The importance of nutrient co‐limitation in regulating algal community composition,productivity and algal‐derived DOC in an oligotrophic marsh in interior Alaska

1. Compared to lakes and streams, we know relatively little about the factors that regulate algae in freshwater wetlands. This discrepancy is particularly acute in boreal regions, where wetlands are abundant and processes related to climate change (i.e. increased permafrost collapse and soil weathering) are expected to increase nutrient inputs into aquatic systems. To investigate how accelerated nutrient inputs might affect algal structure and function in northern boreal wetlands, we added nitrogen, phosphorus and silica to mesocosms in an oligotrophic marsh in interior Alaska. 2. We conducted two in situ mesocosm enrichment experiments during consecutive summer growing seasons, each lasting 24 days. In 2007, we investigated the effects of +N, +P, +Si and +N+P+Si enrichment on benthic algal biomass (chlorophyll‐a, ash‐free dry mass, biovolume), chemistry (N : P ratio) and community composition. In 2008, we expanded our first experiment to investigate the effects +N+P, +N+Si, +P+Si and +N+P+Si on the same algal parameters as well as productivity (mg C m^?2 h^?1). 3. In both experiments, we measured water‐column dissolved organic carbon (DOC) inside treatment enclosures and related changes in DOC to standing algal biomass. 4. Benthic algal accrual did not increase following 24 days of enrichment with any nutrient alone or with P and Si together (+P+Si), but increased significantly with the addition of N in any combination with P and Si (+N+P, +N+Si, +N+P+Si). 5. Algal productivity (20 mg C m^?2 h^?1) increased between three‐ and seven‐fold (57–127 mg C m^?2 h^?1) with the addition of N in combination with any other nutrient (+N+P, +N+Si, +N+P+Si). Water‐column DOC concentration was significantly higher inside N‐combination treatments compared to the control during each season, and DOC increased linearly with benthic algal biomass in 2007 (r² = 0.89, P < 0.0001) and 2008 (r² = 0.74, P < 0.0001). 6. Taxonomic composition of the wetland algal community responded most strongly to N‐combination treatments in both seasons. In 2007, there was a significant shift from Euglena and Mougeotia in the control treatment to Chroococcus and Gloeocystis with +N+P+Si enrichment, and in 2008, a Mougeotia‐dominated community was replaced by Gloeocystis in the +N+P treatment and by Nitzschia in +N+Si and +N+P+Si treatments. 7. Together, these data provide several lines of evidence for co‐limitation, and the central importance of N as a co‐limiting nutrient for the wetland algal community. Changes in algal dynamics with increased nutrient concentrations could have important implications for wetland food webs and suggest that algae may provide a functional link between increasing nutrient inputs and altered wetland carbon cycling in this region.     

VARIABILITY IN CELL SIZE,NUTRIENT DEPLETION,AND GROWTH RATES OF THE SOUTHERN OCEAN DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE) AFTER PROLONGED IRON LIMITATION1

Diatoms are the main primary producers in the Southern Ocean, governing the major nutrient cycles. Fragilariopsis kerguelensis (O’Meara) Hust. is the most abundant diatom species in the Southern Ocean and its paleo‐oceanographic record is frequently used to reconstruct the past position and nutrient characteristics of the Antarctic polar front. Here we report on the responses of F. kerguelensis on prolonged exposure to a range of iron concentrations, allowing a characterization of morphological and nutrient‐depletion changes in relation to iron status. Under iron limitation, F. kerguelensis grew slower, cells became smaller, chains became shorter, and the nutrient‐depletion ratios changed. Prolonged exposure to iron limitation caused F. kerguelensis to decrease its surface area and volume 2‐fold, and to increase its surface‐to‐volume ratio by 25%. With the decrease in growth rates, silicon (Si) and phosphorus (P) depletion per cell remained fairly constant, but when normalized per surface area (Si) or per cell volume (P), depletion increased. In contrast, nitrogen (N) depletion per cell decreased significantly together with the decrease in growth rates but was constant when normalized per cell volume. The different response in Si, P, and N depletion resulted in changes in the nutrient‐depletion ratios, most notably in the Si:N ratio, which significantly increased, and in the N:P ratio, which significantly decreased with decreasing growth rates. It is concluded that under iron limitation, variation in cell size and/or nutrient depletion ultimately can cause changes in oceanic biogeochemical nutrient cycles. It enables the use of cell size of F. kerguelensis as a paleo‐oceanographic proxy.     

Can Hydrodynamics Change Phosphorus Strategies of Diatoms?—Nutrient Levels and Diatom Blooms in Lotic and Lentic Ecosystems

Diatom blooms occur in many water bodies worldwide, causing significant ecological and social concerns. In order to understand the mechanisms of diatom blooms formation, the effects of varying phosphorus (P) concentration and hydrodynamics on the growth of diatoms were studied by combining results from field observations and laboratory experiments. The field investigation showed that spring diatom blooms (Cyclotella meneghiniana and Stephanodiscus hantzschii) occurred in Lake Taihu and Hanjiang River with similar environmental factors such as water temperature, pH, and dissolved oxygen in 2008. Concentrations of total phosphorus (TP), total nitrogen, and ammonia nitrogen (NH₄-N) in Lake Taihu were significantly higher than the concentrations in the Hanjiang River. Laboratory experiments were conducted to evaluate growth and physiological responses of four lotic diatoms (Cyclotella atomus, Fragilaria crotonensis, Nitzschia palea, and S. hantzschii, isolated from the Hanjiang River) and three lentic diatoms (C. meneghiniana, Melosira varians, and Stephanodiscus minutulus, isolated from Lake Taihu, Lake Donghu, and Guanqiao Pond, respectively) to various P concentrations under small-scale turbulent and standing conditions. Our results showed that, with turbulence, lotic diatoms C. atomus, F. crotonensis, N. palea, and S. hantzschii demonstrated a significant increase in affinity for P compared with lentic diatoms C. meneghiniana, M. varians, and S. minutulus. Moreover, lotic diatoms C. atomus, F. crotonensis, and N. palea had higher growth rates and photosynthetic efficiencies with small-scale turbulence than with standing conditions both in P-limited and P-replete conditions. Lotic species S. hantzschii and three lentic diatoms (C. meneghiniana, M. varians, and S. minutulus) grew well under standing conditions. Our results may explain our field observation that the occurrence of diatom blooms in lakes is often associated with higher TP concentrations whereas in rivers, diatom blooms occur at a wide range of TP concentrations under flows. Therefore, different hydrodynamics and nutrient concentrations determined the dominant diatom species, according to their habitat-dependent physiological characteristics.     

Time‐dependent effects of fertilization on plant biomass in floating fens

Abstract. A cross‐over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m^?2 N, 5 g.m^?2 P, both elements and unfertilized control). The above‐ground biomass production of vascular plants was co‐limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N‐fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P‐fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N‐fertilization vascular plant growth was still co‐limited by N and P, but one year after P‐fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N‐fertilizer had mostly ‘disappeared’ from N‐fertilized plots, whereas the availability of P remained markedly enhanced in P‐fertilized plots. In addition, P‐fertilization enhanced the uptake of N by plants the following year. The time‐dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P‐fertilization on the availability of N; (4) contrasting effects of N‐ and P‐fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above‐ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non‐limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer‐term experiments are needed, so that indirect effects have time to develop and be detected.     

Grazer species effects on epilithon nutrient composition

1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory. 2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon. 3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment. 4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.     

THE EFFECT OF NITROGEN SOURCE ON THE GROWTH AND TOXICITY OF THREE POTENTIALLY HARMFUL DINOFLAGELLATES

Increases in population and agriculture in coastal areas can result in increased nutrient inputs and alterations in the ratios of organic to inorganic nutrients in coastal waters. Such changes in coastal nutrient regimes can affect phytoplankton community structure by creating conditions favorable for growth and dominance of algae that were not dominant before. The effect that changes in ratios and concentrations of nutrients have on toxicity of harmful algal species is not well known. There seems to be a relationship; however, between nutrient stress and toxin production among harmful phytoplankton producing low‐N toxins, e.g. Diarrhetic Shellfish Poisoning (DSP) toxins. Even less is known about the relationship between organic nutrient uptake and toxin production. Benthic species and species in coastal areas are probably exposed to greater fluxes of dissolved organic nitrogen (DON). In this study, benthic and planktonic species of Prorocentrum were grown on L1 media with the sole N‐source varying among treatments as nitrate, ammonium, urea, L‐glutamic acid, and high molecular weight natural DON. An ELISA specific to the DSP toxins, okadaic acid and 35‐methylokadaic acid, was used to determine toxin production by each species when grown on the different N sources. Preliminary results indicate that some organic forms of N support growth as well as inorganic forms for Prorocentrum minimum, P. mexicanum, and P. hoffmannianum.     

Temperature and Photoperiod Interactions with Phosphorus-Limited Growth and Competition of Two Diatoms

In lakes, trophic change and climate change shift the relationship between nutrients and physical factors, like temperature and photoperiod, and interactions between these factors should affect the growth of phytoplankton species differently. We therefore determined the relationship between P-limited specific growth rates and P-quota (biovolume basis) of Stephanodiscus minutulus and Nitzschia acicularis (diatoms) at or near light saturation in axenic, semi-continuous culture at 10, 15 and 20 °C and at 6, 9 and 12 h d⁻¹ photoperiod. Photoperiod treatments were performed at constant daily light exposure to allow comparison. Under these conditions, we also performed competition experiments and estimated relative P-uptake rates of the species. Temperature strongly affected P-limited growth rates and relative P uptake rates, whereas photoperiod only affected maximum growth rates. S. minutulus used internal P more efficiently than N. acicularis. N. acicularis was the superior competitor for P due to a higher relative uptake rate and its superiority increased with increasing temperature and photoperiod. S. minutulus conformed to the Droop relationship but N. acicularis did not. A model with a temperature-dependent normalised half-saturation coefficient adequately described the factor interactions of both species. The temperature dependence of the quota model reflected each species’ specific adaptation to its ecological niche. The results demonstrate that increases in temperature or photoperiod can partially compensate for a decrease in P-quota under moderately limiting conditions, like during spring in temperate lakes. Thus warming may counteract de-eutrophication to some degree and a relative shift in growth factors can influence the phytoplankton species composition.     

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.     

Influence of experimental,environmental, and geographic factors on nutrient‐diffusing substrate experiments in running waters

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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.     

Elemental concentrations and correlations in winter micropopulations of Stephanodiscus rotula: an autecological study over a period of cell size reduction and restoration

The population of Stephanodiscus rotula in a temperate eutrophic lake was studied over a 2 month period (January to early March). Although during the study period nutrient concentrations in the lake water remained far in excess of phytoplankton requirements, no notable increase in chlorophyll levels was recorded. This suggests that algae were limited by shortage of light and low temperatures. Frequency distribution analysis of cell diameters showed a dramatic size reduction at the end of winter, followed by restoration of higher values by early spring. Electron probe X-ray microanalysis spectra from Stephanodiscus cells routinely showed peaks of Ca, K, Si, P, S, Fe, Al, Mn, Mg, Na and Cl, with substantial variation in elemental concentrations both between and within samples. End-of-winter reduction in the cell size coincided with a considerable depletion of intracellular chemical levels of Si, P, Cl and K and could be related to the concurrent decrease in dissolved organic C and increase in intracellular Al. Correlation and factor analysis of intracellular elemental concentrations showed that statistical elemental associations within Stephanodiscus cells were mainly determined by three factors, with P, Cl, Si and K showing higher loadings on the first, Ca, Mn and S on the second, and Fe, Ca and Al on the third factor. Significant correlations among the elements of the first association may indicate the importance of P (ATP), K (through involvement in P metabolism) and Cl (possibly charge balance) in the active Si uptake during the study period.     

Tracing Si–N–P ecosystem-pathways: is relative uptake in riparian vegetation influenced by soil waterlogging,mowing management and species diversity?

Despite the growing concern about the importance of silicon (Si) in controlling ecological processes in aquatic ecosystems, little is known about its processing in riparian vegetation, especially compared to nitrogen (N) and phosphorus (P). We present experimental evidence that relative plant uptake of N and P compared to Si in riparian vegetation is dependent on mowing practices, water-logging and species composition. Results are obtained from a controlled and replicated mesocosm experiment, with a full-factorial design of soil water logging and mowing management. In our experiments, the Si excluding species Plantago lanceolata was dominant in the mown and non-waterlogged treatments, while Si accumulating meadow grasses and Phalaris arundinacea dominated the waterlogged treatments. Although species composition, management and soil moisture interacted strongly in their effect on relative Si:N and Si:P uptake ratios, the uptake of N to P remained virtually unchanged over the different treatments. Our study sheds new light on the impact of riparian wetland ecosystems on nutrient transport to rivers. It indicates that it is essential to include Si in future studies of the impact of riparian vegetation on nutrient transport, as these are often implemented as a measure to moderate excessive N and P inputs.     

TEMPERATURE EFFECTS ON SILICON LIMITED GROWTH OF THE LAKE MICHIGAN DIATOM STEPHANODISCUS MINUTUS (BACILLARIOPHYCEAE)1

The effect of temperature on the silicon limited growth and nutrient kinetics of Stephanodiscus minutus Grun. was examined using batch and semicontinuous culture methods. Short-term batch culture methods gave maximum growth rates which were essentially constant over the temperature range of 10° to 20°C (μ₃= 0.71–0.80 d^?1). The half-saturation constant for growth (K_s) was significantly lowest at 10°C (K_s= 0.31 μM Si; 0.22–0.41), and higher at both 15°C (K_s= 1.03 μM Si; 0.68–1.47) and 20°C (K_s= 0.88 μM Si; 0.60–1.22). Two methods were used to evaluate the semicontinuous experiments. The Droop relationship showed that the minimum cell quota was about 1.50 × 10^?7 nmol Si cell^?1, but there was much overlap in the results at all three temperatures. The Monod growth relationship for the semicontinuous experiments gave estimates of K_s which were lowest at 15°C (K_s= 0.12 μM Si), and higher at 10°C (K_s= 0.68 μM Si) and 20°C (K_s= 1.24 μM Si), although 95% confidence intervals overlapped. The maximum growth rate estimates for the semicontinuous experiments were similar at 10° and 15°, and higher at 20°C, but the number of points used in making the calculations makes the results less reliable than those from batch cultures. Generally, there were no consistent significant differences in the silicon limited growth of S. minutus over the temperature range studied. Our values of K_s for S. minutus are the lowest recorded for a freshwater diatom, and are consistent with the distribution of this species in nature. Generally, this species becomes abundant in areas with high phosphorus loading and very low silicon levels (low Si:P loading rates). Stephanodiscus species are also fossil indicators of eutrophication in north temperate lakes.     

Elemental composition of strawberry plants inoculated with the plant growth‐promoting bacterium Azospirillum brasilense REC3, assessed with scanning electron microscopy and energy dispersive X‐ray analysis

The elemental composition of strawberry plants (Fragaria ananassa cv. Macarena) inoculated with the plant growth‐promoting bacterium Azospirillum brasilense REC3, and non‐inoculated controls, was studied using scanning electron microscopy (SEM) and energy dispersive X‐ray (EDS) analysis. This allowed simultaneous semi‐quantification of different elements in a small, solid sample. Plants were inoculated and grown hydroponically in 50% or 100% Hoagland solution, corresponding to limited or optimum nutrient medium, respectively. Bacteria‐inoculated plants increased the growth index 45% and 80% compared to controls when grown in 100% and 50% Hoagland solution, respectively. Thus, inoculation with A. brasilense REC3 in a nutrient‐limited medium had the strongest effect in terms of increasing both shoot and root biomass and growth index, as already described for Azospirillum inoculated into nutrient‐poor soils. SEM‐EDS spectra and maps showed the elemental composition and relative distribution of nutrients in strawberry tissues. Leaves contained C, O, N, Na, P, K, Ca and Cu, while roots also had Si and Cl. The organic fraction (C, O and N) accounted for over 96.3% of the total chemical composition; of the mineral fraction, Na had higher accumulation in both leaves and roots. Azospirillum‐inoculated and control plants had similar elemental quantities; however, in bacteria‐inoculated roots, P was significantly increased (34.33%), which constitutes a major benefit for plant nutrition, while Cu content decreased (35.16%).     

ELEMENTAL AND BIOCHEMICAL COMPOSITION OF RHINOMONAS RETICULATA (CRYPTOPHYTA) IN RELATION TO LIGHT AND NITRATE‐TO‐PHOSPHATE SUPPLY RATIOS1

The biochemical basis for variations in the critical nitrogen‐to‐phosphorus (N:P) ratio, which defines the transition between N‐ and P‐limitation of growth rate, is currently not well understood. To assess this issue, we cultured the cryptophyte Rhinomonas reticulata NOVARINO in chemostats with inflow nitrate‐to‐phosphate ratios ranging from 5 to 60 mol N·(mol P)^?1 at two light intensities. The nitrate‐to‐phosphate ratio marking the transition between N‐ and P‐limitation was independent of light intensity and was between 30 and 45 mol N/mol P. In N‐limited cells, the particulate N:P ratio was stable at around 23 mol N/mol P over a range of inflow nitrate‐to‐phosphate from 5 to 30, whereas in P‐limited cells this ratio was around 90 mol N/mol P at inflow nitrate‐to‐phosphate ratios of 45 and 60. Cell phosphorus decreased with increasing nitrate‐to‐phosphate ratio up to the critical nitrate‐to‐phosphate ratio for each light intensity, above which they remained stable. The C:P of R. reticulata cells increased with increasing inflow nitrate‐to‐phosphate from around the Redfield value (106 mol C/mol P) to around 700. There was a significant effect of light on C:P in the N‐ limited cells, with higher C:P under high light conditions that was not observed in the P‐limited chemostats. Cellular RNA was not influenced by light but was greatly influenced by the type of nutrient limitation. In contrast, chl a, C, N, and protein were not influenced by the nitrate‐to‐phosphate in the inflow medium. Total protein per RNA was independent of light intensity but exhibited a maximum at inflow nitrate‐to‐phosphate of 30. Our results suggest a strong “two‐level” homeostatic mechanism of cellular N and P content in R. reticulata with two distinct states that are determined by the type of nutrient limitation and not by light.     

Enhancement of extraplastidic oil synthesis in Chlamydomonas reinhardtii using a type‐2 diacylglycerol acyltransferase with a phosphorus starvation–inducible promoter

When cultivated under stress conditions, many plants and algae accumulate oil. The unicellular green microalga Chlamydomonas reinhardtii accumulates neutral lipids (triacylglycerols; TAGs) during nutrient stress conditions. Temporal changes in TAG levels in nitrogen (N)‐ and phosphorus (P)‐starved cells were examined to compare the effects of nutrient depletion on TAG accumulation in C. reinhardtii. TAG accumulation and fatty acid composition were substantially changed depending on the cultivation stage before nutrient starvation. Profiles of TAG accumulation also differed between N and P starvation. Logarithmic‐growth‐phase cells diluted into fresh medium showed substantial TAG accumulation with both N and P deprivation. N deprivation induced formation of oil droplets concomitant with the breakdown of thylakoid membranes. In contrast, P deprivation substantially induced accumulation of oil droplets in the cytosol and maintaining thylakoid membranes. As a consequence, P limitation accumulated more TAG both per cell and per culture medium under these conditions. To enhance oil accumulation under P deprivation, we constructed a P deprivation‐dependent overexpressor of a Chlamydomonas type‐2 diacylglycerol acyl‐CoA acyltransferase (DGTT4) using a sulphoquinovosyldiacylglycerol 2 (SQD2) promoter, which was up‐regulated during P starvation. The transformant strongly enhanced TAG accumulation with a slight increase in 18 : 1 content, which is a preferred substrate of DGTT4. These results demonstrated enhanced TAG accumulation using a P starvation–inducible promoter.     

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.     

Differences in nutrient limitation and grazer suppression of phytoplankton in seepage and drainage lakes of the Adirondack region,NY, U.S.A

1. For seepage and drainage lakes of the Adirondack mountain region (NY, U.S.A) hydrologic regime is correlated with physical and chemical differences that can affect phytoplankton and planktonic food webs (e.g. presence and influence of wetlands, dissolved organic carbon concentration, anoxia, nutrient cycling). We conducted short‐term (48 h), in situ enclosure experiments to evaluate the relative importance of macrozooplankton grazing and nutrient limitation of phytoplankton biomass in small Adirondack seepage and drainage lakes (N = 18, 1–137 ha). Epilimnetic dissolved organic carbon (DOC) concentrations and pH values represented the diversity of the region. We measured chlorophyll a changes in response to grazer removal (> 120 μm) and nutrient addition (～ 10× ambient N, P, or N + P), and evaluated changes with respect to in situ light, temperature, NO₃, NH₄, SRP, and crustacean assemblage characters. 2. Nutrient addition stimulated significant increase in chlorophyll a concentration at 11 of 18 sites (GLM, Tukey–Kramer). Phytoplankton of clearwater drainage lakes were P‐limited, whereas clearwater and brownwater seepage lakes responded to additions of N and/or N + P. Relative light availability explained half the variance in response to nutrient addition in drainage (r2 = 0.48), but not seepage lake experiments (P > 0.05). 3. We observed responses to grazer removal at eight of 18 sites, usually clearwater drainage lakes. Crustacean grazing may be as significant as nutrient limitation of [chl a] for many drainage lake phytoplankton assemblages. Responses were related to in situ density of zooplankton only in drainage lakes. Light explained some variability in response to grazer removal for drainage (r2 = 0.35) and seepage lake experiments (r2 = 0.35). 4. These experiments provide evidence that hydrology may ultimately play an important role in determining nutrient and grazer regulation of phytoplankton. Proximate mechanisms affecting our results may be associated with differences in wetland vegetation, [DOC], and nutrient cycling.     

Relative importance of competition with Daphnia (Cladocera) and nutrient limitation on Anuraeopsis (Rotifera) population dynamics in a laboratory study

• 1 In the laboratory, the growth and reproduction of Anuraeopsis fissa were measured when fed on Scenedesmus species grown in nutrient‐sufficient, nitrogen‐limited and phosphorus‐limited media and in the presence or absence of one adult Daphnia longispina per vial.
• 2 Poor food quality may reduce the effect of competition on rotifers. Competition from Daphnia was stronger with nutrient sufficient algae than with nutrient limited algae. P‐limitation significantly reduced Anuraeopsis population growth rate and fecundity. The effect of nutrient limitation on Anuraeopsis was stronger than that of competition with Daphnia. The Anuraeopsis population declined with P‐limited food in both the presence and absence of Daphnia.
• 3 Exploitative competition by Daphnia on Anuraeopsis was stronger in the nutrient‐sufficient treatment than in the N‐limited one. Density, fecundity and population growth rate of Anuraeopsis were negatively affected by Daphnia in the nutrient‐sufficient treatment, while only fecundity was reduced by Daphnia in the N‐limited treatment. Consequently, in the N‐limited treatment, mortality should be lower in the presence of Daphnia. This result could suggest that Anuraeopsis lives longer when short of nitrogen.
• 4 Nutrient limitation may affect to the competitive interactions between zooplankton species. P‐limitation decreased the quality of algae as food for Anuraeopsis while N‐limitation decreased the susceptibility of this rotifer species to exploitative competition by Daphnia.

     

APPLICATION OF THE PDMPO TECHNIQUE IN STUDYING SILICA DEPOSITION IN NATURAL POPULATIONS OF FRAGILARIA CROTONENSIS (BACILLARIOPHYCEAE) AT DIFFERENT DEPTHS IN A EUTROPHIC RESERVOIR1

At weekly intervals from July to October 2006, we measured silica deposition in the summer diatom assemblage at various depths in the eutrophic ?ímov Reservoir (Czech Republic) using PDMPO, the 2‐(4‐pyridyl)‐5{[4‐(2‐dimethylaminoethyl‐aminocarbamoyl)‐methoxy]phenyl}oxazole labeling technique. Fluorescence microscopy coupled with image analysis allows quantifying silicon (Si) deposition over time and a simple distinction between cells that are actively depositing Si and those that are not. Diatom assemblage was exclusively dominated by Fragilaria crotonensis Kitton, which formed pronounced subsurface maxima (2–6.5 m). Concentrations of the main nutrients (Si and phosphorus, P) were low over the whole season; however, at depth, the nutrient availability was higher than at the surface. Fragilaria silica deposition rates were eight times higher at the surface than at depth. Half the population was involved in silica deposition at the surface, while only 20% active cells were doing so at depth. At the surface, silica deposition was limited by P deficiency; the effect of dissolved Si (DSi) was not statistically significant. Silica deposition at depth was significantly constrained by low light availability despite the 1% average light attenuation at depth, which is supposed sufficient for photosynthesis. This study represents the first attempt to employ the PDMPO technique coupled with quantitative image analysis of PDMPO fluorescence in freshwater ecology. On the basis of our results, PDMPO probe appears to be an appropriate proxy for the study of resource limitation in natural diatom populations.