Spatial heterogeneity of diatom silicification and growth in a eutrophic reservoir

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MONITORING RAPID VALVE FORMATION IN THE PENNATE DIATOM NAVICULA SALINARUM (BACILLARIOPHYCEAE)1

After each division of a diatom cell, a new siliceous hypovalve is formed inside the silica deposition vesicle (SDV). We present the sequence of this early formation of the new valve in the pennate marine diatom Navicula salinarum (Grunow) Hustedt, visualized by using the fluorescent probe 2‐(4‐pyridyl)‐5‐((4‐(2‐dimethylaminoethylamino‐carbamoyl)methoxy)phenyl)oxazole (PDMPO). Our observations confirm that two‐dimensional expansion of the growing valve is a rapid process of no more than 15 min; three‐dimensional completion of the valve appears to be slower, lasting most of the time valve formation takes. The results are relevant to studies of the timing of molecular processes involved in valve formation (i.e. the bio‐ and morphogenesis of the SDV) in relation to uptake and transport of silicic acid. Use of this probe helps us to identify specific developmental stages for further detail analysis of diatom basilica formation, which eventually could lead to obtaining enriched SDV fractions.     

Consistent patterns in diatom assemblages and diversity measures across water‐depth gradients from eight Boreal lakes from north‐western Ontario (Canada)

1. Until recently, the distribution of diatom species assemblages and their attributes (e.g. species richness and evenness) in relation to water depth have been identified but not quantified, especially across several lakes in a region. Here, we examined diatom assemblages in the surface sediment across a water‐depth gradient in eight small, boreal lakes in north‐western Ontario, minimally disturbed by human activities. 2. Surface‐sediment diatom assemblages were collected within each lake along a gentle slope from near‐shore to the centre deep basin of the lake, at a resolution of ～1 m water depth. Analysis of sedimentary samples provided an integrated view of assemblages that were living in the lake over several years and enabled a high‐resolution analysis of many lakes. The study lakes ranged in water chemistry, morphology and size and are located along an east–west transect approximately 250 km long in north‐western Ontario (Canada). 3. The majority of diatom species were distributed along a continuum of depth, with those taxa having similar habitat requirements forming distinct, though overlapping, assemblages. Three major zones of diatom assemblages in each lake were consistently identified: (i) a near‐shore assemblage of Achnanthes (sensu lato), Nitzschia, Cymbella (sensu lato) and other benthic species; (ii) a mid‐depth assemblage of small Fragilaria (sensu lato)/small Aulacoseira and various Navicula taxa; and (iii) a deep‐water assemblage of planktonic origin (mainly Discotella spp.). 4. The depth of the transition between assemblage zones varied between the eight lakes. The boundary between the deep‐water planktonic zone and the mid‐depth benthic zone varied according to water chemistry and was probably related to light attenuation. The boundary was deeper in lakes with the lower dissolved organic carbon and total phosphorus (TP) (i.e. less light attenuation) and vice versa. 5. Generally, species richness, species evenness and turnover rate of species as a function of depth were significantly lower in the planktonic assemblage zone in comparison with the two zones nearer the shore. Reproducibility of species and assemblage distributions across the depth gradient of the lakes illustrated that, despite potential for sediment transport, detailed ecological characterisation of diatom species can be gleaned from sedimentary data. Such data are often lacking, particularly for near‐shore benthic species.     

A STUDY OF Si DEPOSITION SYNCHRONY IN RHIZOSOLENIA (BACILLARIOPHYCEAE) MATS USING A NOVEL 32Si AUTORADIOGRAPHIC METHOD

A ³²Si autoradiographic technique using a liquid photographic emulsion was developed for the study of diatom silica deposition in culture or in natural water samples. The method was used in the Central North Pacific to study silica deposition by diatoms of the genus Rhizosolenia. The species examined form centimeter-sized aggregates commonly referred to as mats. The Rhizosolenia mats examined were composed of a matrix of R. fallax Sundström chains, embedded with chains of larger cells, either R. debyana H. Peragallo or R. acuminata H. Peragallo. The autoradiographs revealed distinct rings of labeled intercalary bands and/or labeled valves. A greater proportion of the frustule of the larger species was labeled during the incubations with ³²Si, implying higher rates of silicification by R. debyana and R. accuminata compared to R. fallax. A quantitative consideration of these differences in species-specific Si production combined with abundance and surface area estimates for each species indicates that cells of the larger species carry out the majority of silica production in Rhizosolenia mats. The large cell size (pervalvar axis 240 to 3000 μm) and elongate frustule morphology of Rhizosolenia cells enabled us to localize the deposition of silica along the pervalvar axis. Positions of labeled bands along this axis indicate progress through the Si deposition cycle, and the results suggest that cell division is phased, with either a bimodal or unimodal age distribution of cells within the cell cycle for all species in a mat. Species-specific doubling times from 25 to 60 h were implied by the mean fractions of frustule that were labeled. ³²Si autoradiography revealed unique species-specific differences in diel patterns of cell division and silica deposition and has potential for studies of Si deposition by other diatom species and assemblages.     

CYTOPLASMIC MASSES PRESERVED IN EARLY HOLOCENE DIATOMS: A POSSIBLE TAPHONOMIC PROCESS AND ITS PALEO‐ECOLOGICAL IMPLICATIONS1

In Lake Suigetsu, central Japan, greenish/light‐brown granules identified as cytoplasmic masses had been preserved in siliceous cell walls of freshwater diatoms in annual layers of lacustrine muds since the early Holocene. The lacustrine muds consisted of alternating dark‐colored (rich in diatom valves, clay, and organic matter) and light‐colored (mainly diatom valves) laminae. The greenish/light‐brown granules were predominately preserved in frustules of the genus Aulacoseira preserved in the dark‐colored laminae. The dark‐colored laminae were inferred to have formed annually under stratified water caused by surface water warming in summer that caused the formation of an organic‐rich anoxic layer on the lake bottom that favored granule preservation. The good preservation of cytoplasmic masses in dark‐colored laminae suggested a cause for diatom assemblage periodicity, a phenomenon that was commonly noted in temperate lakes: the cells containing these masses could be potential seed stocks for subsequent spring blooms. Frustules of the most abundant granule‐containing species, Aulacoseira nipponica (Skvortzow) Tuji, in the dark‐colored laminae of the Early Holocene muds were abundant in the overlying light‐colored laminae, suggesting that these species reproduced abundantly in springtime yielding a massive diatom bloom.     

Diatom elemental and morphological changes in response to iron limitation: a brief review with potential paleoceanographic applications

Diatoms are a major group of phytoplankton that account for approximately 40% of the ocean carbon fixation and the vast majority of biogenic silica production through the construction of their cell walls (termed frustules). These frustules accumulate and are partially preserved in the ocean sediments. Diatom growth and nutrient utilization in high‐nitrate, low‐chlorophyll regions of the world’s oceans are mostly regulated by iron availability. Diatoms acclimate to iron limitation by decreasing cell size. The associated increase in surface area‐to‐volume ratio and decrease in diffusive boundary layer thickness may improve nutrient uptake kinetics. In parallel, cellular silicon (Si) contents are elevated in iron‐limited diatoms relative to nitrogen (N) and carbon (C). Variations in degree of silicification and nutritional requirements of iron‐limited diatoms have been hypothesized to account for higher cellular Si and/or lower cellular N and C, respectively. However, in some diatoms, frustule silicification does not significantly change when cells are iron‐limited. Instead, changes in the Si‐containing valve surface area relative to volume within these diatoms is hypothesized to be responsible for the variations in the cellular Si : N and Si : C ratios. In particular, some examined iron‐limited pennate diatoms have reduced widths relative to their lengths (i.e. lower length‐normalized widths, LNW) compared to iron‐replete cells. In the pennate diatom Fragilariopsis kerguelensis, the mean LNWs of valves preserved in sediments throughout the Southern Ocean (a well‐characterized iron‐limited region) is positively correlated with satellite‐derived, climatological net primary productivity in the overlying waters. Because of the specific morphological changes in pennate diatom frustules in response to iron availability, the valve morphometerics (e.g. LNWs) can potentially be used as a diagnostic tool for iron‐limited diatom growth and relative changes in the Si : N (and Si : C) ratios in extant diatom assemblages as well as those preserved in the sediments.     

UNCOUPLING OF SILICON COMPARED WITH CARBON AND NITROGEN METABOLISMS AND THE ROLE OF THE CELL CYCLE IN CONTINUOUS CULTURES OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) UNDER LIGHT,NITROGEN, AND PHOSPHORUS CONTROL1

The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light‐ (E), nitrogen‐ (N), and phosphorus‐ (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2‐ to 6‐fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions.     

Climate forcing of diatom productivity in a lowland,eutrophic lake: White Lough revisited

1. In cultural landscapes, lake response to climate can be masked by land‐use change and nutrient loss from their catchments. Palaeolimnological methods were used to reconstruct the ecological response of diatoms in a eutrophic lowland lake (White Lough, Co. Tyrone, Northern Ireland) to altered nutrient P loading and precipitation variability over c. 100 years. 2. ²¹⁰Pb‐dated sediment cores were analysed to determine diatom assemblage variability, biogenic silica concentration, geochemical phosphorus concentration and accumulation rate. Manure P and agricultural N surplus data were collated from documentary sources. Long‐term trends in annual temperature and precipitation were derived from the Armagh Observatory. 3. Diatom community turnover from 1890 until c. 1960 was limited, and assemblages were dominated by Aulacoseira subarctica; after this date, changes primarily reflected a eutrophication sequence owing to increased diffuse nutrient inputs associated with intensification of land use (external P loading increased by a factor of three). 4. Diatom and biogenic Si profiles were compared with North Atlantic Oscillation (NAO) records, an index of regional weather patterns. Biogenic Si exhibited a c. 7‐year cycle, which tracked a cycle of similar timescale in the Armagh climate record for dry summers. In turn, this cycle was related to the variation in the NAO. 5. Monitoring data from 1971 to 2007 of nitrate exports from the Blackwater River showed that these too followed a roughly 7‐year cycle at least up to 2000, in which dry summers were followed by sharp increases in nitrate export. It is argued that diatom production in White Lough reflects the cyclic behaviour in nitrate loading and the constraints that nitrogen availability places on the spring diatom bloom in a lake that is dominated by cyanobacteria.     

EFFECT OF ZINC AVAILABILITY ON GROWTH,MORPHOLOGY, AND NUTRIENT INCORPORATION IN A COASTAL AND AN OCEANIC DIATOM1

We investigated the effect of Zn availability on growth rate (μ), cell morphology, and elemental stoichiometry and incorporation rate in two marine diatoms. For the coastal diatom Skeletonema costatum (Grev.) Cleve, the half‐saturation constant (K_S) for growth was 4.1 pM Zn²⁺, and growth ceased at ≤ 2.6 pM Zn²⁺, whereas for the oceanic diatom Thalassiosira oceanica Hasle, K_S was 0.5 pM Zn²⁺, and μ remained at ～40%μ_max even at 0.3 pM Zn²⁺. Under Zn‐limiting (Zn‐L) conditions, S. costatum decreased cell size significantly, leading to an 80% increase in surface area to volume ratio (SA/V) at Zn²⁺ of 3.5 pM compared to Zn‐replete (Zn‐R) conditions (at Zn²⁺ of 13.2 pM), whereas T. oceanica’s morphology did not change appreciably. Cell quotas of C, N, P, Si, and chl a significantly decreased under Zn limitation in S. costatum (at Zn²⁺ of 3.5 pM), whereas Zn limitation in T. oceanica (at Zn²⁺ of 0.3 pM) had little effect on quotas. Elemental stoichiometry was ～85C:10N:9Si:1P and 81C:9N:5Si:1P for S. costatum, and 66C:5N:2Si:1P and 52C:6N:2Si:1P for T. oceanica, under Zn‐R and Zn‐L conditions, respectively. Incorporation rates of all elements were significantly reduced under Zn limitation for both diatoms, but particularly for Si in S. costatum, and for C in T. oceanica, despite its apparent tolerance of low Zn conditions. With [Zn²⁺] in some parts of the ocean being of the same order (～0.2 to 2 pM) as our low Zn conditions for T. oceanica, our results support the hypothesis that in situ growth and C acquisition may be limited by Zn in some oceanic species.     

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m^–2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.     

Response of an algal assemblage to nutrient enrichment and shading in a Hawaiian stream

To investigate the effects of nitrate enrichment, phosphate enrichment, and light availability on benthic algae, nutrient-diffusing clay flowerpots were colonized with algae at two sites in a Hawaiian stream during spring and autumn 2002 using a randomized factorial design. The algal assemblage that developed under the experimental conditions was investigated by determining biomass (ash-free dry mass and chlorophyll a concentrations) and composition of the diatom assemblage. In situ pulse amplitude-modulated fluorometry was also used to model photosynthetic rate of the algal assemblage. Algal biomass and maximum photosynthetic rate were significantly higher at the unshaded site than at the shaded site. These parameters were higher at the unshaded site with either nitrate, or to a lesser degree, nitrate plus phosphate enrichment. Analysis of similarity of diatom assemblages showed significant differences between shaded and unshaded sites, as well as between spring and autumn experiments, but not between nutrient treatments. However, several individual species of diatoms responded significantly to nitrate enrichment. These results demonstrate that light availability (shaded vs. unshaded) is the primary limiting factor to algal growth in this stream, with nitrogen as a secondary limiting factor.     

KINETICS OF SILICIC ACID UPTAKE AND RATES OF SILICA DISSOLUTION IN THE MARINE DIATOM THALASSIOSIRA PSEUDONANA1,2

Tracer techniques using the stable isotope ³⁰Si were used to measure rates of silicic acid uptake and silica dissolution in silicon replete and silicon depleted populations of 2 clones of the marine diatom Thalassiosira pseudonana Hasle & Heimdal. Uptake kinetics were describable using the Michaelis-Menten equation for enzyme kinetics, and no threshold concentration for uptake was evident. The maximum specific uptake rate of the estuarine clone 3H (0.062–0.092 · h^?1) was higher than that of the Sargasso Sea clone 13-1 (0.028–0.031 · h^?1), but half-saturation constants for uptake by the 2 clones were not measurably different (0.8–2.3 μM for 3H; 1.4–1.5 μM for 13-1). There was little or no light dependence of uptake in populations grown under optimal light conditions prior to the experiment. Exponentially growing populations released silicic acid to the medium by dissolution of cellular silica at rates ranging from 6.5 to 15% of the maximum uptake rate.     

Effects of small‐scale environmental variation on metaphyton condition and community composition

1. Metaphyton mats typically originate as benthic algal biofilms that experience higher solar radiation and temperatures, and reduced access to nutrients, once they reach the water surface, but the impacts of these physicochemical changes on metaphyton condition and community composition have received little attention. 2. Using microprobes positioned at 0, 2, 4 and 6 cm depth, we recorded small‐scale differences in water chemistry within metaphyton mats constrained in floating nets during an 8‐week period. Concurrent weekly samples of filamentous algae and their diatom epiphytes were collected from the top, middle and bottom of the mats and were analysed for changes in ash‐free dry mass (AFDM) and chlorophyll‐a, nutrient (N, P, C, Si) content and taxonomic composition. 3. Light intensity, temperature and dissolved oxygen declined both with increasing depth in the mat and over the study period. The autotrophic index (=AFDM/chlorophyll‐a) was greatest at the top of the mats and increased over time; samples also had higher C/P and C/N ratios than samples deeper within the mat. 4. Pithophora was consistently the dominant algal filament throughout the study (representing 85% of all filaments averaged over time and depth); epiphytic diatom cover on Pithophora (calculated as epiphyte area index) declined over time, particularly at the top of the mat. 5. Densities of the diatom epiphytes Gomphonema, Cocconeis and Fragilaria increased with increasing depth within the mat, whereas Cymbella/Encyonema was more common in surface samples. 6. Our results indicate that metaphyton mats are highly dynamic communities, spatially organised in part by small‐scale environmental variation and subject to changes in taxonomic composition following their arrival at the water surface.     

NUTRIENT LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.     

GROWTH AND PHOTOPROTECTION IN THREE DINOFLAGELLATES (INCLUDING TWO STRAINS OF ALEXANDRIUM TAMARENSE) AND ONE DIATOM EXPOSED TO FOUR WEEKS OF NATURAL AND ENHANCED ULTRAVIOLET‐B RADIATION1

Long‐term growth response to natural solar radiation with enhanced ultraviolet‐B (UVB) exposure was examined in two species of dinoflagellates [Alexandrium tamarense (M. Lebour) Balech, At, and Heterocapsa triquetra (Ehrenb.) F. Stein, Ht], including two strains of A. tamarense, one from Spain and another from UK, and one diatom species (Thalassiosira pseudonana Hasle et Heimdal). We examined whether variable photoprotection (mycosporine‐like amino acids [MAAs] and xanthophyll‐cycle pigments) affected photosynthetic performance, phytoplankton light absorption, and growth. Growth rate was significantly reduced under enhanced UVB for the UK strain of At and for Ht (both grew very little) as well as for the diatom (that maintained high growth rates), but there was no effect for the Spanish strain of At. MAA concentration was high in the dinoflagellates, but undetectable in the diatom, which instead used the xanthophyll cycle for photoprotection. The highest cell concentrations of MAAs and photoprotective pigments were observed in the UK strain of At, along with lowest growth rates and F_v/F_m, indicating high stress levels. In contrast, the Spanish strain showed progressive acclimation to the experimental conditions, with no significant difference in growth between treatments. Increase in total MAAs followed linearly the cumulative UVB of the preceding day, and both total and primary MAAs were maintained at higher constitutive levels in this strain. Acclimation to enhanced UVB in the diatom resulted in an increase in PSII activity and reduction in nonphotochemical quenching, indicating an increased resistance to photoinhibition after a few weeks. All four species showed increased phytoplankton light absorption under enhanced UVB. Large intrastrain differences suggest a need to consider more closely intraspecific variability in UV studies.     

THE POSSIBLE OCCURRENCE OF PHOTOSYNTHETIC MICROORGANISMS IN DEEP-SEA SEDIMENTS OF THE NORTH ATLANTIC1,2,3

An assemblage of 12 diatom species and other photosynthetic microorganisms associated with red clay sediments collected from a depth of 6150 m in the North Atlantic was observed to bloom in Antarctic bottom water when exposed to sunlight at sea surface temperature. Although no growth could be detected over the first 3 days of the experiment, nitrate was nearly completely stripped from the water. The maximum growth rate of 4.7 doubling/day was reached between days 4 and 5. Most of the diatom species have been described as littoral or coastal forms, and it is suggested that these organisms are transported to depth by fecal material, turbidity currents, or a combination of the two.     

Luminescence properties of a nanoporous freshwater diatom

Freshwater diatom frustules show special optical properties. In this paper we observed luminescence properties of the freshwater diatom Cyclotella meneghiniana. To confirm the morphological properties we present scanning electron microscopy (SEM) images. X‐ray diffraction (XRD) studies were carried out to visualize the structural properties of the frustules, confirming that silica present in diatom frustules crystallizes in an α‐quartz structure. Study of the optical properties of the silica frustules of diatoms using ultra‐violet‐visible (UV‐vis) spectroscopy and photoluminescence spectroscopy confirmed that the diatom C. meneghiniana shows luminescence in the blue region of the electromagnetic spectrum when irradiated with UV light. This property of diatoms can be exploited to obtain many applications in day‐to‐day life. Also, using time‐resolved photoluminescence spectroscopy (TRPL) it was confirmed that this species of diatom shows bi‐exponential decay. Copyright © 2011 John Wiley & Sons, Ltd.     

VERTICAL MIGRATION OF A MIXED‐SPECIES EUGLENA (EUGLENOPHYTA) ASSEMBLAGE INHABITING THE HIGH‐INTERTIDAL SANDS OF NYE BEACH,OREGON1

Comparatively little is known about the vertical migration of the microphytobenthic community forming visible patches on high‐energy beaches. We collected surface and cored samples to evaluate the timing and extent of downward migration of a multispecies Euglena assemblage inhabiting Nye Beach, Oregon. Euglena density at the surface was highly variable and was not correlated with the time of low tide or instantaneous irradiance measurements; however, triplicate cores collected at low and high tides revealed a tidal rhythm in mean depth. On average, 95% of the assemblage occurred within 1 cm of the surface during low tide, but 54% of the assemblage was collected between 1 and 8 cm below the surface during high tide. A midday shading experiment revealed that short‐term changes in irradiance levels altered the Euglena density at the sediment surface by inducing vertical migration. This response to short‐term fluctuations in light may explain the weak correlation between cell density at the surface and time of day. The high‐intertidal location of these patches prevented the removal of nonmigrating cells by daily high tides, which increased the variability in surface samples and obscured the tidal migration rhythm detected in the core samples. Due in part to the semidiurnal nature of Oregon tides, this study provides in situ confirmation of past mesocosm research indicating that sediment disturbance during daily submersed periods is an important process in maintaining the quasi‐tidal rhythm in the appearance and disappearance of Euglena spp. from the surface of beaches and intertidal sandflats.     

THE Si:C:N RATIO OF MARINE DIATOMS: INTERSPECIFIC VARIABILITY AND THE EFFECT OF SOME ENVIRONMENTAL VARIABLES1

The variability of marine diatom Si:C and Si:N composition ratios was examined to assess their utility as ecological conversion factors. Twenty-seven diatom species grown under an 18:6 h LD cycle and sampled at the end of the light period gave mean ratios, by atoms, of 0.13 ± 0.04 and 1.12 ± 0.33 for Si:C and Si:N ratios, respectively (95% C.I. reported). The mean ratios for 18 species grown under continuous illumination were 0.12 ± 0.03 for Si:C and 0.95 ± 0.23 for Si:N. The mean ratios of the clones grown under constant light were not statistically different from those calculated for the same species grown under an 18:6 h LD photoperiod. The overall mean Si:C and Si:N ratios for the 18:6 h LD and continuous light experiments taken together, weighted by the number of species in each experiment, are 0.13 and 1.05, respectively. The average ratios for the nine nanoplankton species (<20 μm) examined were 0.09 ± 0.03 for Si:C and 0.80 ± 0.35 for Si:N. The eighteen netplankton species (>20 μm) had higher mean ratios, Si:C = 0.15 ± 0.04 and Si:N = 1.20 ± 0.37. Time course sampling throughout a 24 h period revealed twofold variations in both ratios for individual species grown on a 14:10 h LD cycle. Changes in irradiance can also produce factor of two variations, both ratios being higher under low light. Comparisons of these data with those from the literature regarding the effects of temperature and nutrient limitation on diatom elemental composition suggest that use of these ratios to convert field estimates of biogenic silica into nitrogen or carbon units, or to estimate silica production from ¹⁴C data, should yield results accurate to within a factor of three under most circumstances.     

STREAM PERIPHYTON PHOTOACCLIMATION RESPONSE IN FIELD CONDITIONS: EFFECT OF COMMUNITY DEVELOPMENT AND SEASONAL CHANGES1

The photochemical behavior of intact stream periphyton communities in France was evaluated in response to the time course of natural light. Intact biofilms grown on glass substrata were collected at three development stages in July and November, and structural parameters of the biofilms were investigated (diatom density and taxonomy). At each season, physiological parameters based on pigment analysis (HPLC) and pulse‐amplitude‐modulated (PAM) chl fluorescence technique were estimated periodically during a day from dawn to zenith. Regardless of the community studied, the optimal quantum yield of PSII (F_v/F_m), the effective PSII efficiency (Φ_PSII), the nonphotochemical quenching (NPQ), and the relative electron transport rate (rETR) exhibited clear dynamic patterns over the morning. Moreover, microalgae responded to the light increase by developing the photoprotective xanthophyll cycle. The analysis of P‐I parameters and pigment profiles suggests that July communities were adapted to higher light environments in comparison with November ones, which could be partly explained by a shift in the taxonomic composition. Finally, differences between development stages were significant only in July. In particular, photoinhibition was less pronounced in mature assemblages, indicating that self‐shading (in relation to algal biomass) could have influenced photosynthesis in older communities.