High Areal Productivity Research Articles

Biofilm and suspension-based cultivation of microalgae to treat anaerobic digestate food effluent (ADFE)

Anaerobic digestion of organic waste produces effluent (ADE) that requires further treatment. Biofilm-based microalgal cultivation is a favoured approach to ADE treatment. This study compared Chlorella sp. MUR 268 and Scenedesmus sp. MUR 269 in biofilm and suspension cultures to treat anaerobic digestate food effluent (ADFE). Chlorella sp. MUR 268 biofilm had significantly higher biomass (50.38 g m−2) than Scenedesmus sp. biofilm (9.39 g m−2). Conversely, Scenedesmus sp. yielded 1.5 times more biomass (1.2 g L−1) than Chlorella sp. in suspension. Chlorella sp. biofilm had 49.3 % higher areal productivity than suspension, while Scenedesmus sp. showed 87.3 % higher areal growth in suspension. Chlorella sp. MUR 268 and Scenedesmus sp. MUR 269 significantly removed nutrients in ADFE. In suspension, COD, ammoniacal nitrogen, and phosphate were reduced to 94.9, 5.2, and 5.98 mg L−1 for Chlorella sp. MUR 268, and 245, 2.89, and 3.22 mg L−1 for Scenedesmus sp. MUR 269, respectively. In biofilm, Chlorella sp. MUR 268 achieved reductions to 149.9, 1.16, and 3.57 mg L−1, while Scenedesmus sp. MUR 269 achieved 100.2, 6.9 and 2.07 mg L−1. Most of these values are below the recommended effluent discharge standard, highlighting the efficacy of this system in ADFE treatment. Biofilm cultures fixed 68–81 % of removed nitrogen in biomass, while in suspension, only 55–71 % ended in the biomass. Chlorella sp. MUR 268 biofilm fixed 88 % of removed phosphorus, while Scenedesmus sp. MUR 269 suspension fixed more phosphorus (55 %) than the biofilm counterpart (34 %). This biofilm design offers advantages like simplified, cost-effective operation, easy biomass recovery, and reduced water usage.

Syuzevskoye diamond placer: A new stage in studying the diamond potential of Western Urals

Research subject. Geological features of the structure and composition of the Syuzevskoye diamond placer in the Aleksandrovsky region of the Perm Krai. The placer was found in the border zone of two regional tectonic structures of the Urals: the Western Ural outer folding zone and the Central Ural uplift. This placer was developed within the erosion-karst (?), erosion-structural or tectonically determined Chikman-Nyarskaya depression. Materials and methods. The placer was identified in the period from 2017 to 2022 during a geological study of the subsoil area “Glubokiy” (License PEM 02687 KP LLC “Almining”). Mining and enrichment works were carried out according to the original enrichment scheme using the clamshell sampling method, heavy media and electromagnetic separation. The morphometric characteristics of several hundred diamonds were identified and described; reports were prepared; and reserves were assessed. Results. In the section of the Syuzevskoye placer, two diamond-bearing strata are clearly distinguished, separated by poor- and non-diamond-bearing deposits; anomalously high seam thickness for the Ural placers (up to 14-16 m) was established; high areal productivity of the placer (up to 0.7-1.2 ct/m2) and high diamond content (up to 0.5 ct/m3 per sample and an average of 0.09 ct/m3 for the placer). The placer is attributed to the type of placers of tectonic scarp zones. Reserves of alluvial diamonds weighing 430.5 thousand ct of the Syuzevskoye deposit are estimated at C1 + C2 categories in the area limited by the area between exploration lines 107-116 (size 1000 x 750 m); approved by the Minutes of the GKZ Rosnedra No. 7139 dated November 18, 2022 and will be increased. Certificate No. PEM 23 DRK 10147 confirming the discovery of a mineral deposit, issued by the Federal Agency for Subsoil Use of the Russian Federation to LLC Almining on March 29, 2023. Over 90% of diamonds are of gem quality. The average mass of crystals in the reserve calculation loop is 87 mg, and behind the loops is 29 mg. The leading size classes by mass -8 + 4 and -4 + 2 mm are 93.9% and 72.3% in quantity with an average mass of a diamond crystal of 99.9 mg. The shape is dominated by dodecahedroids (66.9%), less fragments of crystals with an unpreserved shape (14.9%) and fragments of crystals (13.1%); octahedrons (2.4%) and tetrahedroids (2.2%) are rare, rhombic dodecahedroids (0.4%) are rare. The preservation of crystals is good: full crystals make up 63.7%, fragments of crystals - 16.3%, fragments - 12.9%. Most of the crystals do not have wear (42.4%), or have weak wear of the tops and ribs (43.0%), medium and strong wear was noted only in 13.7% of the crystals. The average cost of diamonds is estimated at 361.59 USD per 1 ct. Conclusions. A unique deep-seated diamond deposit has been discovered, which is considered as a new type of deep-seated diamond-bearing placers in tectonic ledge zones. The geological criteria for identifying such diamond deposits in the Urals were determined. A new technological scheme for prospecting and exploration of deep-seated diamond placers within erosion-karst and tectonically determined depressions was developed, which is applicable for prospecting for precious metal placers.

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.

Mixotrophic cultivation of Galdieria sulphuraria for C-phycocyanin and protein production

G. sulphuraria is a polyextremophilic microalga that can tolerate low pH, high temperature and high osmotic pressure. We cultivated G. sulphuraria ACUF 064 in chemostat at a biomass concentration of 134 to 243 g·m−2 aiming for maximal pigment content without compromising biomass productivity. Autotrophy was compared to ‘oxygen balanced’ mixotrophy with intracellular recirculation of oxygen and carbon dioxide. No differences were found in C-phycocyanin (C-PC) and protein content between autotrophic and mixotrophic cultures. In mixotrophy the biomass productivity and concentration were doubled compared to the photoautotrophic counterpart. In mixotrophy aeration was not needed and 89% of the substrate carbon was converted into biomass.Mixotrophically grown biomass contained 10% w/w C-PC which, combined with its high areal biomass productivity (49 g·m−2·day−1), sums up as one of the highest C-PC areal productivities ever reported (5 g·m−2·day−1) under 24 h/24 h illumination. C-PC extracted from G. sulphuraria was more stable than the currently used C-PC extracts from Spirulina. No significant loss of color was observed down to a pH of 3 and up to a temperature of 55 °C. G. sulphuraria had a protein content of 62% w/w and compared favorably with FAO dietary recommendation of adults regarding amino acid composition. G. sulphuraria contains a high proportion of essential, sulfur amino acids compared to Chlorella, Spirulina and soybean protein.Due to its attractive amino acid profile and high protein content, G. sulphuraria is a good candidate for food and feed applications to overcome sulfur amino acid deficiencies. In addition, oxygen balanced mixotrophy allows for efficient and productive cultivation of G. sulphuraria biomass.

Prevention of algaculture contamination using pesticides for biofuel production

Compared to terrestrial biofuels, algal biofuels have the advantages of high areal productivity, high oil content and the ability to be cultivated using non-potable water on non-arable land. However, just like terrestrial agricultural crops, algae can be grazed by a wide variety of organisms, particularly in open pond systems. Algal grazers can be controlled using pesticides to make a more sustainable source of biofuels. In this study, we choose Chlorella kessleri as the model freshwater algal producer and select the freshwater rotifer, Brachionus calyciflorus and the freshwater ciliate, Colpoda sp. as the model algal grazers. The pesticides - quinine (QN), niclosamide (NC), 3-trifluoromethyl-4-nitrophenol (TFM), rotenone (ROT) and protons (pH) are used to inhibit algal grazers. C. kessleri is not affected by TFM and NC at concentrations up to 39 and 31 μM respectively, while inhibition of B. calyciflorus is achieved at concentrations of 18 and 14 μM respectively. Colpoda sp. is inhibited by NC at 8 μM, while, QN, TFM and ROT inhibit Colpoda sp. at 30 μM. A pH range between 8.4 and 10.3 inhibits B. calyciflorus and allows C. kessleri to grow well. Compared to previous results using ROT, the findings presented in this study demonstrate that algal grazers may be effectively controlled by using QN, NC and TFM yet may require higher doses than ROT.

Maximising the productivity of the attached cultivation of Ulva tepida in land-based systems

The viability of land-based seaweed cultivation is linked to high areal productivity of biomass and best suited for species that grow vegetatively and unattached. To develop optimised strategies for cultivation in land-based systems for attached species characterised by short growth cycles with periodic reproduction, ropes were seeded with zoids of the green seaweed Ulva tepida and stacked vertically in the water column from one to four layers deep. The layers spanned a culture depth from 50 to 350 mm below the surface of the water. After an initial grow-out period of 10 days, the biomass was continuously harvested on a rolling 4-day cycle which maintained the biomass in an actively growing state, over the 62-day cultivation period. Compared with the single layer treatment the vertical stacking of seeded ropes, in combination with frequent harvesting, increased the areal productivity of U. tepida by 40% with productivities >20 g dw m−2 day−1. Notably, productivities were similar between stacking treatments with two, three, or four layers due to a decreasing availability of light with an increasing number of layers and an increasing depth in the water column. Consequently, the most effective method of cultivation was to stack two layers of ropes in combination with the vertical movement of the layers through the water column with frequent harvesting to reduce the onset of reproduction.

Industrial production of Phaeodactylum tricornutum for CO2 mitigation: biomass productivity and photosynthetic efficiency using photobioreactors of different volumes

The photosynthetic efficiency (PE) and potential of Phaeodactylum tricornutum for CO2 mitigation in industrial tubular photobioreactors (PBRs) of different volumes were evaluated. A preliminary assay was performed at lab-scale to optimize the salt concentration of the culture medium. Interestingly, salinity did not affect the growth of P. tricornutum at concentrations of 2.5, 5, 10, and 20 g L−1. Higher volumetric productivities were achieved in the 2.5-m3 tubular PBR (0.235 g L−1 day−1), followed by 35- and 10-m3 PBRs. Maximum areal productivities corresponded to 48.5, 45.0, and 12.8 g m−2 day−1 for the 35-, 10-, and 2.5-m3 PBRs, respectively. PE was thus higher in the 35- and 10-m3 PBRs (2.21 and 2.08%, respectively). The 10- and 35-m3 PBR showed CO2 mitigation efficiencies of 60 and 41%, respectively, of the CO2 introduced into the PBR, corresponding to 2.3 and 2.5 g of fixed CO2 per g of biomass. Overall, cultivation of P. tricornutum couples high PE and areal productivity when the industrial PBRs were used, particularly PBRs of larger volumes. This improved PE performance with larger PBR volumes strongly suggests that large-scale cultivation of this diatom holds great potential for industrial CO2 mitigation.

Performance and emission analysis of single cylinder SI engine using bioethanol-gasoline blend produced from Salvinia Molesta

Rapid depletion of world’s crude oil reserve, rising global energy demand and concerns about greenhouse gases emission have led to the high-level interest in biofuels. The biofuel, bioethanol is found as an alternative fuel for SI engines as it has similar properties those of gasoline. Higher areal productivity with fast growth rate of microalgae and aquatic weeds makes them promising alternative feedstocks for bioethanol production. In this study, bioethanol produced from S.molesta (aquatic weed) using combined pre-treatment and hydrolysis followed by fermentation with yeast was used to make bioethanol-gasoline blend. The quantity of bioethanol produced from S.molesta was 99.12% pure. The physical properties such as density and heating value of bioethanol were 792.2 kg/m3 and 26.12 MJ/kg, respectively. In this work, the effects of bioethanol-gasoline (E5) fuel blends on the performance and combustion characteristics of a spark ignition (SI) engine were investigated. In the experiments, a single-cylinder, four-stroke SI engine was used. The tests were performed using electric dynamometer while running the engine at the speed (3200 rpm), and seven different load (0, 0.5, 1, 1.5, 2, 2.5 and 3 kW). The results obtained from the use of bioethanol-gasoline fuel blends were compared to those of gasoline fuel. The test results showed an increase of 0.3% in brake thermal efficiency for E5. From the emission analysis, reduced emissions of 39 ppm unburned hydrocarbon, 1.55% carbon monoxide and 2% smoke opacity, respectively was observed with E5 at full load. An increase in CO2 by 0.17% and NOx by 86.7 ppm was observed for E5 at full load.

Optimization of lipid extraction from Salvinia molesta for biodiesel production using RSM and its FAME analysis.

The higher areal productivity and lipid content of microalgae and aquatic weed makes them the best alternative feedstocks for biodiesel production. Hence, an efficient and economic method of extracting lipid or oil from aquatic weed, Salvinia molesta is an important step towards biodiesel production. Since Salvinia molesta is an unexplored feedstock, its total lipid content was first measured as 16% using Bligh and Dyer's method which was quite sufficient for further investigation. For extracting more amount of lipid from Salvinia molesta, methanol: chloroform in the ratio 2:1 v/v was identified as the most suitable solvent system using Soxhlet apparatus. Based on the literature and the preliminary experimentations, parameters such as solvent to biomass ratio, temperature, and time were identified as significant for lipid extraction. These parameters were then optimized using response surface methodology with central composite design, where experiments were performed using twenty combinations of these extraction parameters with Minitab-17 software. A lipid yield of 92.4% from Salvinia molesta was obtained with Soxhlet apparatus using methanol and chloroform (2:1 v/v) as solvent system, at the optimized conditions of temperature (85°C), solvent to biomass ratio (20:1), and time (137min), whereas a predicted lipid yield of 93.5% with regression model. Fatty acid methyl ester (FAME) analysis of S. molesta lipid using gas chromatograph mass spectroscopy (GCMS) with flame ionization detector showed that fatty acids such as C16:0, C16:1, C18:1, and C18:2 contributed more than 9% weight of total fatty acids. FAME consisted of 56.32, 28.08, and 15.59% weight of monounsaturated, saturated, and polyunsaturated fatty acids, respectively. Higher cetane number and superior oxidation stability of S. molesta FAME could be attributed to its higher monounsaturated content and lower polyunsaturated content as compared to biodiesels produced from C. vulgaris, Sunflower, and Jatropha.

The Selective Use of Hypochlorite to Prevent Pond Crashes for Algae-Biofuel Production.

Although algae-biofuels have many advantages including high areal productivity, algae can be preyed upon by amoebas, protozoans, ciliates, and rotifers, particularly in open pond systems. Thus, these higher organisms need to be controlled. In this study, Chlorella kessleri was used as the algal culture and Brachionus calyciflorus as the source of predation. The effect of sodium hypochlorite (bleach) was tested with the goal of totally inhibiting the rotifer while causing minor inhibition to the alga. The 24-hr LC(50) for B. calyciflorus in spring water was 0.198 mg Cl/L while the 24-hr LC(50) for C. kessleri was 0.321 mg Cl/L. However, chlorine dissipates rapidly as the algae serves as reductant. Results showed a chlorine dosage between 0.45 to 0.6 mg Cl/L and a dosing interval of two hours created the necessary chlorine concentrations to inhibit predation while letting the algae grow; thus giving algae farmers a tool to prevent pond crashes.

Pavlova lutheri is a high-level producer of phytosterols

Plant sterols are well-known for their ability to reduce low density lipoprotein (LDL)-cholesterol and promote cardiovascular health, as well as anti-inflammatory, anti-atherogenicity, anti-cancer and anti-oxidative activities and protection against nervous system disorders. As considerable amounts of phytosterols are present in microalgae, we screened out of hundreds of Australian isolates, ten microalgal species for sterol profiles and total sterol content. The top three sterol producers at 0.4–2.6% dry weight were Pavlova lutheri, Tetraselmis sp. M8 and Nannochloropsis sp. BR2. To further increase sterol accumulation, the highest sterol producer, P. lutheri was subjected to nutrient and salinity changes. Although no significant immediate effects were observed for altered total sterol contents, individual sterols varied and, significantly higher total sterol amounts (up to a 2-fold increase) were found after prolonged cultivation. Total sterol accumulation of 5.1% dry weight was achieved in P. lutheri. Combined with the high areal productivity, commercial phytosterol production from microalgae could be considered.

Taking advantage of rotifer sensitivity to rotenone to prevent pond crashes for algal-biofuel production

Although algae-biofuels have many advantages including high areal productivity, algae can be preyed upon by amoeba, ciliates, and rotifers, particularly in open pond systems. Thus, these algal predators need to be controlled. In this study, Chlorella kessleri was used as the algae and the freshwater rotifer Brachionus calyciflorus as the predator. The effect of the pesticide, rotenone, was tested with the goal of totally inhibiting the rotifer while causing only minor inhibition to the alga. The rotenone 24-hr LC50 for B. calyciflorus was 0.074μM while C. kessleri growth was uninhibited at 0.8μM. The rotenone EC50 for population growth of the marine alga Tetraselmis suecica was 0.56μM, in contrast to 0.13 and 0.26μM for the marine rotifers Brachionus rotundiformis and Brachionus manjavacas, respectively. Rotifer reproduction was inhibited at even lower rotenone concentrations than those causing mortality. In co-culture, a rotenone concentration of 0.1μM caused near complete B. calyciflorus mortality while leaving C. kessleri growth unaffected. Rotenone applications to produce 1milliongallons of biodiesel in one year are estimated to cost $842. This provides algae farmers an important and inexpensive tool to manage grazing predators in algae mass cultures and avoid pond crashes.

The Selective Use of Hypochlorite to Prevent Pond Crashes for Algae-Biofuel Production.

Although algae-biofuels have many advantages including high areal productivity, algae can be preyed upon by amoebas, protozoans, ciliates, and rotifers, particularly in open pond systems. Thus, these higher organisms need to be controlled. In this study, Chlorella kessleri was used as the algal culture and Brachionus calyciflorus as the source of predation. The effect of sodium hypochlorite (bleach) was tested with the goal of totally inhibiting the rotifer while causing minor inhibition to the alga. The 24-hr LC50 for B. calyciflorus in spring water was 0.198 mg Cl/L while the 24-hr LC50 for C. kessleri was 0.321 mg Cl/L. However, chlorine dissipates rapidly as the algae serves as reductant. Results showed a chlorine dosage between 0.45 to 0.6 mg Cl/L and a dosing interval of two hours created the necessary chlorine concentrations to inhibit predation while letting the algae grow; thus giving algae farmers a tool to prevent pond crashes. Water Environ. Res., 87 (2015).

Higher biomass productivity of microalgae in an attached growth system, using wastewater.

Although most algae cultivation systems are operated in suspended culture, an attached growth system can offer several advantages over suspended systems. Algal cultivation becomes light-limited as the microalgal concentration increases in the suspended system; on the other hand, sunlight penetrates deeper and stronger in attached systems owing to the more transparent water. Such higher availability of sunlight makes it possible to operate a raceway pond deeper than usual, resulting in a higher areal productivity. The attached system achieved 2.8-times higher biomass productivity and total lipid productivity of 9.1 g m(-2) day(-1) and 1.9 g m(-2) day(-1), respectively, than the suspended system. Biomass productivity can be further increased by optimization of the culture conditions. Moreover, algal biomass harvesting and dewatering were made simpler and cheaper in attached systems, because mesh-type substrates with attached microalgae were easily removed from the culture and the remaining treated wastewater could be discharged directly. When the algal biomass was dewatered using natural sunlight, the palmitic acid (C16:0) content increased by 16% compared with the freeze-drying method. There was no great difference in other fatty acid composition. Therefore, the attached system for algal cultivation is a promising cultivation system for mass biodiesel production.

Emergy analysis of oil production from microalgae

The use of microalgae and cyanobacteria for the production of biofuels and other raw materials is considered a very promising sustainable technology due to the high areal productivity, potential for CO2 fixation and use of non-arable land. The production of oil by microalgae in a large scale plant was studied using emergy analysis. The joint transformity calculated for the base scenario was 1.32E + 5 sej/J, the oil transformity was 3.51E + 5 sej/J, the emergy yield ratio (EYR) was 1.09 and environmental loading ratio was 11.10 and the emergy sustainability index (ESI) was 0.10, highlighting some of the key challenges for the technology such as high energy consumption during harvesting, raw material consumption and high capital and operation costs. Alternatives scenarios and the sensitivity to process improvements were also assessed, helping prioritize further research based on sustainability impact.

Isolation and Evaluation of Oil-Producing Microalgae from Subtropical Coastal and Brackish Waters

Microalgae have been widely reported as a promising source of biofuels, mainly based on their high areal productivity of biomass and lipids as triacylglycerides and the possibility for cultivation on non-arable land. The isolation and selection of suitable strains that are robust and display high growth and lipid accumulation rates is an important prerequisite for their successful cultivation as a bioenergy source, a process that can be compared to the initial selection and domestication of agricultural crops. We developed standard protocols for the isolation and cultivation for a range of marine and brackish microalgae. By comparing growth rates and lipid productivity, we assessed the potential of subtropical coastal and brackish microalgae for the production of biodiesel and other oil-based bioproducts. This study identified Nannochloropsis sp., Dunaniella salina and new isolates of Chlorella sp. and Tetraselmis sp. as suitable candidates for a multiple-product algae crop. We conclude that subtropical coastal microalgae display a variety of fatty acid profiles that offer a wide scope for several oil-based bioproducts, including biodiesel and omega-3 fatty acids. A biorefinery approach for microalgae would make economical production more feasible but challenges remain for efficient harvesting and extraction processes for some species.

PHOTOPHYSIOLOGY OF A TURF ALGAL COMMUNITY: INTEGRATED RESPONSES TO AMBIENT LIGHT AND STANDING BIOMASS(1).

This study investigated the variation in the relationship between photosynthesis and ambient light (P-E curves) for turf algal communities on a temperate reef off the coast of South Australia, analyzing the integrated effects of ambient light and standing biomass. The photophysiology of turfs was studied in situ on a seasonal basis, examining algal communities growing on artificial substrate (plates) at depths of 4 m and 10 m. P-E curves and estimates for the photokinetic parameters (Pm , Rd , α, Ek , and Ec ) were obtained through oxygen evolution methods, using an automated underwater respirometer. Photoacclimation responses to changes in ambient light were strongly affected by the biomass of the community. Pm showed an inverse relationship to standing biomass, irrespective of depth and season, which was considered to be a response to self-shading and boundary layer effects. Biomass effects imposed a high variance on estimates for all photosynthetic parameters, overshadowing differences observed for season and depth. Biomass also affected photoinhibition on turf communities, where significant afternoon depression of photosynthesis was observed in sparse turf patches when compared to denser patches. High areal productivity rates were maintained across all seasons with a significant decrease only being observed during winter.

Optimization of a flat plate glass reactor for mass production of Nannochloropsis sp. outdoors

The relationships between areal (g m −2 per day) and volumetric (g l −1 per day) productivity of Nannochloropsis sp. as affected by the light-path (ranging from 1.3 to 17.0 cm) of a vertical flat plate glass photobioreactor were elucidated. In general, the shorter the length of the light-path (LP), the smaller the areal volume and the higher the volumetric productivity. The areal productivity in relation to the light-path, in contrast, yielded an optimum curve, the highest areal productivity was obtained in a 10 cm LP reactor, which is regarded, therefore, optimal for mass production of Nannochloropsis. An attempt was made to identify criteria by which to assess the efficiency of a photobioreactor in utilizing strong incident energy. Two basic factors which relate to reactor efficiency and its cost-effectiveness have been defined as (a) the total illuminated surface required to produce a set quantity of product and (b) culture volume required to produce that quantity. As a general guide line, the lower these values are, the more efficient and cost-effective the reactor would be. An interesting feature of this analysis rests with the fact that an open raceways is as effective in productivity per illuminated area as a flat-plate reactor with an optimal light path, both cultivation systems requiring ca. 85 m 2 of illuminated surface to produce 1 kg dry cell mass of Nannochloropsis sp. per day. The difference in light utilization efficiency between the two very different production systems involves three aspects — first, the open raceway requires ca. 6 times greater volume than the 10 cm flat plate reactor to produce the same quantity of cell-mass. Second, the total ground area (i.e. including the ground area between reactors) for the vertical flat plate reactor is less than one half of that occupied by an open raceway, indicating the former is more efficient, photosynthetically, compared with the latter. Finally, the harvested cell density is close to one order of magnitude higher in the flat plate reactor, which carries economic significance. The advantage of vertical lamination of photoautotrophic cells provided by vertical plate reactors, is thereby clearly seen. The optimal population density (i.e. which results in the highest areal productivity) in the 10 cm plate reactor was obtained by a daily harvest of 10% of culture volume, yielding an annual average of ca. 12.1 g dry wt. m −2 per day (on the basis of the overall illuminated reactor surfaces, i.e. front and back) or 240 mg l −1 per day.

Productivity of Enewetak Atoll reef flats predicted from mass transfer relationships

The productivity of a coral reef flat at Enewetak Atoll is calculated by assuming phosphate (P) uptake is mass transfer limited, and then scaling P uptake to carbon (C) fixation. The approach is to treat the coral reef flat as a rough surface in fully-developed turbulent flow. Input parameters to the equations are water velocity, depth, roughness height of the reef, P concentration and the C:P ratio of autotrophs. A 9-day continuous record of water velocity and depth over the windward reef flats of Enewetak Atoll is used to compare an estimate of productivity from the model to the measured productivity. The calculated productivity is 542 ± 79mmol C m −2 day −1 and the measured productivity is 500 mmol C m −2 day −1. Although there are few experiments to verify empirical scaling of engineering equations to reef communities, these results indicate that neither alternative external supplies of P, nor biological mechanisms of recycling appear to be required to support the high areal productivity of the Enewetak reef flats. A high flow rate of tropical surface water across reef flats can account for the elevated productivity of these communities.