Effect of light intensity and ammonium-N on carotenogenesis ofTrentepohlia odorata andDunaliella bardawil

AxenicTrentepohlia odorata was cultured at three different NH₄Cl levels (3.5 × 10^–2, 3.5 × 10^–3, 3.5 × 10^–4 M) and three different light intensities (48, 76, 122 µmol m^–2 s^–1). Chloride had no effect on growth over this range of concentration. High light intensity and high NH₄Cl concentration enhanced the specific growth rate. The carotenoid content increased under a combination of high light intensity and low N concentration. WhenD. bardawil was exposed to the same combination of growth conditions, there was an increase in its carotenoid content. The light saturation and the light inhibition constants (K _s andK _i, respectively) for growth, and the saturation constant (K _m) for NH₄Cl were determined. TheK _s andK _i values were higher inT. odorata (66.7 and> 122 mol m^–2 s^–1, respectively) than inD. bardawil (5.1 and 14.7 µmol m^–2 s^–1, respectively). TheK _m value determined at 122 µmol m^–2 s^–1, however, was lower inT. odorata (0.048 µM) than inD. bardawil (0.062 µM).Author for correspondence     

Light-limited algal growth in Lake Loosdrecht: steady state studies in laboratory scale enclosures

Phytoplankton growth in the shallow, turbid Lake Loosdrecht (The Netherlands) is importantly influenced by light availability, and thus the concentrations of the various light-attenuating materials. The system is highly eutrophic and supports an algal biomass of ca. 160 mg Chl m^–3. A model is proposed here which predicts algal growth in the lake as a function of the light received and subsequent attenuation in the water column by phytoplankton, tripton and background colour. The model is based on an energy balance which relates growth rate to the true growth yield on light energy and the energy demand for cell maintenance. The coefficients for energy conversion (Y = 0.002 gDW kJ^–1) and cell maintenance (µ_e = 0.031 day^–1) were determined from steady state growth kinetics of Prochlorothrix hollandica in light-limited laboratory flow systems with the same depth as the lake and receiving summer average conditions of irradiance. Light attenuation by phytoplankton and tripton were quantified using specific attenuation coefficients: 0.011 m² mg^–1 Chl for the phytoplankton and 0.23 m² g^–1 DW for tripton.The growth studies demonstrated that Lake Loosdrecht can support a much higher algal biomass in the absence of non-algal particulate matter. The proposed model is used to predict chlorophyll a concentrations in dependence on growth rate and levels of tripton. Since approximately 75% of the sestonic dry weight in Lake Loosdrecht may be attributed to tripton, it is concluded that the algal biomass is markedly lowered by the abundance of tripton in the water column. A knowledge of the sources and fate of tripton in the lake is thus of fundamental importance in modelling phytoplankton dynamics.     

In situ nutrient assays of periphyton growth in a lowland Costa Rican stream

Nutrient limitation of primary production was experimentally assessed using an in situ bioassay technique in the Quebrada Salto, a third-order tropical stream draining the northern foothills of the Cordillera Central in Costa Rica. Bioassays employed artificial substrata enriched with nutrients that slowly diffuse through an agar-sand matrix (Pringle & Bowers, 1984). Multiple comparisons of regression coefficients, describing chlorophyll-a accrual through time for different nutrient treatments, revealed positive micronutrient effect(s). Micronutrient treatment combinations (Fe, B, Mn, Zn, Co, Mo, EDTA), supplemented with and without nitrate and phosphate, exhibited significantly greater chlorophyll-a accrual over all other treatments (P < 0.05), supporting over three times that of the control after 14-d of substratum colonization. Neither of the major nutrients (N or P) produced a significant stimulation, although the N treatment displayed 50% more chlorophyll-a than the control after 14-d. Similarly, Si, EDTA, and Si + N + P treatments did not exhibit chlorophyll-a response curves that were significantly different from the control. During the experiment, mean NH₄-N and (NO₂ + NO₃)-N concentrations in the Salto were 2.0 µM (28.6 µg · l^–1) and 7.2 µM (100.2 µg · l ^–1), respectively. High concentrations of PO₄-P ( = 2.0 µM; 60.9 µg · l^–1) and TP ( = 3.0 µM; 94.0 µg · l^–1) were also found, and consequently low molar N:P ratios = 4.7). Despite the potential for N limitation in the system, both N and P appear to be at growth saturating levels. This may be due to micronutrient limitation and/or light limitation of periphyton growth in densely shaded upstream portions of the stream.     

Biofiltering efficiency in removal of dissolved nutrients by three species of estuarine macroalgae cultivated with sea bass (Dicentrarchus labrax) waste waters 2. Ammonium

Three estuarine macroalgae (Ulva rotundata,Enteromorpha intestinalis, Gracilariagracilis) of economic potential were cultivated in the laboratory toassess their biofiltering capacities for ammonium in waste effluents from a seabass (Dicentrarchus labrax) cultivation tank. The studywasdeveloped to investigate the functioning of N nutrition of the three species.Atlow water flow (< 2 volumes d^–1) the three species strippedefficiently the ammonium dissolved in the waste water from the fish tank, withaminimum biofiltering efficiency estimate of 61% in unstarved cultures ofG. gracilis at a water flow of 2 volumesd^–1. Maximum velocity for ammonium uptake (89.0 molNH₄ ⁺ g^–1 dry wth^–1) was found in U. rotundata,whereas G. gracilis showed the highest affinity for thisnutrient. The net ammonium uptake rate was significantly affected by the waterflow, being greatest at the highest flow assayed (2 volumesd^–1). Variations of tissue N and C:N ratios during aflow-through experiment suggested that N was not limiting macroalgal growth.However, when ammonium was supplied at a flow rate of 0.5 volumesd^–1, specially in a three-stage design, the marked reductionintissue N and the biomass C:N:P ratios suggested a more general nutrientdeficiency. A significant correlation was found between growth rates and the Nbiomass gained in the cultures. The three-stage design under low water flow(0.5volumes d^–1) showed that the highest ammonium uptake rates (upto 80.9 mol NH₄ ⁺ g^–1 dry wtd^–1 in U. rotundata) were found inthe first stage, with decreasing rates in the following ones. As a result, lowincrements or even losses of total N biomass in these stages were found,suggesting that ammonium was excreted from the algae. We conclude that thesespecies present a potential ability to biofilter the ammonium dissolved inwastewater from a D. labrax cultivation tank, suggesting thatscaling up the biofiltration designs, future practises using these macroalgaemay be implemented in the local fish farms, resulting in both environmental andeconomical advantages.     

Ammonium uptake by Chondrus crispus Stackhouse (Gigartinales,Rhodophyta) in culture

Cultivated Chondrus crispus was used in N-NH₄ uptake experiments in the laboratory. An elevation of temperature increased the apparent rate of uptake, especially up to 11 °C. Uptake in the dark was found to be 83 % of that in the light. The apparent uptake decreased with increasing internal N pool; rates were 26.5, 22.2 and 20.2 µg N g dry wt^–1 min^–1 for internal N pools of 2.7, 3.5 and 4.6%, respectively. Apparent uptake increased with the substrate N concentration. The resulting curve has two components: an active uptake and a diffusion component at high (> 5000 µg N L^–1) external N levels. Ks and V _max were calculated by deducting the diffusion component from the uptake curve: these were of 497 µg N L ^–1 and 14.4 µg N g dry wt^–1 min^–1. respectively, and reflect a low substrate affinity. This could be the result of 10 years of continuous culture of C. crispus. Uptake was similarly followed in the culture tanks and showed comparable results; nighttime would be the most appropriate time to supply nutrients.     

Response of Audouinella saviana (Meneghini) Woelkerling (Nemaliales,Rhodophyta) cultures to monochromatic light

Audouinella saviana (Meneghini) Woelkerling was cultured at a constant temperature (24 °C) and different irradiances (from 1 µmol to 30 µmol photons m^–2 s^–1) of blue (430–470 nm) and green (500–560 nm) light in order to study its adaptive response. Modifications in colour, morphology and ultrastructure of the thalli, together with changes in pigment composition and in the spectral properties of chlorophyll a and R-phycoerythrin, were observed both by means of light and electron microscopy (TEM, SEM) and spectrophotometric and spectrofluorimetric analyses. In this paper we report the adaptive response of the seaweed to blue and green radiation by focussing on the cell wall and on the photosynthetic apparatus, particularly on phycobilisomes in situ and on R-PE after extraction. PBSs were fully structured only under blue light at low irradiance whilst they were absent under green light, whatever the irradiance, in spite of the high R-PE content. This fact, together with the spectral changes of R-PE, suggests adaptation at a molecular level, presumably referable to changes in aggregation state.     

Physiological responses of Pterocladia and Gelidium (Gelidiales,Rhodophyta) from the Azores,Portugal

Manometric studies were conducted on Pterocladia capillacea, Gelidium latifolium and Gelidium spinulosum from the Azores, Portugal to determine optimal values of temperature, light and salinity for growth. Physiological responses were considered in relation to vertical distribution patterns of these species commonly observed throughout the Azores. Optimal parameters for the growth of Pterocladia capillacea, Gelidium latifolium and G. spinulosum were 17 to 25 °C, a photon flux density between 200 and 300 µmol m^–2 s^–1 and salinities of 25 to 35.     

Combined effect of temperature and light intensity on growth and extracellular polymeric substance production by the cyanobacterium Arthrospira platensis

The effects of light intensity and temperature on Arthrospira platensis growth and production of extracellular polymeric substances (EPS) in batch culture were evaluated using a three-level, full-factorial design and response surface methodology. Three levels were tested for each parameter (temperature: 30, 35, 40°C; light intensity: 50, 115, 180 μmol photons m⁻² s⁻¹). Both growth and EPS production are influenced mainly by the temperature factor but the interaction term temperature*light intensity also had a significant effect. In addition, conditions optimising EPS production are different from those optimising growth. The highest growth rate (0.414 ± 0.003 day⁻¹) was found at the lowest temperature (30°C) and highest light intensity (180 μmol photons m⁻² s⁻¹) tested, no optima were detectable within the given test range. Obviously, optima for growth must be at a temperature lower than 30°C and a light intensity higher than 180 μmol photons m⁻² s⁻¹. For EPS production, light intensity had a positive linear effect (optimum obviously higher than 180 μmol photons m⁻² s⁻¹), but for the temperature parameter a maximum effect was detectable at 35°C.     

Ulva rigida (Ulvales,Chlorophyta) tank culture as biofilters for dissolved inorganic nitrogen from fishpond effluents

Ulva rigida was cultivated in 7501 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d^–1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m³ Tm^–1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the Ulva tank biofiltering system. Maximum yields (40 g DW m^–2 d^–1) were obtained at a density of 2.5 g FW 1^–1 and at a DIN inflow rate of 1.7 g DIN m^–2 d^–1. Maximum DIN uptake rates were obtained during summer (2.2 g DIN M^–2 d^–1), and minimum in winter (1.1 g DIN m^–2 d^–1) with a yearly average DIN uptake rate of 1.77 g DIN m^–2 d^–1 At yearly average DIN removal efficiency (2.0 g DIN m^–2 d^–1, if winter period is excluded), 153 m² of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish.Abbreviations DIN= dissolved inorganic nitrogen (NH _inf4 ^sup+ + NO _inf3 ^sup– + NO _inf2 ^sup– ); - FW= fresh weight; - DW= dry weight; - PFD= photon flux density; - V= DIN uptake rate     

Outdoor pond cultivation of the subtropical marine red algaGracilaria tenuistipitata in brackish water in Sweden. Growth,nutrient uptake,co-cultivation with rainbow trout and epiphyte control

Pond cultivation of the subtropical, euryhaline macroscopic red algaGracilaria tenuisipitata var.liui Zhanget Xia was carried out in brackish seawater (6–7) in the Gryt archipelago on the east coast of Sweden, using four outdoor tanks of 30–40 m³. Growth rate and nutrient uptake in batch culture were measured with the aim of estimating the water purification capacity ofG. tenuisipitata in outdoor conditions. Its ability to withstand epiphytic infections was also studied. An average growth rate of 4 biomass increase per day was recorded during two seasons with a maximum growth rate of 9 d^–1. The initial biomass was usually 1 kgFW m^–3 (FW, fresh weight). The nutrient uptake capacity was on average ca. 1 g N_i kgFW^–1 d^–1 and 0.08 g P_i kgFW^–1 d^–1 and the uptake rates for NH₄ ⁺-N were higher than those for NO₃ ^–-N. Both the growth rate and the nutrient uptake rate were highest at the highest water temperature. Co-cultivation with rainbow trout (Oncorhynchus mykiss) was tested: with trout fodder as the only nutrient inputG. tenuistipitata could grow and maintain low levels of N_i and P_i with optimum efficiency at a trout: alga ratio of 1:1 (w:w). Epiphytic growth of filamentous green and brown algae was limited, probably as a result of the high pH values caused by inorganic carbon uptake byG. tenuistipitata. The growth ofEnteromorpha intestinalis, the only significant epiphyte, was completely inhibited and the majority of plants died by a few days treatment with 100 µg 1^–1 Cu²⁺, a concentration that did not severely affectG. tenuistipitata. We conclude thatG. tenuistipitata can be cultivated in outdoor ponds in southern Sweden during 5–6 months of the year using aerated or unaerated batch cultures and that wastewater from trout cultivation may be used as a nutrient source, resulting in purification with respect to N and P.     

Decrease of polypeptides in the PS I antenna complex with increasing growth irradiance in the red alga Porphyridium cruentum

Thylakoids isolated from cells of the red alga Porphyridium cruentum exhibit an increased PS I activity on a chlorophyll basis with increasing growth irradiance, even though the stoichiometry of Photosystems I and II in such cells shows little change (Cunningham et al. (1989) Plant Physiol 91: 1179–1187). PS I activity was 26% greater in thylakoids of cells acclimated at 280 mol photons · m^–2 · s^–1 (VHL) than in cells acclimated at 10 mol photons · m^–2 · s^–1 (LL), indicating a change in the light absorbance capacity of PS I. Upon isolating PS I holocomplexes from VHL cells it was found that they contained 132±9 Chl/P700 while those obtained from LL cells had 165±4 Chl/P700. Examination of the polypeptide composition of PS I holocomplexes on SDS-PAGE showed a notable decrease of three polypeptides (19.5, 21.0 and 22 kDa) in VHL-complexes relative to LL-complexes. These polypeptides belong to a novel LHC I complex, recently discovered in red algae (Wolfe et al. (1994a) Nature 367: 566–568), that lacks Chl b and includes at least six different polypeptides. We suggest that the decrease in PS I Chl antenna size observed with increasing irradiance is attributable to changes occurring in the LHC I-antenna complex. Evidence for a Chl-binding antenna complex associated with PS II core complexes is lacking at this point. LHC II-type polypeptides were not observed in functionally active PS II preparations (Wolfe et al. (1994b) Biochimica Biophysica Acta 1188: 357–366), nor did we detect polypeptides that showed immunocross-reactivity with LHC II specific antisera (made to Chlamydomonas and Euglena LHC II).Abbreviations Bis-Tris bis(2-hydroxyethyl)imino-tris(hydroxymethyl)methane - DCPIP 2,6-dichlorophenol indophenol - -dm dodecyl--d-maltoside - HL high light of 150 mol photons · m^–2 · s^–1 - LGB lower green band - LHC I light-harvesting complex of PS I - LHC II light-harvesting complex of PS II - LL low light of 10 mol photons · m^–2 · s^–1 - ML medium light of 50 mol photons · m^–2 · s^–1 - MES 2-(N-morpholino) ethanesulfonic acid - P700 reaction center of PS I - PFD photon flux density - Trizma tris(hydroxymethyl)aminomethane - UGB upper green band - VHL very high light of 280 mol photons · m^–2 · s^–1     

Nitrate and phosphate removal by<Emphasis Type="Italic"> Spirulina platensis</Emphasis>

The cyanobacterium Spirulina platensis was used to verify the possibility of employing microalgal biomass to reduce the contents of nitrate and phosphate in wastewaters. Batch tests were carried out in 0.5 dm³ Erlenmeyer flasks under conditions of light limitation (40 mol quanta m^–2 s^–1) at a starting biomass level of 0.50 g/dm³ and varying temperature in the range 23–40°C. In this way, the best temperature for the growth of this microalga (30°C) was determined and the related thermodynamic parameters were estimated. All removed nitrate was used for biomass growth (biotic removal), whereas phosphate appeared to be removed mainly by chemical precipitation (abiotic removal). The best results in terms of specific and volumetric growth rates ( =0.044 day^–1, Q _x =33.2 mg dm^–3 day^–1) as well as volumetric rate and final yield of nitrogen removal ( =3.26 mg dm^–3 day^–1, =0.739) were obtained at 30°C, whereas phosphorus was more effectively removed at a lower temperature. In order to simulate full-scale studies, batch tests of nitrate and phosphate removal were also performed in 5.0 dm³ vessels (mini-ponds) at the optimum temperature (30°C) but increasing the photon fluence rate to 80 mol quanta m^–2 s^–1 and varying the initial biomass concentration from 0.25 to 0.86 g/dm³. These additional tests demonstrated that an increase in the inoculum level up to 0.75 g/dm³ enhanced both NO₃ ^– and PO₄ ^3– removal, confirming a strict dependence of these processes on biomass activity. In addition, the larger surface area of the ponds and the higher light intensity improved removal yields and kinetics compared to the flasks, particularly concerning phosphorus removal ( =0.032–0.050 day^–1, Q _x =34.7–42.4 mg dm^–3 day^–1, =3.24–4.06 mg dm^–3 day^–1, =0.750–0.879, =0.312–0.623 mg dm^–3 day^–1, and =0.224–0.440).     

In vitro culture of adult and juvenile bud explants of Passiflora species

Cultivar E23, an F1 hybrid of P. edulis and P. edulis f. flavicarpa is usually propagated by shoot-tip grafting. Various media were tested to evaluate the potential of E23 for in vitro propagation. Adult tissue was difficult to culture and did not respond to media containing low (<10 µM) concentrations of growth regulators. Growth of adult buds on intact stem sections was promoted by 1 week of dark incubation on MS basal medium plus 150 µM 2iP, 200 µM adenine sulphate and 17.1 µM IAA (3 mg l^–1), and further developed into shoots on MS medium plus 4.9 µM 2iP (1 mg l^–1) and 5.7 µM IAA (1 mg l^–1). By contrast, juvenile shoots of E23, and Passiflora species: edulis f. flavicarpa, edulis, alata, caerulea, mollissima, coccinea, herbertiana and suberosa grew rapidly on MS medium plus 10 µM kinetin and 5 µM IAA. Rapid multiplication was achieved on MS plus 20 µM BA, 10 µM kinetin, 5 µM IAA, and roots initiated on MS plus 5 µM IAA.Abbreviations IAA indole-3-acetic acid - 2iP N6-iso pentenyl adenine - BA N6-benzyl adenine     

Responses of strawberry plantlets cultured in vitro under superbright red and blue light-emitting diodes (LEDs)

Unrooted strawberry cv. `Akihime' shoots with three leaves obtained from standard mixotrophic cultures were cultured in the ``Culture Pack'-rockwool system with sugar-free MS medium under CO<sub>2</sub>-enriched condition. To examine the effect of superbright red and blue light-emitting diodes (LEDs) on in vitro growth of plantlets, these cultures were placed in an incubator, ``LED PACK', with either red LEDs, red LEDs1blue LEDs or blue LEDs light source. To clarify the optimum blue and red LED ratio, cultures were placed in ``LED PACK 3' under LED light source with either 100, 90, 80, or 70% red + 0, 10, 20, 30% blue, respectively, and also under standard heterotrophic conditions. To determine the effects of irradiation level, cultures were grown under 90% red LEDs + 10% blue LEDs at 45, 60 or 75 mol m^–2 s^–1 . Plantlet growth was best at 70% red + 30% blue LEDs. The optimal light intensity was 60 mol m^–2 s^–1. Growth after transfer to soil was also best after in vitro culture with plantlets produced were 70% red LEDs + 30% blue LEDs.     

Modelling continuous culture of Rhodospirillum rubrum in photobioreactor under light limited conditions

Rhodospirillum rubrum was grown continuously and photoheterotrophically under light limitation using a cylindrical photobioreactor in which the steady state biomass concentration was varied between 0.4 to 4 kg m^–3 at a constant radiant incident flux of 100 W m^–2. Kinetic and stoichiometric models for the growth are proposed. The biomass productivities, acetate consumption rate and the CO₂ production rate can be quantitatively predicted to a high level of accuracy by the proposed model calculations. Nomenclature: C _X, biomass concentration (kg m^–3) D, dilution rate (h^–1) Ea, mean mass absorption coefficient (m² kg^–1) I , total available radiant light energy (W m^–2) K, half saturation constant for light (W m^–2) R _W, boundary radius defining the working illuminated volume (m) r _X, local biomass volumetric rate (kg m^–3 h^–1) <r _X>, mean volumetric growth rate (kg m^–3 h^–1) V _W, illuminated working volume in the PBR (m^–3). Greek letters: , working illuminated fraction (–) _M, maximum quantum yield (–) bar, mean energetic yield (kg J^–1).     

Biofiltering efficiency in removal of dissolved nutrients by three species of estuarine macroalgae cultivated with sea bass (Dicentrarchus labrax) waste waters 1. Phosphate

The potential of three estuarine macroalgae (Ulvarotundata, Enteromorpa intestinalis andGracilaria gracilis) as biofilters for phosphate ineffluents of a sea bass (Dicentrarchus labrax) cultivationtank was studied. These seaweeds thrive in Cádiz Bay and were alsoselected because of their economic potential, so that environmental andeconomicadvantages may be achieved by future integrated aquaculture practices in thelocal fish farms. The study was designed to investigate the functioning of Pnutrition of the selected species. Maximum velocity of phosphate uptake (2.86mol PO₄ g^–1 dry wth^–1) was found in U. rotundata.This species also showed the highest affinity for this nutrient. At low flowrates (< 2 volumes d^–1), the three species efficientlyfiltered the phosphate dissolved in the waste water, with a minimum efficiencyof 60.7% in U. rotundata. Net phosphate uptake rate wassignificantly affected by the water flow, being greatest at the highest rateassayed (2 volumes d^–1). The marked decrease in tissue P shownby the three species during a flow-through experiment suggested that growth wasP limited. However, due to the increase in biomass, total P biomass increasedinthe cultures. A significant correlation was found between growth rates and thenet P biomass gained in the cultures. A three-stage design under low water flow(0.5 volumes d^–1) showed that the highest growth rates (up to0.14 d^–1) and integrated phosphate uptake rates(up to 5.8 mol PO₄ ^3– g^–1dry wt d^–1) were found in E.intestinalis in the first stage, with decreasing rates in thefollowing ones. As a result, phosphate become limiting and low increments oreven losses of total P biomass in these stages were found suggesting thatphosphate was excreted from the algae. The results show the potential abilityofthe three species to reduce substantially, at low water flow, the phosphateconcentration in waste waters from a D. labrax cultivationtank, and thus the quality of effluents from intensive aquaculture practices.     

Influence of environmental factors on the growth in culture of a New Zealand strain of the fast-spreading algaHydrodictyon reticulatum (water-net)

Hydrodictyon reticulatum (L.) Lagerh. is a recent addition to the New Zealand flora and is expanding its distribution rapidly. Proliferations of the alga now constitute an economic nuisance in waters which have not previously suffered filamentous algal blooms. To better understand the current and likely future spread of the alga and to identify possible management options the alga's growth requirements have been investigated. A strain isolated from New Zealand tolerated temperatures between 5 and 40 °C and salinities from 0 to 5. Optimal growth was at 25 °C, 150 mol photon m^–2s^–1 and in freshwater. Nett photosynthesis was saturated at photon flux densities of 100 and 160 mol m^–2s^–1 at 12 and 20 °C, respectively. Growth rate was linearly related to internal N concentration and hyperbolically to internal P concentration. Minimum cellular nutrient contents, by weight, were 1% N and 0.2% P. Growth was saturated at contents of 5% N and 0.5% P under the conditions of culture (20 °C, 150 mol photon m^–2s^–1). The alga maintained optimal cellular N content at low ambient nitrate concentrations (100 mg m^–3) half optimum content at 18 mg m^–3. Affinity for filtrable reactive phosphorus was not unusually high compared to other filamentous algae. We suggest that this alga is occupying a niche in New Zealand which has been precluded from other filamentous nuisance algae by low N concentration and N:P ratio. The significance of these findings in setting environmental targets for management of this nuisance alga is discussed.Author for correspondence     

A laboratory study of phosphorus and nitrogen excretion of Euchlanis dilatata lucksiana

Phosphorus (PO₄-P) and nitrogen (NH₄-N) excretion rates of Euchlanis dilatata lucksiana, a rotifer, isolated from Lake Loosdrecht (The Netherlands) and cultured in the lake water at 18–19 °C, were measured in the laboratory.In a series of experiments, the effects of experiment duration on the P and N excretion rates were examined. The rates measured in the first half-hour were about 2 times higher for P and 2–4 times for N than the rates in the subsequent three successive hours which were quite comparable.Eight experiments were carried out in triplicate, 4 each for P and N excretion measurements, using animals of two size ranges: 60–125 µm and > 125 µm. The specific excretion rates varied from 0.06 to 0.18 µg P.mg^–1 DW.h^–1 and 0.21 to 0.76 µg N.mg^–1 DW.h^–1. Generally an inverse relationship was observed between the specific excretion rates and the mean individual weight. The excretion rates of Euchlanis measured by us are lower than those reported for several other rotifer species, most of which are much smaller than Euchlanis.Extrapolating the excretion rates of Euchlanis to the other rotifer species in Lake Loosdrecht, and accounting for their density, size and temperature, rotifer excretion appears to be a significant, potential nutrient (N,P) source for phytoplankton growth in the lake. The excretion rates for the rotifers appear to be about two thirds of the total zooplankton excretion, even though the computed rotifer mean biomass is about one-third of the total zooplankton biomass.     

Light acclimation and photoinhibition in threeSpirulina platensis (cyanobacteria) isolates

Three isolates ofSpirulina platensis (Norst) Geitler marked BP, P4P and Z19/2 were compared with respect to their response and acclimation capability to high photon flux densities (HPFD). Cultures exposed to HPFD (1500–3500 mol photon m^–2 s^–1) exhibited a marked decrease in light-dependent O₂ evolution rate. P4P was more sensitive to HPFD than the two other isolates. All three isolates recovered from photoinhibition when placed under low PFD. The BP isolate was able to recover also in the dark but to a lower extent and at a lower rate, while no recovery was observed in the other two isolates under dark conditions. No recovery was observed when protein synthesis was inhibited using chloramphenicol. Cultures grown at 200 mol photon m^–2 s^–1 differed from cultures grown at 120 mol photon m 2 s-1 by their lower maximal photosynthetic rate (P _max ) and higher light saturation (I _k ) value, while being more resistant to HPFD stress. The ability ofSpirulina isolates to acclimate and withstand HPFD may provide useful information for the selection of strains useful for outdoor mass cultivation.Author for correspondence     

Photoinhibition as a control on photosynthesis and production of Sphagnum mosses

The effect of high light intensity on photosynthesis and growth of Sphagnum moss species from Alaskan arctic tundra was studied under field and laboratory conditions. Field experiments consisted of experimental shading of mosses at sites normally exposed to full ambient irradiance, and removal of the vascular plant canopy from above mosses in tundra water track habitats. Moss growth was then monitored in the experimental plots and in adjacent control areas for 50 days from late June to early August 1988. In shaded plots total moss growth was 2–3 times higher than that measured in control plots, while significant reductions in moss growth were found in canopy removal plots. The possibility that photoinhibition of photosynthesis might occur under high-light conditions and affect growth was studied under controlled laboratory conditions with mosses collected from the arctic study site, as well as from a temperate location in the Sierra Nevada, California. After 2 days of high-light treatment (800 mol photons m^–2 s^–1) in a controlled environmental chamber, moss photosynthetic capacity was significantly lowered in both arctic and temperate samples, and did not recover during the 14-day experimental period. The observed decrease in photosynthetic capacity was correlated (r ²=0.735, P<0.001) with a decrease in the ratio of variable to maximum chlorophyll fluorescence (F _v/F _m) in arctic and temperate mosses. This relationship indicates photoinhibition of photosynthesis in both arctic and temperate mosses at even moderately high light intensities. It is suggested that susceptibility to photoinhibition and failure to photoacclimate to higher light intensities in Sphagnum spp. may be related to low tissue nitrogen levels in these exclusively ombrotrophic plants. Photoinhibition of photosynthesis leading to lowered annual carbon gain in Sphagnum mosses may be an important factor affecting CO₂ flux at the ecosystem level, given the abundance of these plants in Alaskan tussock tundra.