Areal Productivity Research Articles

Culture depth effect on Scenedesmus sp. growth, photo-physiology and nutrient removal rate in anaerobically digested abattoir effluent

Here, we evaluated the effect of culture depth on the growth and nutrient removal rate of Scenedesmus sp. grown in anaerobically digested abattoir effluent (ADAE) and cultivated semi-continuously in raceway ponds at depths of 14 cm, 17 cm, 20 cm, and 23 cm during Austral winter and summer. Culture medium pH was kept constant at pH 6.5 for all treatments throughout the experimental period using a pH–stat system. Algal cultures grown in summer had 2.3- 2.7 times higher biomass productivity than the same grown in winter. In both seasons, maximum volumetric productivity of this alga was achieved at 14 cm depth (14 cm depth⩾ 17 cm depth⩾ 20 cm depth⩾ 23 cm depth). However, areal biomass productivity of culture grown at 23 cm depth was 12% and 29% higher than that of culture grown at 14 cm depth in winter and summer, respectively. In addition, nitrogen, phosphorus and COD areal removal rates were significantly higher in cultures operated at 23 cm among all treatments in both seasons. The effective quantum yield (Fq'/Fm') in summer was 23 cm depth = 20 cm depth > 17 cm depth = 14 cm depth while it followed 14 cm depth⩾ 17 cm depth⩾ 20 cm depth⩾ 23 cm depth in winter, indicating significance of operational conditions on algal photosynthesis. The outcome of this study shows that, irrespective of the season, operating the culture in higher depths significantly increased areal biomass productivity as well as areal nutrient removal rates when treating ADAE.

High cell density culture of Neochloris oleoabundans in novel horizontal thin-layer algal reactor: Effects of localized aeration, nitrate concentration and mixing frequency

A horizontal thin-layer algal reactor was designed, constructed and tested for high cell density culture of an oil-rich green alga, Neochloris oleoabundans, at a thickness of 6 mm. A hollow fiber membrane sparger was used for localized aeration with CO2-enriched air, which allowed control of culture pH at a narrow range of pH 7.0–7.5. The optimal nitrate concentration under the investigated conditions was 345 mg-N/L with a cell density of ∼6.6 × 108 cells/mL and biomass concentration of ∼8 g/L. The volumetric and areal biomass productivities were 1.81 g/L/day and 10.85 g m− 2/day, respectively, and the maximum volumetric and areal productivities 3.57 g/L/day and 21.42 g m− 2/day, respectively. Intermittent mixing at a frequency of once every two hours (five min mixing each time) produced a cell growth profile similar to that of continuous mixing. Such intermittent mixing resulted in 98.38 % saving on mixing energy and 33.9 % reduction in water evaporation. It was observed that metabolic stress such as high pH and high nitrate concentration could inhibit cell division while causing cell size to enlarge abnormally. The results indicate that it is technically feasible to have high cell density culture of photoautotrophic microalgae with intermittent mixing and no culture circulation, opening the door towards the development of open algal farming systems of low capital, operational, and maintenance costs.

Outdoor annual algae productivity improvements at the pre-pilot scale through crop rotation and pond operational management strategies

The Development of Integrated Screening, Cultivar Optimization, and Verification Research (DISCOVR) collaborative consortium operated pre-pilot scale outdoor ponds to deliver much-needed multi-year, long-term and consistent, algae cultivation data relevant to understanding the current state of technology in terms of expected seasonal algae biomass productivity. Over the course of four years from 2018 to 2021, twelve identical 4.2 m2 mini-ponds were run in triplicate sets to test strains and operational strategies demonstrated in small-, indoor photobioreactors, in pursuit of increasing overall algae areal productivity and projected farm yield. Fourteen different cultivars derived from a strain screening pipeline were tested. Through deliberate seasonal crop rotation and improvements in operational strategies, annual biomass productivity increased from 11.6 to 17.6 g m−2 day−1, a > 50 % increase over the 2018 baseline. Both brackish and marine strains were included and four out of the fourteen strains consistently yielded high productivity across multiple years; brackish strains Monoraphidium minutum (26BAM) and Scenedesmus obliquus (UTEX393), and marine strains Tetraselmis striata (LANL1001) and Picochlorum celeri (TG2). These freely available datasets, which represent nearly complete annual daily coverage of cultivation metrics including weather, pond temperature and pH, nutrients, and productivity, are unique in the public domain and seek to fill agronomic and operational knowledge gaps to help in the eventual commercialization of algal biofuels and bioproducts.

Multiphysics simulations of concentric-tube internal loop airlift photobioreactors for microalgae cultivation

In this work, a comprehensive multiphysics model is developed in-house to numerically predict microalgae productivity in concentric-tube internal loop airlift photobioreactors (PBRs). The program integrates solar radiation, light attenuation in microalgae suspensions, cell growth kinetics, and hydrodynamics. The simulations of light attenuation and cell growth kinetics are based on experimental data measured in a PBR. The gas–liquid bubbly flow hydrodynamics is simulated using computational fluid dynamics (CFD) (Ansys FLUENT). The program is employed to predict the annual biomass areal and volumetric productivities of PBRs with various geometrical and operating parameters. The governing equations for the model integration are numerically solved using the finite-volume method for spatial discretization and an explicit scheme for time integration with a time step of PBR liquid circulation time. The effects of plant latitude, shading, clearance between adjacent PBRs, PBR dimension, downtime, and harvesting biomass threshold concentration on the annual biomass productivities are investigated. Under the assumptions made in this study, it is found that the 2.5m-high PBRs yield higher annual areal biomass productivity than the 1.5m- and 2m-high PBRs. The highest productivity is found at a plant latitude of approximately 15°. The optimal harvesting threshold concentration for the highest annual areal productivity is in the range of 0.4–0.7gl−1 depending on the downtime. The optimal concentration increases with longer downtime. Small clearances between adjacent PBRs are preferred to offer higher areal productivity. For PBRs positioned with small clearances, PBRs with smaller diameter-to-height ratios are more advantageous since the higher land utilization efficiency outweighs the increased shading loss.

Development of integrated screening, cultivar optimization, and verification research (DISCOVR): A coordinated research-driven approach to improve microalgal productivity, composition, and culture stability for commercially viable biofuels production

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 laboratories - Pacific 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.

DISCOVR strain pipeline screening – Part II: Winter and summer season areal productivities and biomass compositional shifts in climate-simulation photobioreactor cultures

Assessing the seasonal biomass productivity and compositional shift dynamics under simulated outdoor culture conditions of the top 21 algae strains selected during Tier I flask screening is an important step in the further prioritization of strains with regard to outdoor pond cultivation.These top 21 strains were subjected to Tier II testing in the PNNL Laboratory Environmental Algae Pond Simulator (LEAPS) photobioreactors, simulating light and temperature conditions of 20 cm deep outdoor ponds during the Arizona winter and summer season. All strains were grown in two consecutive nutrient-replete batch culture experiments at their particular optimal medium salinity to quantify their respective seasonal linear-phase areal biomass productivities. To determine biomass compositional shifts in response to nutrient-depletion, the LEAPS cultures were allowed to enter a 9-day nutrient depletion phase at the end of the second batch run. The following strains were evaluated in winter-season climate-simulated cultures and are listed in the order from highest (7.9 g m−2 day−1) to lowest (2.3 g m−2 day−1) areal N-replete biomass productivity: Monoraphidium minutum 26B-AM, Tetraselmis striata LANL1001, Chlorella vulgaris LRB AZ-1201, Micractinium reisseri NREL14-F2, Monoraphidium sp. MONOR1, Chlorella vulgaris NREL4-C12, Scenedesmus obliquus UTEX393, Scenedesmus acutus LRB-AP-0401, Nannochloropsis oceanica CCAP849/10, and Stichococcus minutus CCALA727. The following strains were evaluated in summer-season climate-simulated cultures and are listed in the order from highest (31.8 g m−2 day−1) to lowest (14.2 g m−2 day−1) areal N-replete biomass productivity: Picochlorum renovo NREL39-A8, Scenedesmus obliquus UTEX393, Porphyridium cruentum CCMP675, Picochlorum celeri TG2-WT-CSM/EMRE, Chlorella sorokiniana DOE1116, Stichococcus minor CCMP819, Picochlorum oklahomensis CCMP2329, Chlorella sorokiniana DOE1412 (UTEXB3016), Scenedesmus rubescens NREL46B-D3, Picochlorum soloecismus DOE101, Tetraselmis striata LANL1001, Scenedesmus obliquus DOE 0152.z, and Agmenellum quadruplicatum UTEX2268. All LEAPS cultures experienced a significant reduction in areal biomass productivity in response to nutrient-depletion, from 7 to 16 % in the winter season simulation and from 1 to 60 % in the summer season simulation. For 10 of the strains tested, the carbohydrate content more than doubled upon nutrient depletion, and for 9 strains, the lipid content increased by over 50 % of the initial content.

Cold-adapted culturing of the microalga Monoraphidium sp. in thin-layer raceway pond for biomass production

Three cultivation regimes were tested in cold-adapted cultures of the green microalga Monoraphidium in an outdoor thin-layer raceway pond: cultivation under sunlight; its combination with continuous supplementary illumination; and nitrogen depletion using both light sources. The highest volumetric and areal productivity, 0.16 g L−1 d−1 and 3.22 g m−2 d−1, respectively corresponding to the specific growth rate μ of 0.191 d−1 were achieved when sunlight was combined with supplementary illumination. The maximum total fatty acid content, 20.29 % of DW, rich in oleic acid, 54 % of total fatty acid content, was achieved under nitrogen depletion stress. An outstanding amount of lutein, 26.39 mg lutein g−1 DW, was detected grown under sunlight in the first trial. From the harvested and fermented biomass in the second trial 236 mLN g−1oTS of methane was generated.

Year-round sustainable biomass production potential of Nannochloris sp. in outdoor raceway pond enabled through strategic photobiological screening.

Microalgae cultivation utilizes the energy of sunlight to reduce carbon dioxide (CO2) for producing renewable energy feedstock. The commercial success of the biological fixation of carbon in a consistent manner depends upon the availability of a robust microalgae strain. In the present work, we report the identification of a novel marine Nannochloris sp. through multiparametric photosynthetic evaluation. Detailed photobiological analysis of this strain has revealed a smaller functional antenna, faster relaxation kinetics of non-photochemical quenching, and a high photosynthetic rate with increasing light and temperatures. Furthermore, laboratory scale growth assessment demonstrated a broad range halotolerance of 10-70 parts per thousand (PPT) and high-temperature tolerance up to 45°C. Such traits led to the translation of biomass productivity potential from the laboratory scale (0.2-3.0 L) to the outdoor 50,000 L raceway pond scale (500-m2) without any pond crashes. The current investigation revealed outdoor single-day peak areal biomass productivity of 43gm-2 d-1 in summer with an annual (March 2019-February 2020) average productivity of 20gm-2 d-1 in seawater. From a sustainability perspective, this is the first report of successful round-the-year (> 347days) multi-season (summer, monsoon, and winter) outdoor cultivation of Nannochlorissp. in broad seawater salinity (1-57 PPT), wide temperature ranges (15-40°C), and in fluctuating light conditions. Concurrently, outdoor cultivation of this strain demonstrated conducive fatty acid distribution, including increased unsaturated fatty acids in winter. This inherent characteristic might play a role in protecting photosynthesis machinery at low temperatures and inhigh light stress. Altogether, our marine Nannochloris sp. showed tremendous potential for commercial scale cultivation to produce biofuels, food ingredients, and asustainable source for vegetarian protein.

Spatial, seasonal, and historical variation of phytoplankton production in Lake Michigan

There have been few direct measurements of phytoplankton production made in Lake Michigan since invasive dreissenid mussels became established in the lake. Here we report the results of 64 measurements of phytoplankton primary production made in Lake Michigan during 2016 and 2017. We conducted two lake-wide surveys, one in the spring 2016 isothermal period and one after summer stratification in 2017 and examined seasonal production with bi-weekly sampling between May and November 2017 at an offshore station in the southwestern part of the lake. We assessed nearshore-offshore gradients by sampling at three transect locations on three occasions in 2017. Spring 2016 production and production:biomass (P:B) ratios (reflective of growth rates) were similar across the lake and were higher than those reported before dreissenid mussels became established, suggesting that despite decreases in phytoplankton biomass, growth rates remain high. Summer 2017 production and growth rates increased from south to north. Areal production in 2017 peaked in late summer. Mean 2017 summer production (499 ± 129 mg C m−2 day−1) was lower than values reported prior to the mussel invasion, and the fraction of total production occurring in the deep chlorophyll layer was about half that measured pre-mussels. At the offshore site picoplankton accounted for almost 50 % of the chlorophyll. As spring P:B ratios have increased and summer P:B and seston carbon:phosphorus ratios have not changed, we conclude that the decrease in phytoplankton production in Lake Michigan is due primarily to grazing by mussels rather than to stronger nutrient limitation.

Integrated Model for Design, Optimization, Scale-Up, Culture Scheme, and Production Evaluation of a Bioreactor and Its Application for Improving Algal Biomass and Carbon Fixation in Bioreactors

A comprehensive numerical model system for the bioreactor design, structure and cultivation condition optimization, scale-up, and carbon fixation performance evaluation was proposed. Three basic models of computational fluid dynamics, particle and mass transfer, and two improved models of algae growth and light accumulation were integrated innovatively. The algae growth model directly predicted the algal production performance of the bioreactor, and the error was less than 15% compared with the experimental results. The light accumulation model was proposed to simulate the spatial distribution of light utilization with the changes of algal concentration and light intensity. The model system was applied to scale up the flat plate photobioreactor with inclined baffles, and the results showed that the photobioreactor with 9 cm light path had a maximum ideal areal productivity (505.94 g/m2) and better performance. Based on the light accumulation model, the strategy of changing the surface light intensity during the cultivation was proposed to maintain the light utilization and increase the production. Under the optimized surface light intensity, the maximum algal concentration and the average carbon fixation rate of the bioreactor were 43.41 and 248.35% higher than those of the constant light intensity. The proposed model system is a reliable and efficient experimental alternative tool for algal production evaluation and carbon fixation application of the bioreactor.

Microalgae Biomass and Lipids as Feedstock for Biofuels: Sustainable Biotechnology Strategies

Microalgae exhibit remarkable potential as a feedstock for biofuel production compared with other sources, owing to their high areal productivity, low environmental effect, and negligible influence on food security. However, the primary obstacle to the commercialization of algae-based biofuels is the high economic cost due to the low-yield lipid content in the microalgae biomass. Maximizing biomass and lipid production is crucial to improve the economic viability of microalgae for biofuels. Identifying appropriate algal strains, particularly from indigenous environments, and developing those ‘platform strains’ using mutagenesis and genetic-engineering techniques is preferable. The provided discussion of conventional methods to increase microalgae’s biomass and lipid productivity mostly entailed adjusting environmental (such as temperature, light, and salinity) and nutritional (such as nitrogen and phosphorus) parameters. This review illustrated a comprehensive overview of biotechnological approaches and the recent strategies to enhance the lipid productivity of microalgae. The research also emphasized the need to streamline engineering strategies with the aid of recent advancements in DNA-manipulation techniques to hinder the existing biological intricacies in lipogenesis. This review also discussed the current economic and commercialization of this algal biorefinery along with the drawbacks.

Developing food waste biorefinery: using optimized inclined thin layer pond to overcome constraints of microalgal biomass production on food waste digestate

Diversion of food waste from landfill through anaerobic digestion is a sustainable form of energy production (biogas) and the waste effluent (digestate) can be utilised as nutrient supply for microalgae cultivation. However, digestate has very high nutrient concentrations and is highly turbid, making it difficult to utilize as a nutrient source with conventional microalgae cultivation systems. Here we compared the efficiencies of a conventional open raceway pond (ORWP) and an improved inclined thin layer photobioreactor (ITLP) for the utilization and treatment of food waste derived digestate by Chlorella sp. The ITLP improved on volumetric and areal productivities by 17 and 3 times over the ORWP, with values of 0.563 and 31.916 g m −2 day −1 respectively. Areal nutrient removal via microalgae biomass were 2359.759 ± 64.75 and 260.815 ± 7.16 mg m −2 day −1 for nitrogen and phosphorous respectively in the ITLP, which are 2.8 times higher than obtained in the ORWP. The ITLP’s superiority stems from its ability to support a much higher average biomass yield of 6.807 g L −1, which was 7 times higher than in the ORWP. Mean irradiance in-situ was higher in the ITLP, irradiance distribution and utilization by the culture in the ITLP was 44% more efficient than in the ORWP. Our results indicate that the ITLP is a far more productive system than conventional raceway ponds. This demonstrates that integration of ITLP microalgae cultivation using digestate has the potential to make digestate management yield net benefit in food waste biorefinery settings.

Optimisation of Operational Conditions during the Production of Arthrospira platensis Using Pilot-Scale Raceway Reactors, Protein Extraction, and Assessment of their Techno-Functional Properties.

The aim of the present study was to identify the optimum combination of dilution rate and depth of the culture to maximise the Arthrospira platensis BEA005B (Spirulina) productivity using 80 m2 raceway reactors. By varying these two main operational conditions, the areal biomass productivity of the reactors varied by over 55%. The optimum combination, optimised using a surface response methodology, was a depth of 0.10 m and a dilution rate of 0.33 day−1, which led to a biomass productivity of 30.2 g·m−2·day−1 on a dry weight basis when operating the reactors in semi-continuous mode. The composition of the produced biomass was 62.2% proteins, 42.5% carbohydrates, 11.6% ashes, and 8.1% lipids. The isolated proteins contained all the essential amino acids (except for tryptophan, which was not determined); highlighting the content of valine (6.8%), histidine (8.3%), and lysine (7.5%). The functional properties of the proteins were also assessed, demonstrating huge potential for their use in the development of innovative and sustainable foods.

Microalgae-based Bioenergy with Carbon Capture and Storage quantified as a Negative Emissions Technology

In this paper, microalgae are considered to produce bioenergy through firing. Emitted CO2 would then be captured and stored. This “negative emissions” process, called Microalgae-based Bioenergy with Carbon Capture and Storage (MBECCS), is quantified for the first time, and compared to “classical” Bioenergy with Carbon Capture and Storage (BECCS) using plants. Our goal is to remove 10 Gt per year of CO2 from the atmosphere using MBECCS only. The needed area for cultivation is 4 times less than with BECCS. Because of the high water consumption, marine microalgae should be used. On the other hand, nutrients taken from world wastewater are insufficient, and remain an issue. Planktonic and biofilm microalgae are considered: the latter offer a better areal productivity and require less energy since they are in a concentrated form.For both planktonic and biofilm microalgae, the process produces net heat. Concerning electrical efficiency, we show that it is better for biofilms, as expected. Moreover, for low productivities, MBECCS consumes electricity, comparably to Direct Air Capture, another energy-consuming negative emissions process. Finally, for higher productivities, we show that MBECCS produces net electricity, comparably to other renewable sources such as wind turbines and to BECCS.

Development of plastic flatbed-based algal culture system deployable on non-arable land

The industrial use of microalgae as bulk commodities relies on large scale and economical production. We recently reported a flat, open and shallow (FOS) culture method. In the present study, microalgae (Chlorella sorokiniana) were cultivated in various modes to evaluate key parameters and assess biomass and productivity. Using batch mode with 1-5 cm culture depth, the highest volumetric and areal productivity were 0.37 g L−1 d−1 and 9.08 g m−2 d−1, respectively. Using fed-batch mode, the microalgae density reached 6.40 g L−1. The yield remained stable after the microalgae were harvested 11 times in the semi-continuous mode. The system was then upgraded and scaled up to 4000 L for outdoor experiments in autumn, spring and summer, and the productivities were 6.80, 7.20 and 7.60 g m−2 d−1, respectively. Technoeconomic analysis showed that compared with open pond systems, this method is more efficient in terms of investment, water, electricity and operation, thus having great potential for deployment on non-arable land for massive algal production.

Resistance of DISCOVR algae strains to deleterious species

The goal of the DISCOVR consortium is to improve the annualized areal production of open algal production systems through a process of algal strain evaluation and selection combined with testing in outdoor pilot scale production systems. The role of Sandia National Laboratories, in this evaluation process, has been to screen the selected subset of algal strains for resistance to grazing. Sandia tested 9 marine and 8 freshwater strains of algae against a panel of 14 grazer species, (5 marine, 5 freshwater and 4 euryhaline). A total of 153 algal/grazer pairs at two grazer concentrations for a total of 306 assays in triplicate. The relative resistance of each algal strain to each grazer strain was expressed as the decimal fraction of its specific growth rate 29in the presence of the grazer divided by the growth rate of the algae alone. Of the strains tested, those with the greatest resistance to the grazer panel were the freshwater strains Acutodesmus obliquus UTEX 393 and Scenedesmus obliquus. DOE 152z, and the marine strains Micractinium sp 14-F2 and Scenedesmus sp. 46B-D3. Additionally, these assays revealed the unexpected levels of variation in grazer resistance between taxonomically related organisms.

Pilot-Scale Cultivation of the Snow Alga Chloromonas typhlos in a Photobioreactor.

The most studied and cultivated microalgae have a temperature optimum between 20 and 35°C. This temperature range hampers sustainable microalgae growth in countries with colder periods. To overcome this problem, psychrotolerant microalgae, such as the snow alga Chloromonas typhlos, can be cultivated during these colder periods. However, most of the research work has been carried out in the laboratory. The step between laboratory-scale and large-scale cultivation is difficult, making pilot-scale tests crucial to gather more information. Here, we presented a successful pilot-scale growth test of C. typhlos. Seven batch mode growth periods were compared during two longer growth tests in a photobioreactor of 350 L. We demonstrated the potential of this alga to be cultivated at colder ambient temperatures. The tests were performed during winter and springtime to compare ambient temperature and sunlight influences. The growth and CO2 usage were continuously monitored to calculate the productivity and CO2 fixation efficiency. A maximum dry weight of 1.082 g L−1 was achieved while a maximum growth rate and maximum daily volumetric and areal productivities of 0.105 d−1, 0.110 g L−1 d−1, and 2.746 g m−2 d−1, respectively, were measured. Future tests to optimize the cultivation of C. typhlos and production of astaxanthin, for example, will be crucial to explore the potential of biomass production of C. typhlos on a commercial scale.

Assessment of Eicosapentaenoic Acid (EPA) Production from Filamentous Microalga Tribonema aequale: From Laboratory to Pilot-Scale Study.

It has long been explored to use EPA-rich unicellular microalgae as a fish oil alternative for production of the high-value omega-3 fatty acid eicosapentaenoic acid (EPA, 20:5, n-3). However, none of the efforts have ever reached commercial success. This study reported a filamentous yellow-green microalga Tribonema aequale that possesses the ability to grow rapidly and synthesize significant amounts of EPA. A series of studies were conducted in a glass column photobioreactor under laboratory culture conditions and in pilot-scale open raceway ponds outdoors. The emphasis was placed on the specific nutrient requirements and the key operational parameters in raceway ponds such as culture depth and mixing regimes. When optimized, T. aequale cells contained 2.9% of EPA (w/w) and reached a very high biomass concentration of 9.8 g L−1 in the glass column photobioreactor. The cellular EPA content was increased further to 3.5% and the areal biomass and EPA productivities of 16.2 g m−2 d−1 and 542.5 mg m−2 d−1, respectively, were obtained from the outdoor pilot-scale open raceway ponds, which were the record high figures reported thus far from microalgae-based EPA production. It was also observed that T. aequale was highly resistant to microbial contamination and easy for harvesting and dewatering, which provide two additional competitive advantages of this filamentous microalga over the unicellular counterparts for potential commercial production of EPA and other derived co-products.

High density cultivation and CO2 uptake by panel arrays of the macrophytic red alga Gracilaria vermiculophylla in a 100 L raceway pond

New approaches are needed for land-based cultivation of macrophytic red algae that reduce costly aeration requirements for biomass suspension and enable high-density cultivation. The goal of this study was to demonstrate the high-density cultivation of the carbohydrate-rich macrophytic red alga Gracilaria vermiculophylla on vertical arrays of panels deployed in an open channel raceway configuration similar to those developed for mass cultivation of microalgae. A clonal culture of G. vermiculophylla, consisting of branched, cylindrical thallus tissues of 8–10 cm length, was mechanically blended using a Waring blender into 2–3 cm fragments and then fluidically injected onto a 3 mm polypropylene mesh support. Immobilized G. vermiculophylla mesh panels were spaced 6.5 cm apart and aligned parallel to flowing seawater medium at nominal bulk velocity of 20 cm s−1 in a 100 L raceway pond of 20 cm liquid depth. This raceway was equipped with real-time measurement of CO2 concentration in the inlet and outlet gas for determination of CO2 uptake dynamics. Specific rates for CO2 uptake became saturated at 8000 ppm CO2. To match CO2 demand by the biomass under nutrient-replete conditions at 21 °C, the inlet gas CO2 was increased from 1000 to 4000 ppm (day 7–14), and then to 8000 ppm (day 14–23) at 0.010 L gas L−1 liquid min−1 gas flow. Over the 23 day cultivation, biomass on the panel increased by a factor of 48, with final biomass loading exceeding 10 kg FW m−2 panel area, and cumulative CO2 capture of 65%. The cumulative average areal productivity within the panel zone of the raceway exceeded 60 g AFDW m−2 day−1, and final biomass density nearing 7.2 g AFDW L−1 (47 g FW L−1) was achieved after 23 days. Overall, these outcomes demonstrate the potential for land-based raceway cultivation of clonal red macroalgae of present and future commercial significance.

Global estimates of the extent and production of macroalgal forests

AbstractAimMacroalgal habitats are believed to be the most extensive and productive of all coastal vegetated ecosystems. In stark contrast to the growing attention on their contribution to carbon export and sequestration, understanding of their global extent and production is limited and these have remained poorly assessed for decades. Here we report a first data‐driven assessment of the global extent and production of macroalgal habitats based on modelled and observed distributions and net primary production (NPP) across habitat types.LocationGlobal coastal ocean.Time periodContemporary.Major taxa studiedMacroalgae.MethodsHere we apply a comprehensive niche model to generate an improved global map of potential macroalgal distribution, constrained by incident light on the seafloor and substrate type. We compiled areal net primary production (NPP) rates across macroalgal habitats from the literature and combined this with our estimates of the global extent of these habitats to calculate global macroalgal NPP.ResultsWe show that macroalgal forests are a major biome with a global area of 6.06–7.22 million km2, dominated by red algae, and NPP of 1.32 Pg C/year, dominated by brown algae.Main conclusionsThe global macroalgal biome is comparable, in area and NPP, to the Amazon forest, but is globally distributed as a thin strip around shorelines. Macroalgae are expanding in polar, subpolar and tropical areas, where their potential extent is also largest, likely increasing the overall contribution of algal forests to global carbon sequestration.