called AUDACity was developed and demonstrated for continuously delivering ~100 g per day of direct air-captured (DAC) CO2 into aqueous alkaline solutions within a 4.2 m2 outdoor raceway pond in a moisture-driven process. Anion-exchange resin particles contained within elongated mesh tube packets were found to reduce drying and CO2 loading time ~4-fold over larger mesh bags, which allowed the sorbent to clump and reduce surface area exposed to air, as measured in a closed loop wind tunnel. The system was used for cultivating the cyanobacterium Synechocystis sp. PCC 6803 in a 4.2 m2 outdoor raceway pond. The culture grown in the AUDACity pond did not show visible contamination whereas a control pond without AUDACity showed contamination from amoeba and diatoms. Biofouling of the sorbent packets during cultivation reduced its CO2 binding capacity by ~50%, which was partially restored to 75% of its initial capacity after sequentially washing in 0.04% bleach, 1 mM HCl and then 0.5 M NaHCO3.

A moisture-driven air capture (DAC) system was designed and demonstrated. A laboratory-scale system delivering ~1 g CO2 per day was demonstrated in a laminar flow hood and a small pilot-scale system that could deliver ~100 g CO2 daily was operated outdoors in a 4.2 m2 (areal surface area) raceway pond. Elongated mesh tube packets were designed to contain AER beads with high surface area for contacting the air and were found to reduce drying and CO2 loading time ~4-fold over larger mesh bags. Whereas this system was designed for CO2 delivery for cultivating photosynthetic microbes, its potential uses are much broader and include CO2 use in the food and beverage industry, conversion to fuels and chemicals, and sequestration. Techno-economic assessments for a practical scenario based on current results are \$670/tonne to capture CO2 into an alkaline solution and an additional \$280/tonne to extract CO2 from solution, purify and compress to 15 MPa for sequestration. An aspirational scenario modelling reasonable improvements to develop AER sorbents with a capacity of 4 mmol CO2 per gram of sorbent and water uptake of 50 wt.%, which leads to sorbent drying and loading within 1 h, shows a potential to reach \$51/tonne to capture CO2 into an alkaline solution and an additional \$109/tonne to get to 15 MPa for sequestration. Life cycle analysis shows the aspirational moisture-driven process uses up to 87% less energy than thermal and/or vacuum swing DAC by using energy from water evaporation; however, ~330 wt.% water uptake by the sorbent contained in a hydrophilic mesh packets leads to ~33-fold higher water use than the thermodynamic limits, which emphasizes future research is needed to increase sorbent hydrophobicity while maintaining and further increasing ion exchange capacity needed to bind CO2.

A moisture-driven air capture system was developed and demonstrated for cultivating cyanobacteria and microalgae at the flask (50 mL), bench (12 L) and small pilot (840 L) scale. Purolite A501 anion exchange resin beads were found to be biocompatible and rapidly deliver air-captured CO2 when immersed directly in an alkaline cultivation medium containing cyanobacteria or microalgae. Flask-scale cultivation trials showed A501 could sustain rapid growth (190 mg/L/d) of the cyanobacterium Synechocystis sp. PCC 6803 strain engineered to produce laurate. A bench-scale system installed in a laminar flow hood was able to deliver 2 g CO2/d into abiotic alkaline cultivation medium and 0.5 g/d in the presence of Synechocystis to support vigorous growth (39 mg/L/d) limited by the CO2 delivered by the sorbent. A small pilot-scale system installed in a 4.2 m2 outdoor raceway pond in Mesa, Arizona was able to deliver 100 g CO2/d into abiotic alkaline cultivation medium. Exopolysaccharides and other products excreted by Synechocystis 6803 covered the sorbent beads, reducing their capacity to 25

Despite extensive research on algal bioproducts, there is limited understanding of how pond contamination affects their economics and environmental impacts. This work compared the costs and environmental impacts of algal biofuels across different pond failure scenarios. Pond failure was simulated by a reliability model based on pond mean-time-to-failure (MTTF). The reliability model was integrated with a process model to analyze the impacts of pond failure on the operations of algal farms and biorefineries. Process model outputs were used for techno-economic analysis and life cycle assessment to determine the minimum fuel selling price (MFSP), global warming potential (GWP), and freshwater consumption impacts of algal biofuels for five MTTF scenarios of 20, 54, 80,120, and 350 days, assuming an average mean-time-to-reset of 7 days. Results show that higher MTTFs reduce the cost and environmental impact of algal biofuels, but with diminishing returns. The average MFSPs for the 20-day, 54-day, and 350-day MTTF scenarios were $3.52, $2.54, and $2.10 per liter of gasoline equivalent, respectively. The GWP for the same scenarios were131, 96, and 83 g CO2eq MJ(-1), respectively. This study highlights the significant impact of larger seed trains, required under low MTTFs, on the costs and greenhouse gas emissions of algal biofuels. Moreover, the work shows that algal biofuels fail to be cost-competitive with conventional fuels, even when productivities are increased from 17 to 35 g m(-2) d(-1). This work is the first to explore the implications of pond failure on the sustainability of algal biofuels and provides valuable insights to algae farmers on how to reduce the costs and financial risks of algal cultivation through process design and pond management strategies.

To address major knowledge gaps and barriers to the commercial development of algal biomass for biofuels and co -products, a collaborative consortium, Development of Integrated Screening, Cultivar Optimization, and Verification Research (DISCOVR) , was established in 2016. Funded by the U.S. Department of Energy (DOE) Bioenergy Technologies Office (BETO), this consortium constitutes a partnership between four DOE national laboratoriesPacific Northwest National Laboratory (PNNL), Los Alamos National Laboratory (LANL), the National Renewable Energy Laboratory (NREL), and Sandia National Laboratories (SNL) - and the Arizona Center for Algae Technology and Innovation (AzCATI) at Arizona State University. To address the barriers of strain selection for achieving high seasonal productivities with a suitable composition and culture resilience, a tiered strain down -selection pipeline is implemented. At Tier I, the temperature and salinity tolerance of strains is determined in flask cultures; at Tier II, the areal biomass productivity and composition is determined in climate -simulation photobioreactors; at Tier III, the productivity and culture stability are determined in outdoor raceways. The top performing strains move forward to long-term testing at the algae testbed site at AzCATI to generate annual biomass productivity data. Concurrent to the strain down -selection in the DISCOVR pipeline, hypotheses for increasing biomass productivity, shifting biomass composition to enhance intrinsic value, and improving culture stability and resistance to pests are also tested. Techno-economic analyses are carried out to determine whether promising findings from laboratory studies or proposed modifications in outdoor pond cultivation conditions translate into reductions in the minimum biomass selling price (MBSP). In the three years following the launch of DISCOVR, annual biomass productivity has increased from 11.7 to 17.6 g m - 2 day -1 , resulting in an MBSP decrease from 824 to 611 $ ton -1 .