Extracellular polysaccharide production in outdoor mass cultures of Porphyridium sp. in flat plate glass reactors

This work concerns an attempt to develop large scalecultivation of Porphyridium sp. outdoors. Theimpact on cell growth and production of solublesulphated polysaccharides of light-path length (LP)was studied in flat plate glass reactors outdoors. TheLP of the plate reactors ranged from 1.3–30 cm,corresponding to culture volumes of 3–72 L. The sidewalls of all reactors were covered, ensuring similarilluminated surfaces for all reactors. Maximal daytemperature was maintained at 26 ±1 ^°C.Growth conditions of pH (7.5), stirring (withcompressed air) and mineral nutrients, were optimal.Maximal volumetric concentration of the soluble sulfated polysaccharide (1.32 g L^-1) was obtained in winter with the smallest light-pathreactor (1.3 cm ) at a cell density of 1.37 ×10¹¹cells L^-1. Under these conditions, theviscosity of the culture medium was also highest,being inversely proportional to the culture'slight-path. Highest areal concentration of solublepolysaccharides (60 g m^-2) and areal cell density(3.01 × 10¹²m^-2) was recorded in the 20 cmLP reactor, progressively lower values being obtainedas the light path became shorter. A similar patternwas obtained for the areal productivity ofpolysaccharides, the highest being 4.15 g m^-2day^-1 (considering the total illuminated reactorsurface), produced in the 20-cm LP reactor.The main sugar composition (i.e. xylose, galactose andglucose) of the sulfated polysaccharides was similarin all reactors. As viscosity increased with timeduring culture growth, there was a substantial declinein bacterial population. Cultivation throughout mostof the year provided good evidence that a light pathlength of 20 cm in flat plate reactors under theseconditions is optimal for maximal areal solublepolysaccharide production of Porphyridium sp.     

Mass cultivation of Nannochloropsis sp. in annular reactors

A study was made on the mass cultivation of Nannochloropsis sp. in newly designed annular reactors operated under natural, artificial or combined illumination. The annular reactor consists of two 2-m-high Plexiglas cylinders of different diameter placed vertically one inside the other so as to form an annular culture chamber. Artificial illumination is supplied by lamps placed inside the inner cylinder. Two annular reactors of different diameter (50 and 91 cm), light path (4.5 and 3.0 cm) and illuminated surface area (5.3 and 9.3 m²) were experimented with. The effect of two different artificial light sources (fluorescent tubes and metal halide lamps) on culture productivity was investigated in both systems. The highest productivity on a per reactor basis (about 34 g (d. wt) reactor^–1 24 h^–1) was achieved with the larger reactor illuminated by a 400-W metal halide lamp. From February to May a 91-cm reactor illuminated only with natural light was operated in parallel with a 91-cm reactor subjected to combined illumination. Under natural illumination productivity increased from 16.6 g (d. wt) reactor^–1 d^–1 in February to 34.1 g (d. wt) reactor^–1 d^–1 in May. Under combined illumination productivity was 41.3 g (d. wt) reactor^–1 d^–1 in February and increased up to 48.3 g (d. wt) reactor^–1 d^–1 in May. Although the culture exposed to combined illumination always attained higher yields, the productivity gap between the two cultures decreased gradually along the season as solar radiation and minimum night temperatures increased. A 1200-L plant made of ten 50-cm annular reactors was set up and operated for two years with combined illumination yielding an average of 270 g of dry Nannochloropsis sp. biomass per day. More than 2000 L of concentrate suspension (50 g (d. wt) L^–1) of Nannochloropsis sp. were produced and successfully used by fish hatcheries as live feed for rotifers and for rearing seabream larvae with the green-water technique. This study indicates that the annular reactor can be profitably used for long-term cultivation of Nannochloropsis in temperate climates. Besides reliability and ease of operation, the main advantage of the system is that it can be used under natural illumination, yet artificial light can be also supplied to maintain high productivity levels in winter or on cloudy days.     

Optimizing the population density inIsochrysis galbana grown outdoors in a glass column photobioreactor

Particularly high population densities are readily sustainable in newly designed glass column reactors. The optimal density ofIsochrysis galbana in these columns in summer was 4.6 g L^–1 dry algal mass at which value the highest sustainable productivity obtained was a record of 1.6 g L^–1 d^–1. The population density exerted a direct effect on productivity: The higher the light intensity, the more pronounced was the dependence of the output rate on the population density, variations of 10%± from the optimal density resulting in a significant decline in productivity. The population density had also a very significant effect on the course of photoadaptation which took place during the first days after transferring the cultures from the laboratory to the outdoors. The output rate was lower by 5 to 35% on the first day of such transfer as compared to the light-adapted control. The higher the cell density, the faster was the process of photoadaptation as indicated by the rise of the productivity and O₂ tension to the control level. The potential for excess light damages was most prominent in the column reactors used, in which the light path was much reduced compared with that in open raceways. Significant photoinhibition took place at below optimal population density (2.8–3.8 g L^–1), and when cell density was further reduced (1.9 to 1.1 g L^–1), exposure to full sunlight caused photooxidative death within a few hours. The pattern of O₂ concentration in the culture that emerged along the day served as a useful indicator of photolimitation.Author for correspondence     

Characterization of growth and arachidonic acid production of Parietochloris incisa comb. nov (Trebouxiophyceae, Chlorophyta)

Arachidonic acid (AA) is a precursor of biologically activeprostaglandines and leukotrienes. The commercial source for AA at present is afungus, but the recently discovered coccoid green alga,Parietochlorisincisa comb. nov., in which over 90% of total AA is deposited intriacylglycerols, makes this species a potential candidate for commercialproduction of AA. We investigated the effect of the light-regime on cell-AAcontent and on culture productivity, by manipulating the intensity of the lightsource, the length of the light-path (LPL), and the population density ofcultures grown in flat plate glass reactors under both controlled laboratoryconditions (continuously illuminated) as well as outdoors. The effect ofnitrogen deprivation on culture content of AA and its productivity was alsostudied.In all experiments, the longer light-path reactors with the highest arealvolumes (L m^–2) yielded the highest culture-AA or the highestamount of AA harvested per illuminated reactor surface. Highest culture contentof AA was obtained in cultures exposed to strong light andnitrogen-deprivation.In contrast, highest cell-AA content was obtained in cultures receiving thelowest light-dose. Maximum culture content of AA obtained in the laboratory was2667 mg L^–1, reached after a 38-day growth period(of which the final 17 days took place in nitrogen-free medium), undercontinuous exposure to 2000 mol photon m^–2s^–1. Maximal culture content of AA outdoors wassignificantly lower compared with the maximum obtained in the laboratory.     

Effect of light-path length in outdoor flat plate reactors on output rate of cell mass and of EPA in Nannochloropsis sp.

The effect of light-path length (i.e. reactor width or thickness) of flat plate glass reactors on outdoor production of eicosapentaenoic acid (EPA) and cell mass of Nannochloropsis sp. was tested, using a range of light-paths from 1.3 to 17.0 cm. Volumetric productivity of cell mass and optimal, as well as maximal cell density which represents the highest sustainable cell density under the experimental conditions, decreased with increase in light-path. Daily areal output rate (g dry weight m⁻² day⁻¹) increased with increased light-path, in contrast with results obtained in similar reactors with Spirulina cultures, in which areal output rates increased when the light-path was reduced. Maximal areal productivity of Nannochloropsis sp. (12.8 and 22.4 g ash-free dry weight per day per m² of irradiated reactor surfaces, in winter and summer, respectively), reflecting maximal efficiency in light utilization, was obtained with the long light-paths, i.e. 10.4 and 17.0 cm. Increasing the light-path from 1.3 to 17.0 cm resulted in an increase in areal EPA productivity, from 66.7 to 278.2 mg m⁻² day⁻¹ in winter and from 232.1 to 515.7 mg m⁻² day⁻¹ in summer. This enhancement in areal productivity of EPA stems from increased productivity of cell mass which was associated with the increase in light-path. We concluded that the optimal light-path, which must be defined for each algal species, represents an important parameter which determines optimal culture density (i.e. resulting in the highest output rate of cell mass per irradiated reactor surface), as well as productivity of cell mass and cell products. Under our conditions the optimal light-path for culturing Nannochloropsis in vertical reactors was ca 10 cm.     

Growth characteristics of ultrahigh-density microalgal cultures

The physiological characteristics of cultures of very high cell mass (e.g. 10 g cell mass/L), termed “ultrahigh cell density cultures” is reviewed. A close relationship was found between the length of the optical path (OP) in flat-plate reactors and the optimal cell density of the culture as well as its areal (g m⁻² day⁻¹) productivity. Cell-growth inhibition (GI) unfolds, as culture density surpasses a certain threshold. If it is constantly relieved, a 1.0 cm OP reactor could produceca. 50% more than reactors with longer OP,e.g. 5 or 10 cm. This unique effect, discovered by Hu et al. [3], is explained in terms of the relationships between the frequency of the light-dark cycle (L-D cycle), cells undergo in their travel between the light and dark volumes in the reactor, and the turnover time of the photosynthetic center (PC). In long OP reactors (5 cm and above) the L-D cycle time may be orders of magnitude longer than the PC turnover time, resulting in a light regime in which the cells are exposed along the L-D cycle, to long, wasteful dark periods. In contrast, in reactors with an OP ofca. 1.0 cm, the L-D cycle frequency approaches the PC turnover time resulting in a significant reduction of the wasteful dark exposure time, thereby inducing a surge in photosynthetic efficiency. Presently, the major difficulty in mass cultivation of ultrahigh-density culture (UHDC) concerns cell grwoth inhibition in the culture, the exact nature of which is awaiting detailed investigation.     

n-3 Polyunsaturated fatty acid productivity of the marine microalgaIsochrysis galbana. Growth conditions and phenotypic selection

Two set of isolates were obtained in an isolation/selection programme to select eicosapentaenoic acid (EPA) and/or docosahexaenoic acid (DHA) rich strains ofIsochrysis galbana. EPA content was improved up to an average of 5.3% (d.wt) for the second set of isolates. On the other hand, with the aeration rate, pH and irradiance kept at low levels, the growth rate was slow and EPA synthesis was enhanced, but productivity increased when growth rates were maximum. A model relating steady-state dilution rates in chemostat cultures ofI. galbana to internal average irradiance is proposed. The greatest productivities were obtained between 0.0295 h^–1 and 0.0355 h^–1 with 300 mg m^–3 h^–1 for EPA and 130 mg m^–3 h^–1 for DHA.     

Variation in some photosynthetic characteristics of microalgae cultured in outdoor thin-layered sloping reactors

In outdoor thin-layer sloping reactors algae are batch cultured and harvested at biomass concentrations of about 15 g (dw) I^-1 whereafter a portion is used as inoculum for the next cycle. Light saturation curves of the oxygen evolution (PII curves) of the algae were measured using diluted aliquots of suspension taken from the reactors. The maximum specific photosynthetic rates (P ^B _max) and the light intensity at the onset of saturated photosynthesis (I _k ) decreased whilst the maximum specific photosynthetic efficiency ( ^B ) increased with an increase in the biomass concentration, during the production cycle. These differences reflect transition from light- to dark-acclimated state of the algae that occurs as a result of an increase of the suspension concentration during the production cycle. During these experiments the thin-layered smooth sloping cultures (TLSS, culture depth 5–7 mm) had higher photosynthetic rates per volume than the thin-layered baffled sloping cultures (TLBS, culture depth 5–15 mm). This was ascribed to the higherP ^B _max values of the algae grown in the TLSS cultures, allowing them to utilise high incident irradiancies more effectively. However, the areal productivity of the TLBS was higher than the TLSS indicating a higher photosynthetic efficiency of the TLBS reactors. The specific productivity decreased rapidly with an increase in the biomass concentration, but the yield remained linear during the batch production cycle, even at high areal densities.     

The optimal growth conditions for the biomass production of Isochrysis galbana and the effects that phosphorus, Zn2+, CO2, and light intensity have on the biochemical composition of Isochrysis galbana and the activity of extracellular CA

The effects of phosphorus, Zn²⁺, CO₂, and light intensity on growth, biochemical composition, and the activity of extracellular carbonic anhydrase (CA) in Isochrysis galbana were investigated. A significant change was observed when the concentration of phosphorus in the medium was increased from 5 μmol/L to 1000 μmol/L affecting I. galbana’s cell density, biochemical composition, and the activity of extracellular CA. Phosphorous concentration of 50 μmol/L to 500 μmol/L was optimal for this microalgae. The Zn²⁺ concentration at 10 μmol/L was essential to maintain optimal growth of the cells, but a higher concentration of Zn²⁺ (≥ 1000 μmol/L) inhibited the growth of I. galbana. High CO₂ concentrations (43.75 mL/L) significantly increased the cell densities compared to low CO₂ concentrations (0.35 mL/L). However, the activity of extracellular CA decreased significantly with an increasing concentration of CO₂. The activity of extracellular CA at a CO₂ concentration of 43.75 mL/L was approximately 1/6 of the activity when the CO₂ concentration was at 0.35 mL/L CO₂. Light intensity from 4.0 mW/cm² to 5.6 mW/cm² was beneficial for the growth, biochemical composition and the activity of extracellular CA. The lower and higher light intensity was restrictive for growth and changed its biochemical composition and the activity of extracellular CA. These results indicate that phosphorus, Zn²⁺, CO₂, and light intensity are important factors that impact growth, biochemical composition and the activity of extracellular CA in I. galbana.     

Continuous ethanol production byZymomonas mobilis andSaccharomyces cerevisiae in biofilm reactors

Continuous ethanol fermentations were performed in duplicate for 60 days withZymomonas mobilis ATCC 331821 orSaccharomyces cerevisiae ATCC 24859 in packed-bed reactors with polypropylene or plastic composite-supports. The plastic composite-supports used contained polypropylene (75%) with ground soybean-hulls (20%) and zein (5%) forZ. mobilis, or with ground soybean-hulls (20%) and soybean flour (5%) forS. cerevisiae. Maximum ethanol productivities of 536 gL^–1 h^–1 (39% yield) and 499 gL^–1 h^–1 (37% yield) were obtained withZ. mobilis on polypropylene and plastic composite-supports of soybean hull-zein, respectively. ForZ. mobilis, and optimal yield of 50% was observed at a 1.92h^–1 dilution rate for soybean hull-zein plastic composite-supports with a productivity of 96gL^–1h^–1, whereas with polypropylene-supports the yield was 32% and the productivity was 60gL^–1h^–1. With aS. cerevisiae fermentation, the ethanol production was less, with a maximum productivity of 76gL^–1h^–1 on the plastic composite-support at a 2.88h^–1 dilution rate with a 45% yield. Polypropylene-support bioreactors were discontinued due to reactor plugging by the cell mass accumulation. Support shape (3-mm chips) was responsible for bioreactor plugging due to extensive biofilm development on the plastic composite-supports. With suspensionculture continuous fermentations in continuously-stirred benchtop fermentors, maximum productivities of 5gL^–1h^–1 were obtained with a yield of 24 and 26% withS. cerevisiae andZ. mobilis, respectively. Cell washout in suspensionculture continuous fermentations was observed at a 1.0h^–1 dilution rate. Therefore, for continuous ethanol fermentations, biofilm reactors out-performed suspension-culture reactors, with 15 to 100-fold higher productivities (gL^–1h^–1) and with higher percentage yields forS. cerevisiae andZ. mobilis, respectively. Further research is needed with these novel supports to evaluate different support shapes and medium compositions that will permit medium flow, stimulate biofilm formation, reduce fermentation costs, and produce maximum yields and productivities.This is Journal Paper No. J-16357 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 3253     

Outdoor cultivation ofArthrospira platensis during autumn and winter in temperate climates

Arthrospira (Spirulina) platensis M2 was grown outdoors in 50-mm diameter tubular reactors under the climatic conditions of central Italy (Florence) from September to December 1995 and in March 1996. Except for September, the cultures temperature was regulated. Mean productivities of 0.83, 0.44 and 0.61 g dry wt L^–1 d^–1 were achieved in autumn (September–October), winter (November–December) and March, respectively. In autumn and winter, the photosynthetic efficiency of the cultures and the degree of correlation between productivity and solar irradiance were significantly greater than in summer. The effect of cell density and aeration rate on productivity was evaluated in September. The productivity of cultures operated at high supra-optimal population density was about 30% less at high aeration rate (1.0 LL^–1 min^–1), and 50% less at standard aeration rate (0.17 LL^–1 min^–1), than that of control cultures kept at optimal population density and standard aeration rate. The reduction of productivity in high-density cultures was due to lower daylight output rates and higher night biomass losses (the latter were particularly relevant under standard aeration conditions). The main factor limiting productivity in closed reactors during autumn was the night temperature. Heating the cultures during daylight hours on sunny days did not cause any significant increase of the yields, since under sunlight the unheated cultures also reached the optimal temperature for growth early in the morning. On cloudy days, the day-time temperature of the unheated cultures remained well below the optimum, however this had only a limited effect on productivity since algal growth was mainly light-limited.     

Growth inhibition of the eight species of microalgae by growth inhibitor from the culture of Isochrysis galbana and its isolation and identification

A type of growth inhibitor was successfully isolated and purified from cell-free filtrates of cultural medium at the death phase of Isochrysis galbana, and its chemical structure was confirmed by the methods of FABMS, UV, ¹H-NMR, ¹³C-NMR and 2D NMR, which was 1-[hydroxyl-diethyl malonate]-isopropyl dodecenoic acid, C₂₂H₃₈O₇. The results showed that the growth-inhibitor strongly inhibited the growth of Isochrysis galbana, and the growth of the eight species of microalgae (Dunaliella salina, Platymonas elliptica, Chlorella vugralis, Nitzschia closterium, Chaetoceros muelleri, Chaetoceros gracilis, Nitzschia closterium minutissima, Phaeodactylum tricornutum) also could be regulated by the growth-inhibitor in a concentration-dependent manner. The further investigation found that the synthesis process of chlorophyll and protein in the cells of all test microalgae could be inhibited by the growth inhibitor, and the content of chlorophyll and protein significantly decreased.     

Photosynthetic characteristics of <Emphasis Type="Italic">Spirulina platensis</Emphasis> grown in commercial-scale open outdoor raceway ponds: what do the organisms tell us?

At least six important factors that determine productivity in mass algal cultures have been identified. These are (1) the culture depth or optical cross section, (2) turbulence, (3) nutrient content and supply, (4) cultivation procedure, (5) biomass concentration and areal density, and (6) photo-acclimation. Since the efficient capturing of light energy relates to high productivities and efficiencies, all potential losses and inefficiencies need to be managed and eliminated. Photoinhibition could reduce areal productivities by up to 30% and more, where photoinhibition is the decline in photosynthetic rates at supra-optimal irradiancies. It is, however, unclear whether this occurs in high density and turbulent mass algal cultures. Using chlorophyll a fluorescence, it was possible to show how the maximum quantum efficiency of dark adapted cells (Φ_max) decreased at midday conditions of high irradiancies. Neither photochemical (qP) nor, to a lesser extent, non-photochemical quenching (qN), could explain the midday depression. Using chlorophyll a fluorescence transient analyses it was shown that, although light absorption increased towards midday, the captured energy was essentially lost as heat dissipation. This was clearly shown in low-density cultures where the average light per cell was high, compared to denser cultures where the effects of high light exposure were significantly reduced. In low-density cultures, more than 60% of the reaction centres (RCs) became “silent”, meaning that they neither reduce Q_A, nor return their excitation energy to the antenna. At higher cell densities, losses due to photoinhibition and the number of “silent RCs” were much reduced. Elucidation of the relationship between active RCs and productivity should be a priority for optimising photobioreactor productivity. This paper was presented at the 10th International Conference of the International Society for Applied Phycology in Kunming, China.     

Ethanol production by immobilized whole cells ofZymomonas mobilis in a continuous flow expanded bed bioreactor and a continuous flow stirred tank bioreactor

Continuous ethanol fermentation by immobilized whole cells ofZymomonas mobilis was investigated in an expanded bed bioreactor and in a continuous stirred tank reactor at glucose concentrations of 100, 150 and 200 g L^–1. The effect of different dilution rates on ethanol production by immobilized whole cells ofZymomonas mobilis was studied in both reactors. The maximum ethanol productivity attained was 21 g L^–1 h^–1 at a dilution rate of 0.36 h^–1 with 150 g glucose L^–1 in the continuous expanded bed bioreactor. The conversion of glucose to ethanol was independent of the glucose concentration in both reactors.     

The effect of different algal densities of Scenedesmus acuminatus on the population growth of Moina micrura Kurz (Crustacea: Anomopoda,Moinidae)

Six densities (0.5 × 10⁶, 1.0 × 10⁶, 1.5 × 10⁶, 2.0 × 10⁶, 3.0 × 10⁶, and 4.0 × 10⁶ cells ml^–1) of the micro-alga Scenedesmus acuminatus, were fed to the cladoceran, Moina micrura, in 40-litre glass aquaria. Moina population increased with increasing cell densities of Scenedesmus only up to treatment 3 (i.e. 1.5 × 10⁶ cells ml^–1) where a peak population of 11303 individuals per litre was obtained. Moinapopulation growth was inhibited at higher algal densities. The percentage of egg-bearing females and the number of eggs per egg-bearing females, followed a similar pattern. Comparatively, the peak production density of approximately 11000 Moina per litre, is interesting from a mass production point of view and indicates that S. acuminatus is a satisfactory micro-alga food for M. micrura.     

Citric acid production and morphology of Aspergillus niger as functions of the mixing intensity in a stirred tank and a tubular loop bioreactor

The relationship between Aspergillus niger morphology and citric acid production was investigated in two reactor systems with different configurations, a tubular loop and a stirred tank bioreactor, with operating volumes of 6 and 8 dm³, respectively. Morphology was quantified by image analysis. In each system, morphology, characterized by the parameter P (mean convex perimeter of clumps), and citric acid production, were agitation-dependent and closely linked. Increased agitation caused a reduction of clump sizes and results when both reactors demonstrate that the parameter P should not exceed a threshold value in order to achieve increased productivities. The results obtained from the two reactors were in agreement, both qualitatively and quantitatively. Reducing the fundamentally different mixing conditions of the two bioreactors to the order of the dimensionless mixing parameter relative mixing time (τ_m), results showed that the loop simulated the stirred tank. Also, relationships valid for one system accurately described the results obtained from the other system, demonstrating the validity of the relationship between morphology and productivity for the particular fermentation, regardless of the reactor type. Previous attempts to evaluate the use of loop configurations as scale-up tools and their performance as bioreactors, neglected the morphology of the producer micro-organisms. This study demonstrated the close link between morphology and productivity for citric acid production by A. niger, and identified a morphology parameter that was used successfully to characterize the process performance.     

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.     

Light-path length and population density in photoacclimation of Nannochloropsis sp. (Eustigmatophyceae)

Photoacclimation in the marine eustigmatophyte Nannochlropsis sp., used extensively as a food chaincomponent in aquaculture, was studied both in thelaboratory and outdoors. Cell-chlorophyll andcarotenoids were used as markers to assessphotoacclimation to strong light, as well as todecreasing growth irradiance due to cellproliferation. Focusing on practical aspects involvedin mass cultivation, three different approaches wereused as follows: (a) cultures initially exposed to lowlight (150 mol photon m^-2 s^-1) thentransferred to strong light (1000 to 3000 molphoton m^-2 s^-1); (b) initially low celldensity cultures grown in reactors of differentlight-paths, exposed to strong PFD, in the laboratoryand outdoors; (c) initially low or high cell densitycultures exposed to strong light. As has already beenestablished in many reports, cell-chlorophyllrepresented a sensitive parameter in assessing cellresponse to changes in the intensity of the lightsource as well as to modifications in the light regimeto which the cells were exposed. Cell-chlorophyllconcentration sharply decreased initially upontransferring the culture from low PFD cell^-1 tohigh PFD cell^-1 due to either culture dilution(i.e. decrease in cell density and mutual shading) orto an increase in PFD. After some 7 days ofphotoacclimating to 2000 and 3000 mol photonm^-2 s^-1, chlorophyll a content began to riseat a much faster rate than cell number, which alsoincreased in response to the higher irradiance.Cell-chlorophyll in the culture exposed to 2000mol photon m^-2 s^-1 increased afteracclimation earlier and at a faster rate than in theculture exposed to 3000 mol photon m^-2s^-1, indicating the later irradiance affected astronger stress. The length of the reactor's lightpath exerted a decisive effect on cell response tostrong light through its influence on the light regimein the culture. Upon a sharp increase in PFD,carotenoids in the 1-cm reactor increased in muchhigher rate than chlorophyll, compared with the 3-cmlight path reactors. This marked difference in cellresponse to a shift-up in light was attributed to thevast variations in the light regime associated withdifferences in the length of the light path and areal density. Growth oflow cell density cultures ceased temporarily upontransfer to strong light, in contrast with high celldensity cultures transferred to strong light, whichcontinued growth without a lag.     

The influence of soybean planting density on dinitrogen fixation and yield

We investigated the effect of planting density on soybean (Glycine max (L.) Merr.) yield in glasshouse and field experiments. Because net canopy photosynthesis increases with increasing plant density, we hypothesized that increasing planting density would result in increasing rates of dinitrogen fixation in soybeans and higher yields per unit land area.In glasshouse studies, Wayne variety soybeans were planted in 10-cm diameter pots, 1 plant pot^-1 in matrices of 10-, 15-, 20-, 25-, or 30-cm equidistant intervals. Bradyrhizobium japonicum inoculum was added to half of the plants in each treatment. Replicate measurements of total stem height, internode lengths, leaf mass, stem mass, root mass, nodule number, nodule mass, and nitrogenase activity were obtained at 3, 6, and 9 weeks post-emergence. Fruits were harvested and counted at week 14. As planting density increased there were (1) altered morphology and growth rates, (2) increased apparent specific nodule activity (SNA), (3) decreased nodule number and mass, and (4) nearly constant fruit and seed production/plant. Expressed on a unit area basis, nitrogen influx and yield increased geometrically as planting density increased, with maximum values observed for 10-cm plantings.Field studies of Wayne, Stein, Williams, and Gold Harvest soybean varieties were made in 1985. Plots were established containing 100 plants spaced at 10-, 20-, and 30-cm distances. Measurements made during the growing season and at harvest established the same relative trends identified from the glasshouse studies. Increasing plant densities resulted in higher yields per unit land. Varietal differences were almost significant.     

Feeding kinetics of Brachionus plicatilis fed Isochrysis galbana

Clearance and ingestion rates of Brachionus plicatilis were measured using ¹⁴C-labeled Isochrysis galbana Tahiti. Experiments were conducted at 20–22 °C, 20 ppt salinity, and algal concentrations ranging from 0.13–64 mg C 1^–1. Clearance rates were constant and maximal at concentrations <2 mg C 1^–1, with maximum rates ranging from 3.4–6.9 µl ind.^–1 hr^–1. The ingestion rate varied with food concentration, and was described by a rectilinear model. The maximum ingestion rate varied considerably, and was dependent on the growth rate of the rotifers. Depending on the pre-conditions, B. plicatilis ingested about 0.5 to 2 times its body carbon per day at saturating food concentrations.