CARBOHYDRATE RELEASE BY A SUBTROPICAL STRAIN OF SPONDYLOSIUM PYGMAEUM (ZYGNEMATOPHYCEAE): INFLUENCE OF NITRATE AVAILABILITY AND CULTURE AGING1

This paper describes the influence of nitrate availability on growth and release of dissolved free and combined carbohydrates (DFCHOs and DCCHOs) produced by Spondylosium pygmaeum (Cooke) W. West (Zygnematophyceae). This strain was isolated from a subtropical shallow pond, located at the extreme south of Brazil (Rio Grande, RS). Experiments were carried out in batch culture, comparing two initial nitrate levels (10/100 μM) in the medium. Growth was monitored by direct microscopic cell counts and chl a content. Nitrate consumption was determined by ion chromatography, while the production of extracellular carbohydrates was monitored by the phenol‐sulfuric method. The monosaccharide compositions of DFCHOs and DCCHOs were determined in each growth phase by HPLC with pulse amperometric detection (HPLC‐PAD). At the end of the experiment, the total composition of extracellular polysaccharide (EPS) molecules >12 kDa was determined by gas chromatography. Nitrate availability had no influence on S. pygmaeum cell density at any phase. On the other hand, chl a content decreased after a few days growth when the availability of nitrate was restricted, but continued to rise when nitrate was plentiful. Also, nitrate depletion was faster at 10 μM nitrate. No influence of the growth phase or nitrate availability on the total carbohydrates (TDCHOs) released per cell was observed. Only DCCHOs were released by S. pygmaeum, and the composition varied between growth phases, especially at lower nitrate availability. EPS molecules >12 kDa were composed mainly of xylose, fucose, and galactose, as for other desmids. However, a high N‐acetyl‐glucosamine content was found, uniquely among desmid EPSs.     

Positive effects of continuous low nitrate levels on growth and photosynthesis of Alexandrium tamarense (Gonyaulacales, Dinophyceae)

The growth and photosynthesis of Alexandrium tamarense (Lebour) Balech in different nutrient conditions were investigated. Low nitrate level (0.0882 mmol/L) resulted in the highest average growth rate from day 0 to day 10 (4.58 × 10² cells mL^?1 d^?1), but the lowest cell yield (5420 cells mL^?1) in three nitrate level cultures. High nitrate‐grown cells showed lower levels of chlorophyll a‐specific and cell‐specific light‐saturated photosynthetic rate (P_m^{chl a} and P_m^cell), dark respiration rate (R_d^chla and R_d^cell) and chlorophyll a‐specific apparent photosynthetic efficiency (α^chla) than was seen for low nitrate‐grown cells; whereas the cells became light saturated at higher irradiance at low nitrate condition. When cultures at low nitrate were supplemented with nitrate at 0.7938 mmol/L in late exponential growth phase, or with nitrate at 0.7938 mmol/L and phosphate at 0.072 mmol/L in stationary growth phase, the cell yield was drastically enhanced, a 7–9 times increase compared with non‐supplemented control culture, achieving 43 540 cells mL^?1 and 52 300 cells mL^?1, respectively; however, supplementation with nitrate in the stationary growth phase or with nitrate and phosphate in the late exponential growth phase increased the cell yield by no more than 2 times. The results suggested that continuous low level of nitrate with sufficient supply of phosphate may facilitate the growth of A. tamarense.     

Differences in the cellular dry weight per unit biovolume of <Emphasis Type="Italic">Phormidium autumnale</Emphasis> (Cyanobacteria) dependent on growth conditions

The influence of substrate, light intensity, temperature and growth phase on the dry weight per unit biovolume of both living Phormidium autumnale trichomes and living single cells was investigated microinterferometrically. With a Mach–Zehnder Interference Microscope, both the interference-stripe-field method and the phase-shift method were used to measure the optical path differences (OPD) of cells and trichomes. To calculate the cellular dry weight of trichomes, the trichome diameters have to be measured. Widths between 4 and 7 μm were determined. Thick trichomes are characteristic for growth on agar-solidified medium, whereas this was observed in single cases only from trichomes growing on soil surfaces. A reliable prediction of trichome width from growth conditions is not possible. The dry weights per unit biovolume (fg μm⁻³) are independent of the studied parameters during the exponential growth phase (296 ± 22 fg μm⁻³) with exception of the agar-based cultures growing at low light intensity (259 ± 16 fg μm⁻³). During the stationary phase, dry weights per unit biovolume increase independently of growth conditions (353 ± 39 fg μm⁻³). Two separate factors of 0.14 and 0.17 for converting biovolume (mm³) of cells to milligrams carbon could be determined by comparing the growth phase and stationary phase-dependent average values of dry weights per unit biovolume, respectively. These conversion factors could be used as species-specific factors for Phormidium growing on soil surfaces. Irrespective of the method, both the stripe-field and phase-shift method gave similar results. However, the phase-shift method measured lower variances of values. Additionally, detailed quantifying investigations of structures within cells are possible. Thus, the phase-shift method could be a powerful analytical tool in, e.g., ecotoxicological monitoring analyses.     

STRESS‐INDUCED FILAMENT FRAGMENTATION OF CALOTHRIX ELENKINII (CYANOBACTERIA) IS FACILITATED BY DEATH OF HIGH‐FLUORESCENCE CELLS1

Irradiance power and spectral composition as well as nutrient availability strongly influence differentiation of filamentous cyanobacteria. When monitoring the life cycle of Calothrix elenkinii Kossinsk., we found that low nitrogen concentration and growth under green light led to a transient appearance of high‐fluorescence cells that rapidly bleach and disintegrate, thus breaking the parental filament into shorter parts. The dynamics of the process were monitored in a microscope growth chamber by measuring transmission and chl fluorescence of individual cells by a high‐sensitivity camera. Typically, the high‐fluorescence cells appeared near the center of the parental trichome signaled by a rapid 2‐ to 3‐fold rise in their fluorescence emission. By measuring the fluorescence excitation spectra with resolution of individual cells, we showed that the elevated fluorescence emission was largely due to a high absorption by phycoerythrin and energy transfer to chl. Typically, after no more than 20 min, the fluorescence abruptly disappeared with transmission images, indicating loss of pigmentation. The bleaching was a natural process that was not caused by the measuring light. Depending on the mechanical strain, the cell bleaching was followed by breaking of the parental filament. We propose that the high‐fluorescence cells appear as a phase of programmed cell death, allowing the fragmented filaments to escape from unfavorable environmental conditions.     

TOXIN PROFILE,PIGMENT COMPOSITION,AND LARGE SUBUNIT RDNA PHYLOGENETIC ANALYSIS OF AN ALEXANDRIUM MINUTUM (DINOPHYCEAE) STRAIN ISOLATED FROM THE FLEET LAGOON,UNITED KINGDOM1

Paralytic shellfish toxins, pigment composition, and large subunit (LSU) rDNA sequence were analyzed for a clonal culture of Alexandrium minutum Halim isolated in 2000 from the coastal Fleet Lagoon, Dorset, United Kingdom. The HPLC pigment analysis revealed the presence of chl a, peridinin, and diadinoxanthin as major pigments and chl c₁+c₂ and c₃, diatoxanthin, and β‐carotene as minor components. The toxins responsible for paralytic shellfish poisoning were analyzed by HPLC with postcolumn derivatization and fluorescence detection. The paralytic shellfish poisoning toxin profile of the Fleet Lagoon strain of A. minutum in exponential growth phase was dominated by gonyautoxin‐3 up to 54%, whereas gonyautoxin‐2 made up 10% and saxitoxin (STX) 36%. The average toxicity of the culture was 3.8 pg STX Eq·cell^?1, and total toxin content varied from 5.6 fmol·cell^?1 on day 1 to a maximum of 16.8 fmol·cell^?1 during the early stationary phase. Sequence analysis of the LSU rDNA revealed the strain to be closely related to several European strains of A. minutum and one isolated from Australian waters, although most of these do not produce STX. The shallow Fleet Lagoon may provide a favorable environment for A. minutum to bloom, and the presence of highly potent saxitoxins in this strain indicates potential for future shellfish contamination.     

DIMETHYLSULFONIOPROPIONATE PRODUCTION BY PSYCHROPHILIC DIATOM ISOLATES1

Dimethylsulfoniopropionate (DMSP) production by psychrophilic diatom strains, Chaetoceros sp., Navicula sp., and Nitzschia sp., were experimentally analyzed. All strains showed rapid growth (0.3–0.5 d^?1) under cold culture conditions (1.7°C). DMSP concentrations (both as particulate, DMSP_p, and dissolved, DMSP_d) were negligible in cultures of Chaetoceros sp., whereas those for Navicula sp. and Nitzschia sp. increased toward the end of the 56‐day experiments. The ratio of DMSP_p:chl a of the latter two species was approximately 5 in the early exponential growth phase and decreased slightly toward the early stationary phase. During the stationary phase, when chl a and fluorescence remained constant, the ratios in both species increased linearly by up to approximately 6.5 times the value at the start of the stationary phase. This growth‐related DMSP production by diatoms may result in the low concentrations of DMSP_p observed in the early stage of diatom blooms under natural conditions.     

The production of extracellular carbohydrates by estuarine benthic diatoms: the effects of growth phase and light and dark treatment

Epipelic diatoms are important constituents of estuarine microphytobenthic biofilms. Field‐based investigations have shown that the production of carbohydrates by such taxa is ecologically important. However, limited information exists on the dynamics of carbohydrate production by individual species of epipelic diatoms. The production of low and high molecular weight extracellular carbohydrates in axenic cultures of five species of benthic estuarine diatoms, Cylindrotheca closterium (Ehrenberg), Navicula perminuta (Grun.) in Van Heurck, Nitzschia frustulum (Kütz.) Grunow, Nitzschia sigma (Kütz.) Grunow, and Surirella ovata (Kütz.) Grunow, were investigated. All species produced colloidal (water‐soluble) carbohydrates during growth, with maximal production occurring during stationary phase. During logarithmic growth, approximately 20% of extracellular carbohydrates consisted of polymeric material (extracellular polymeric substances [EPS]), but during stationary phase, EPS content increased to 34%–50%. Pyrolysis–mass spectrophotometry analysis showed differences in the composition of EPS produced during logarithmic and stationary phase. All species synthesized glucan as a storage carbohydrate, with maximum glucan accumulation during the transition from log to stationary phase. Short‐term labeling with ¹⁴C‐bicarbonate found that between 30 and 60% of photoassimilates were released as colloidal carbohydrate, with EPS consisting of approximately 16% of this colloidal fraction. When cells were placed in darkness, EPS production increased, and between 85 and 99% of extracellular carbohydrate produced was polymeric. Glucan reserves were utilized in dark conditions, with significant negative correlations between EPS and glucan for N. perminuta and S. ovata. Under dark conditions, cells continued to produce EPS for up to 3 days, although release of low molecular weight carbohydrates rapidly ceased when cells were dark treated. Three aspects of EPS production have been identified during this investigation: (1) production during rapid growth, which differs in composition from (2) EPS directly produced as a result of photosynthetic overflow during growth limiting conditions and (3) EPS produced for up to 3 days in the dark using intracellular storage reserves (glucans). The ecological implications of these patterns of production and utilization are discussed.     

Wachstum und N-Stoffwechsel sowie Anderungen des Isoenzymspektrums der Glutamatdehydrogenase (GDH) in batchvermehrten, licht- und dunkelkultivierten Zellsuspensionen von Pisum sativum

Growth, N-metabolism and isoenzyme pattern of glutamate dehydrogenase in batch-cultures of Pisum sativum cells under light and dark conditions. Cell suspension cultures of Pisum sativum L. derived from root and shoot sections of seedlings have been prepared and cultured in defined nutrient medium. Both the cells and the media were analysed daily for the N-fractions and carbohydrates during the growth period. The data obtained indicate specific correlations between growth and nitrogen and carbohydrate metabolism. At the beginning of the growth cycle ammonia as compared to nitrate was favoured in uptake. An increased uptake of nitrate occurred at the end of the linear growth phase when carbohydrate in the media was depleted. The uptake of sucrose was rapid during the whole growth cycle, only in the range of the linear growth phase the uptake stagnated for 3 or 4 days. During increased biosynthesis of nitrogenous compounds at the beginning of the growth cycle up to seven isoenzymes of the glutamate dehydrogenase could be separated by polyacrylamide gel electrophoresis. The isoenzyme pattern changed during the stationary growth phase, especially when the carbohydrate content in the medium decreased. There is some evidence that the isoenzyme pattern is influenced by carbohydrate metabolism.     

EFFECTS OF NITROGEN AND PHOSPHORUS AVAILABILITY ON THE EXPRESSION OF THE COCCOLITH‐VESICLE V‐ATPASE (SUBUNIT C) Of PLEUROCHRYSIS (HAPTOPHYTA)1

The coccolithophores Emiliania and Pleurochrysis demonstrate increased coccolith production when growth is reduced by nitrate or phosphate limitation. The function of enhanced coccolith production under these conditions and its regulation have not been resolved. Studies at the molecular level are ideally suited to determine the exact relationship between calcification and other cellular functions. In a previous study we provided evidence for the presence of a vacuolar H⁺‐ATPase on coccolith vesicle membranes of P. carterae. These trans‐Golgi–derived vesicles are the sites of coccolith production, with each vesicle containing one coccolith. In this study, expression of the vap gene, encoding subunit c of the vacuolar H⁺‐ATPase, was investigated. Our objective was to explore potential relationships between vap expression, nutrient‐dependent growth, and calcification. Specifically, we monitored vap expression relative to two genes, fcp and pcna, whose expression was previously shown to vary with growth conditions; fcp encodes a fucoxanthin chl a/c‐binding protein, and pcna encodes the proliferating cell nuclear antigen. Relative to the expression of pcna and fcp, vap expression was highest at nutrient concentrations where growth curves and chl a patterns indicated arrest of cell division. Our results indicate that the level of vap expression does not decrease when cell growth diminishes.     

Seasonal Dynamics of Benthic and Planktonic Algae in a Nutrient‐Rich Lowland River (Spree,Germany)

We studied chlorophyll a (chl. a), biovolume and species composition of benthic algae and phytoplankton in the eutrophic lower River Spree in 1996. The chl. a concentration was estimated as 3.5 (2.7–4.5) µg/cm² for epipsammon, 9.4 (7.4–11.9) µg/cm² for epipelon and 6.7 (5.7–7.8) µg/cm² for the epilithon (median and 95% C. L.). The mean total biomass of benthic algae was significantly higher (6.0 µg chl. a/cm²) than the areal chl. a content of the pelagic zone (1.6 µg chl. a/cm²). Although certain phytoplankton taxa were abundant in the periphyton, benthic taxa generally dominated the assemblages. Seasonal dynamics of benthic algae were probably controlled by light and nitrate supply (sand), discharge fluctuations (sand, mud) and invertebrate grazing (stones). This paper shows the importance of benthic algae even in phytoplankton‐rich lowland rivers with sandy or muddy sediments. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

COMBINATION OF FLOW CYTOMETRY AND SINGLE CELL ABSORPTION SPECTROSCOPY TO STUDY THE PHYTOPLANKTON STRUCTURE AND TO CALCULATE THE CHL A SPECIFIC ABSORPTION COEFFICIENTS AT THE TAXON LEVEL1

The phytoplankton community structure of a hypertrophic lake was quantitatively determined with the aid of flow cytometry. The flow cytometry signals were calibrated to obtain cell‐specific information, such as the chl a content and the biovolume per cell. The reliability of this method was tested with laboratory cultures. The results of the phytoplankton structure in a hypertrophic lake with respect to chl distribution in the different algal groups obtained by flow cytometry were compared with the results from HPLC pigment fingerprinting. Both methods yield the percentage contribution of the different algal groups to total chl a. The chl a specific absorption coefficient of the phytoplankton (a^*_Phy) was determined via visible (VIS) spectroscopy of samples taken from a hypertrophic lake (Auensee) in 2003. The results indicated that a^*_Phy of the total cell suspension is dependent on the phytoplankton structure as well as on environmental factors. The linear relationship between a^*_Phy at 675 nm and the product of the chl a content per cell and the biovolume offered the possibility to normalize phytoplankton absorption spectra to acquire the taxon‐specific a^*_Phy. The estimated a^*_Phy (675 nm) values were used to normalize single cell absorption spectra at this wavelength to obtain the a^*_Phy between 400 and 750 nm for representatives of the major algal groups. Our measurements show that the absorption coefficient for the whole phytoplankton community varies within the season. Finally, we used the a^*_Phy and the chl a distribution to calculate the light absorption of each algal group in the hypertrophic lake.     

RELATIONSHIP OF GROWTH AND PHOTOSYNTHESIS WITH COLONY SIZE IN AN EDIBLE CYANOBACTERIUM,GE‐XIAN‐MI NOSTOC (CYANOPHYCEAE)1

Growth and photosynthesis of an edible cyanobacterium, Ge‐Xian‐Mi (Nostoc), were investigated with differently sized colonies. Both photosynthesis and growth were dependent on the colony size. Compared with larger ones, smaller colonies grew faster regardless of the levels of light and temperature for culture and showed higher values of maximal net photosynthetic rate, apparent quantum yield, light‐saturating and compensating points, and dark respiration. The ratios of chl a content and mass to surface area of a colony increased and that of chl a to mass or mass to volume of a colony decreased with increased colonial sizes. A Ge‐Xian‐Mi colony appeared to increase its chl a content per surface area, enhancing the light‐shading effect; however, at the same time it decreased its mass density on a volume basis, minimizing the enhanced effects of shading and diffusion barrier caused by the thickening outer layer with increasing colony size during growth.     

GROWTH AND DOMOIC ACID PRODUCTION BY PSEUDO‐NITZSCHIA SERIATA (BACILLARIOPHYCEAE) UNDER PHOSPHATE AND SILICATE LIMITATION1

Pseudo‐nitzschia seriata (Cleve) H. Peragallo isolated from Scottish west coast waters was studied in batch culture with phosphate (P) or silicate (Si) as the yield‐limiting nutrient at 15°C. This species produced the neurotoxin domoic acid (DA) when either nutrient was limiting but produced more when stressed by Si limitation during the stationary phase. Under P‐limiting conditions, exponential growth stopped after P was reduced to a low threshold concentration. Under Si‐limiting conditions, fast exponential growth was followed by a period of slower exponential growth, until Si became exhausted. A stationary phase was observed in the P‐limited but not the Si‐limited cultures, the latter showing a rapid decrease in cell density after the second exponential growth phase. Si‐limited cultures exhibited a further period of active metabolism (as indicated by increases in chl and carbon per cell) late in the experiment, presumably fueled by regenerated Si. DA production was low in exponential phase under both conditions. In P‐limited cultures, most DA was produced during the immediate postexponential phase, with little or no new DA produced during later cell senescence. In contrast, although a substantial amount of DA was produced during the slower exponential phase of the Si‐limited cultures, DA production was even greater near the end of the experiment, coincident with the period of chl synthesis and increase in carbon biomass. Comparison of the magnitude of toxin production in the two nutrient regimes indicated a greater threat of P. seriata‐generated amnesic shellfish poisoning events under Si rather than P nutrient limitation.     

EFFECTS OF ULTRAVIOLET‐A RADIATION AND NUTRIENT AVAILABILITY ON THE CELLULAR COMPOSITION OF PHOTOPROTECTIVE COMPOUNDS IN GLENODINIUM FOLIACEUM (DINOPHYCEAE)1

The photoprotective response in the dinoflagellate Glenodinium foliaceum F. Stein exposed to ultraviolet‐A (UVA) radiation (320–400 nm; 1.7 W · m²) and the effect of nitrate and phosphate availability on that response have been studied. Parameters measured over a 14 d growth period in control (PAR) and experimental (PAR + UVA) cultures included cellular mycosporine‐like amino acids (MAAs), chls, carotenoids, and culture growth rates. Although there were no significant effects of UVA on growth rate, there was significant induction of MAA compounds (28 ± 2 pg · cell^?1) and a reduction in chl a (9.6 ± 0.1 pg · cell^?1) and fucoxanthin (4.4 ± 0.1 pg · cell^?1) compared to the control cultures (3 ± 1 pg · cell^?1, 13.3 ± 3.2 pg · cell^?1, and 7.4 ± 0.3 pg · cell^?1, respectively). In a second investigation, MAA concentrations in UVA‐exposed cultures were lower when nitrate was limited (P < 0.05) but were higher when phosphate was limiting. Nitrate limitation led to significant decreases (P < 0.05) in cellular concentration of chls (chl c₁, chl c₂, and chl a), but other pigments were not affected. Phosphate availability had no effect on final pigment concentrations. Results suggest that nutrient availability significantly affects cellular accumulation of photoprotective compounds in G. foliaceum exposed to UVA.     

Extracellular polymeric substances of the marine fouling diatom amphora rostrata Wm.Sm

Amphora rostrata was grown under continuous illumination at 27°C in batch cultures using f/2 medium. Cell biomass (measured as chllorophyll a and cell counts) reached a maximum on day 7. Thereafter, cell biomass as chl a showed a small decrease. Planktonic('free') and biofilm extracellular polymeric substances (EPS) from the adherent cells of A. rostrata were studied. Both types of EPS were produced during the logarithmic phase of growth. However, production was higher during the stationary growth phase. Enhanced EPS production was associated with nutrient deficient conditions. Planktonic and biofilm EPS were purified by gel filtration using Sephadex G‐200 and ion exchange chromatography using DEAE‐cellulose. Both polymers showed the presence of a single peak. Capillary gas Chromatographie analysis of both planktonic and biofilm EPS showed that fucose (36.7%) and galactose (27.6%) were the most abundant monosaccharides, with small quantities of rhamnose, xylose, arabinose, mannose and glucose. Other chemical analysis showed the presence of sulphate, uronic acids, hexoamines, pyruvate and proteins in both the planktonic and bio‐film EPS. Uronic acid, pyruvate and sulphate together were found to contribute ～50 to 60% (W/W) to the EPS of A. rostrata. Such a high content of non‐sugar components indicates their importance to the diatom in metal binding, desiccation prevention and flexibility.     

The effects of hydropsychid colonization on algal response to nutrient enrichment in a small Michigan stream, U.S.A.

1. We tested the hypothesis that the indirect effects of colonization by Hydropsyche spp. (Trichoptera: Hydropsychidae) may be greater than direct effects of nutrients on the benthic algal community growth. Two sets of nutrient-releasing substrates (a total of twenty-four) were deployed into a small pristine stream in northern Michigan. Each set was composed of four treatments replicated three times: (i) no nutrient enrichment (C), (ii) 0.5 M phosphate-P enrichment (P), (iii) 0.5 M nitrate-N enrichment (N) and (iv) 0.5 M phosphate-P plus 0.5 M nitrate-N enrichment (P + N). All hydropsychids colonizing on the substrate in one set (twelve substrates) were removed regularly and the other set (twelve substrates) with undisturbed hydropsychids served as the controls. 2. Algal biomass and gross primary productivity were estimated as chlorophyll a (chl a) concentration, algal biovolume, and carbon fixation rate, respectively. There was a significant interactive effect of hydropsychid colonization and P enrichment on algal biomass measured as chl a concentration. With removal of hydropsychids, chl a concentration increased 11-fold in the P enrichment treatments relative to the controls. The effects of P on chl a was, however, not significant in the presence of hydropsychids. Such interactive effects were not observed when algal responses were measured as biovolume and carbon fixation rate (GPP). 3. It is recommended that algal responses to nutrient enrichment should be measured as biovolume or carbon fixation rate in small streams where hydropsychids are commonly present.     

COMPARISON OF METHODS TO DETECT ALLELOPATHIC EFFECTS OF SUBMERGED MACROPHYTES ON GREEN ALGAE1

Detecting allelopathic inhibition of phytoplankton by submerged macrophytes in an ecologically meaningful way is not easy. Multiple‐approach investigations from a laboratory scale to the ecosystem level have been recommended to overcome the shortcomings of individual methods. Whether results of different methods are qualitatively or quantitatively comparable has not yet been tested. Here, we compare the sensitivity of the green algae Desmodesmus subspicatus (Chodat) E. Hegewald et Ant. Schmidt and Stigeoclonium helveticum Vischer to the allelopathic effect of the submerged macrophyte Myriophyllum verticillatum L. The following three approaches were used: (i) coincubation of algae in dialysis membrane tubes in a lake inside and outside a M. verticillatum stand, (ii) coincubation of algae in dialysis membrane tubes in aquaria with and without M. verticillatum, and (iii) single additions of tannic acid (TA), an allelopathically active polyphenol present in this macrophyte, to the algae cultures. For each method, fluorescence‐based (chl a, PSII activity) and particle‐based (cell count, biovolume) parameters were compared after 48 h of incubation. Results revealed quantitative and qualitative differences between methods. Algae incubated in dialysis membrane tubes in aquaria showed a strong decrease in all parameters under the influence of macrophytes. In situ measurements were influenced by adverse growth conditions for the test algae and only detected significant reductions for biovolume. Single additions of TA induced a strong reduction of fluorescence‐based parameters similar to aquarium results, but an increase in the cell count. Even the qualitative transfer of laboratory results to field conditions thus requires caution and a proper selection of parameters.     

Sources and transport of algae and nutrients in a Californian river in a semi-arid climate

1. To elucidate factors contributing to dissolved oxygen (DO) depletion in the Stockton Deep Water Ship Channel in the lower San Joaquin River, spatial and temporal changes in algae and nutrient concentrations were investigated in relation to flow regime under the semiarid climate conditions. 2. Chlorophyll‐a (chl‐a) concentration and loads indicated that most algal biomass was generated by in‐stream growth in the main stem of the river. The addition of algae from tributaries and drains was small (c.15% of total chl‐a load), even though high concentrations of chl‐a were measured in some source waters. 3. Nitrate and soluble‐reactive phosphorus (SRP) were available in excess as a nutrient source for algae. Although nitrate and SRP from upstream tributaries contributed (16.9% of total nitrate load and 10.8% of total SRP load), nutrients derived from agriculture and other sources in the middle and lower river reaches were mostly responsible (20.2% for nitrate and 48.0% for SRP) for maintaining high nitrate and SRP concentrations in the main stem. 4. A reduction in nutrient discharge would attenuate the algal blooms that accelerate DO depletion in the Stockton Deep Water Ship Channel. The N : P ratio, in the main stem suggests that SRP reduction would be a more viable option for algae reduction than nitrogen reduction. 5. Very high algal growth rates in the main stem suggest that reducing the algal seed source in upstream areas would also be an effective strategy.     

CHLOROPHYLL,NITROGEN, AND PHOTOSYNTHETIC PATTERNS DURING GROWTH AND SENESCENCE OF TWO BLUE-GREEN ALGAE1,2

A standardized, multiflask, batch culture system was developed to study the processes of algal senescence in Anacystis nidulans and Phormidium molle Gom, var. tenuior W. et G. West. Growth data over a 3-year period gave reproducible and comparable time-course curves. Although A. nidulans is unicellular and P. molle filamentous, the patterns of change with age were similar. Mean logarithmic doubling times and carbon yields were, respectively, 6.9 hr and 390 mg C/liter for A. nidulans and 7.2 hr and 710 mg C/liter for P. molle. Chlorophyll concentration and photo-synthetic capacity per unit carbon rose rapidly during the logarithmic phase to maximum levels in either late log phase (P. molle) or early linear phase (A. nidulans), then fell throughout the declining growth phase to low levels in the stationary phase. Nitrate was rapidly exhausted from the medium during the period of logarithmic growth and stoichiometrically converted to particulate organic form; very little subsequent fixation of molecular nitrogen occurred. The phycocyanins were rapidly destroyed during the logarithmic phase while the carotenoids remained relatively constant throughout the whole growth period and then slowly declined. Preliminary electron micrographs showed a progressive deterioration in cellular ultrastructure, especially a reduction in the number of photosynthetic thylakoids, commenting in the linear growth phase. Analysis of the results suggests that occurrence of linear growth kinetics and termination of culture growth were caused by exhaustion of nitrate. The observed decreases in chlorophylls, phycocyanins, and photosynthetic capacity during active culture growth show that senescence effects may not be, as assumed, restricted to the stationary phase of growth.