Modeling the C Economy of Anabaena flos-aquae: Estimates of Establishment, Maintenance, and Active Costs Associated with Growth on NH(3), NO(3), and N(2)

Grazing preferences of three species of amoebae on cyanobacteria and green algae

Twenty species/isolates of cyanobacteria and green algae were isolated from cyanobacterial bloom samples in lakes associated with the upper Qu'Appelle River drainage system in southern Saskatchewan, Canada. Three amoebae species (Cochliopodium sp., Vannella sp. and Vermamoeba vermiformis) were also isolated from one of these samples, and were subjected to grazing assays to determine which species of cyanobacteria or algae could potentially serve as a food source. Amoeba grazing rates were quantified based on the diameter of the plaque after 12 days on agar plate assays, and by estimation of the amoeba population growth rate from the rate of increase of plaque area. The common cyanobacterial bloom-formers Dolichospermum sp. and Aphanizomenon flos-aquae supported high growth rates for all three amoebae, while green algae, with the exception of one green alga/amoeba combination, did not support growth of the tested amoebae. Many of the cyanobacterial and algal isolates that did not support amoebae growth were ingested, suggesting that ingestion did not determine grazing success. Overall, while the cyanobacteria Dolichospermum sp. and Aphanizomenon flos-aquae were suitable food sources for the amoebae, the other cyanobacteria were grazed in an unpredictable manner, with some species/strains grazed by some amoebae and some species not grazed at all.

Effects of experimental nitrogen fertilization on planktonic metabolism and CO2 flux in a hypereutrophic hardwater lake

Hardwater lakes are common in human-dominated regions of the world and often experience pollution due to agricultural and urban effluent inputs of inorganic and organic nitrogen (N). Although these lakes are landscape hotspots for CO₂ exchange and food web carbon (C) cycling, the effect of N enrichment on hardwater lake food web functioning and C cycling patterns remains unclear. Specifically, it is unknown if different eutrophication scenarios (e.g., modest non point vs. extreme point sources) yield consistent effects on auto- and heterotrophic C cycling, or how biotic responses interact with the inorganic C system to shape responses of air-water CO₂ exchange. To address this uncertainty, we induced large metabolic gradients in the plankton community of a hypereutrophic hardwater Canadian prairie lake by adding N as urea (the most widely applied agricultural fertilizer) at loading rates of 0, 1, 3, 8 or 18 mg N L^-1 week^-1 to 3240-L, in-situ mesocosms. Over three separate 21-day experiments, all treatments of N dramatically increased phytoplankton biomass and gross primary production (GPP) two- to six-fold, but the effects of N on autotrophs plateaued at ~3 mg N L^-1. Conversely, heterotrophic metabolism increased linearly with N fertilization over the full treatment range. In nearly all cases, N enhanced net planktonic uptake of dissolved inorganic carbon (DIC), and increased the rate of CO₂ influx, while planktonic heterotrophy and CO₂ production only occurred in the highest N treatments late in each experiment, and even in these cases, enclosures continued to in-gas CO₂. Chemical effects on CO₂ through calcite precipitation were also observed, but similarly did not change the direction of net CO₂ flux. Taken together, these results demonstrate that atmospheric exchange of CO₂ in eutrophic hardwater lakes remains sensitive to increasing N loading and eutrophication, and that even modest levels of N pollution are capable of enhancing autotrophy and CO₂ in-gassing in P-rich lake ecosystems.

Photosynthesis in estuarine intertidal microphytobenthos is limited by inorganic carbon availability

The effects of dissolved inorganic carbon (DIC) availability on photosynthesis were studied in two estuarine intertidal microphytobenthos (MPB) communities and in the model diatom species Phaeodactylum tricornutum. Kinetics of DIC acquisition, measured with a liquid-phase oxygen electrode, showed higher K(1/2)(DIC) (0.31 mM) and Vm (7.78 nmol min(-1) µg (Chl a)(-1)) for MPB suspensions than for P. tricornutum (K(1/2)(DIC) = 0.23 mM; Vm = 4.64 nmol min(-1) µg (Chl a)(-1)), suggesting the predominance of species with lower affinity for DIC and higher photosynthetic capacity in the MPB. The net photosynthetic rate of the MPB suspensions reached saturation at a DIC concentration of 1-1.5 mM. This range was lower than the concentrations found in the interstitial water of the top 5-mm sediment layer, suggesting no limitation of photosynthesis by DIC in the MPB communities. Accordingly, carbon isotope discrimination revealed a moderate activity of CO2-concentrating mechanisms in the MPB. However, addition of NaHCO3 to intact MPB biofilms caused a significant increase in the relative maximum photosynthetic electron transport rate (rETR max) measured by imaging pulse-amplitude modulated chlorophyll a fluorescence. These results suggest local depletion of DIC at the photic layer of the sediment (the first few hundred µm), where MPB cells accumulate during diurnal low tides. This work provides the first direct experimental evidence of DIC limitation of photosynthesis in highly productive intertidal MPB communities.

Effects of increasing nitrogen and phosphorus concentrations on phytoplankton community growth and toxicity during Planktothrix blooms in Sandusky Bay, Lake Erie

Sandusky Bay experiences annual toxic cyanobacterial blooms dominated by Planktothrix agardhii/suspensa. To further understand the environmental drivers of these events, we evaluated changes in the growth response and toxicity of the Planktothrix-dominated blooms to nutrient amendments with orthophosphate (PO4) and inorganic and organic forms of dissolved nitrogen (N; ammonium (NH4), nitrate (NO3) and urea) over the bloom season (June - October). We complemented these with a metagenomic analysis of the planktonic microbial community. Our results showed that bloom growth and microcystin (MC) concentrations responded more frequently to additions of dissolved N than PO4, and that the dual addition of NH4 + PO4 and Urea + PO4 yielded the highest MC concentrations in 54% of experiments. Metagenomic analysis confirmed that P. agardhii/suspensa was the primary MC producer. The phylogenetic distribution of nifH revealed that both heterocystous cyanobacteria and heterotrophic proteobacteria had the genetic potential for N2 fixation in Sandusky Bay. These results suggest that as best management practices are developed for P reductions in Sandusky Bay, managers must be aware of the negative implications of not managing N loading into this system as N may significantly impact cyanobacterial bloom size and toxicity.

The influence of water quality variables on cyanobacterial blooms and phytoplankton community composition in a shallow temperate lake

Cyanobacterial blooms and their detrimental effects on water quality have become a worldwide problem. Vancouver Lake, a tidally influenced shallow temperate freshwater lake in Washington state, U.S.A., exhibits annual summer cyanobacterial blooms that are of concern to local resource managers. Our objectives were to describe changes in phytoplankton community composition in Vancouver Lake over seasonal, annual, and interannual time scales, and to identify strong water quality predictors of phytoplankton community structure, with an emphasis on cyanobacterial blooms, from 2007 through 2010. Cluster analysis, indicator species analysis, and non-metric multidimensional scaling were used to identify significantly different phytoplankton community groupings and to determine which environmental factors influenced community changes. From 2007 through 2009, depletion of NO3-N followed by elevated PO4-P concentration was associated with increased biomass and duration of each cyanobacterial bloom. Time-lag analysis suggested that NO3-N availability contributed to interannual changes within the summer phytoplankton community. Specifically, in summer 2010, a distinct cyanobacteria community was not present, potentially due to increased NO3-N and decreased PO4-P and NH4-N availability. Our study provides a comprehensive assessment of species-level responses to water quality variables in a shallow non-stratifying temperate lake, contributes to a better understanding of phytoplankton dynamics, and may aid in predicting and managing cyanobacterial blooms.

Cylindrospermopsis raciborskii: review of the distribution, phylogeography, and ecophysiology of a global invasive species

Cylindrospermopsis raciborskii is a cyanobacterial species extensively studied for its toxicity, bloom formation and invasiveness potential, which have consequences to public and environmental health. Its current geographical distribution, spanning different climates, suggests that C. raciborskii has acquired the status of a cosmopolitan species. From phylogeography studies, a tropical origin for this species seems convincing, with different conjectural routes of expansion toward temperate climates. This expansion may be a result of the species physiological plasticity, or of the existence of different ecotypes with distinct environmental requirements. In particular, C. raciborskii is known to tolerate wide temperature and light regimes and presents diverse nutritional strategies. This cyanobacterium is also thought to have benefited from climate change conditions, regarding its invasiveness into temperate climates. Other factors, recently put forward, such as allelopathy, may also be important to its expansion. The effect of C. raciborskii in the invaded communities is still mostly unknown but may strongly disturb species diversity at different trophic levels. In this review we present an up-to-date account of the distribution, phylogeography, ecophysiology, as well some preliminary reports of the impact of C. raciborskii in different organisms.

Phytoplankton-specific response to enrichment of phosphorus-rich surface waters with ammonium, nitrate, and urea

Supply of anthropogenic nitrogen (N) to the biosphere has tripled since 1960; however, little is known of how in situ response to N fertilisation differs among phytoplankton, whether species response varies with the chemical form of N, or how interpretation of N effects is influenced by the method of analysis (microscopy, pigment biomarkers). To address these issues, we conducted two 21-day in situ mesocosm (3140 L) experiments to quantify the species- and genus-specific responses of phytoplankton to fertilisation of P-rich lake waters with ammonium (NH₄⁺), nitrate (NO₃⁻), and urea ([NH₂]₂CO). Phytoplankton abundance was estimated using both microscopic enumeration of cell densities and high performance liquid chromatographic (HPLC) analysis of algal pigments. We found that total algal biomass increased 200% and 350% following fertilisation with NO₃⁻ and chemically-reduced N (NH₄⁺, urea), respectively, although 144 individual taxa exhibited distinctive responses to N, including compound-specific stimulation (Planktothrix agardhii and NH₄⁺), increased biomass with chemically-reduced N alone (Scenedesmus spp., Coelastrum astroideum) and no response (Aphanizomenon flos-aquae, Ceratium hirundinella). Principle components analyses (PCA) captured 53.2–69.9% of variation in experimental assemblages irrespective of the degree of taxonomic resolution of analysis. PCA of species-level data revealed that congeneric taxa exhibited common responses to fertilisation regimes (e.g., Microcystis aeruginosa, M. flos-aquae, M. botrys), whereas genera within the same division had widely divergent responses to added N (e.g., Anabaena, Planktothrix, Microcystis). Least-squares regression analysis demonstrated that changes in phytoplankton biomass determined by microscopy were correlated significantly (p<0.005) with variations in HPLC-derived concentrations of biomarker pigments (r² = 0.13–0.64) from all major algal groups, although HPLC tended to underestimate the relative abundance of cyanobacteria. Together, these findings show that while fertilisation of P-rich lakes with N can increase algal biomass, there is substantial variation in responses of genera and divisions to specific chemical forms of added N.

Protein turnover and plant RNA and phosphorus requirements in relation to nitrogen fixation

Phosphorus (P) is the proximate (immediate) limiting element for primary productivity in some habitats, and is generally the ultimate limiting element for primary productivity. Although RNA can account for over half of the non-storage P in photosynthetic organisms, some primary producers have more ribosomes than the minimum needed for the observed rate of net protein synthesis; some of this RNA may be needed for protein turnover. Two cases of protein turnover which can occur at a much faster rate than the bulk protein turnover are those of photodamaged photosystem II and O(2)-damaged nitrogenase. While RNA involved in photosystem II repair accounts for less than 1% of the non-storage P in photosynthetic organisms, a maximum, of 12% of non-storage P could occur in RNA associated with replacement of damaged nitrogenase and/or O(2) damage avoidance mechanism in diazotrophic (N(2) fixing) organisms. There is a general trend in published data towards lower P use efficiency (g dry matter gain per day per mol P in the organism) for photosynthetic diazotrophic organisms growing under P limitation with N(2) as their nitrogen source, rather than with NH(4)(+), urea or NO(3)(-). Additional work is needed to examine the generality of a statistically verified decrease in P use efficiency for diazotrophic growth relative to growth on other nitrogen sources and, if this is confirmed, further investigation of the mechanism is needed. The outcome of such work would be important for relating the global distribution of diazotrophy to P availability. There are no known P acquisition mechanisms specific to diazotrophs. Phosphorus (P) is the proximate (immediate) limiting element for primary productivity in some habitats, and is generally the ultimate limiting element for primary productivity. Although RNA can account for over half of the non-storage P in photosynthetic organisms, some primary producers have more ribosomes than the minimum needed for the observed rate of net protein synthesis; some of this RNA may be needed for protein turnover. Two cases of protein turnover which can occur at a much faster rate than the bulk protein turnover are those of photodamaged photosystem II and O(2)-damaged nitrogenase. While RNA involved in photosystem II repair accounts for less than 1% of the non-storage P in photosynthetic organisms, a maximum, of 12% of non-storage P could occur in RNA associated with replacement of damaged nitrogenase and/or O(2) damage avoidance mechanism in diazotrophic (N(2) fixing) organisms. There is a general trend in published data towards lower P use efficiency (g dry matter gain per day per mol P in the organism) for photosynthetic diazotrophic organisms growing under P limitation with N(2) as their nitrogen source, rather than with NH(4)(+), urea or NO(3)(-). Additional work is needed to examine the generality of a statistically verified decrease in P use efficiency for diazotrophic growth relative to growth on other nitrogen sources and, if this is confirmed, further investigation of the mechanism is needed. The outcome of such work would be important for relating the global distribution of diazotrophy to P availability. There are no known P acquisition mechanisms specific to diazotrophs.

DIFFERENCES IN GROWTH AND PHYSIOLOGY OF MARINE SYNECHOCOCCUS (CYANOBACTERIA) ON NITRATE VERSUS AMMONIUM ARE NOT DETERMINED SOLELY BY NITROGEN SOURCE REDOX STATE(1)

The preference of phytoplankton for ammonium over nitrate has traditionally been explained by the greater metabolic cost of reducing oxidized forms of nitrogen. This "metabolic cost hypothesis" implies that there should be a growth disadvantage on nitrate compared to ammonium or other forms of reduced nitrogen such as urea, especially when light limits growth, but in a variety of phytoplankton taxa, this predicted difference has not been observed. Our experiments with three strains of marine Synechococcus (WH7803, WH7805, and WH8112) did not reveal consistently faster growth (cell division) on ammonium or urea as compared to nitrate. Urease and glutamine synthetase (GS) activities varied with nitrogen source in a manner consistent with regulation by cellular nitrogen status via NtcA (rather than by external availability of nitrogen) in all three strains and indicated that each strain experienced some degree of nitrogen insufficiency during growth on nitrate. At light intensities that strongly limited growth, the composition (carbon, nitrogen, and pigment quotas) of WH7805 cells using nitrate was indistinguishable from that of cells using ammonium, but at saturating light intensities, cellular carbon, nitrogen, and pigment quotas were significantly lower in cells using nitrate than ammonium. These and similar results from other phytoplankton taxa suggest that a limitation in some step of nitrate uptake or assimilation, rather than the extra cost of reducing nitrate per se, may be the cause of differences in growth and physiology between cells using nitrate and ammonium.

Heterocyst formation

Heterocysts are microaerobic, N2-fixing cells that form in a patterned array within O2-producing filamentous cyanobacteria. Structural features of heterocysts can be predicted from consideration of their physiology. This review focuses on the spacing mechanism that determines which cells will differentiate, and on the regulation of the progression of the differentiation process. Applicable genetic tools, developed primarily using Anabaena PCC 7120, but employed also with Nostoc spp., are reviewed. These tools include localization of transcription using fusions to lux, lac, and gfp, and mutagenesis with oriV-containing derivatives of transposon Tn5. Mature and developing heterocysts inhibit nearby vegetative cells from differentiating; genes patA, devA, hetC, and the hetMNI locus may hold keys to understanding intercellular interactions that influence heterocyst formation. Regulatory and other genes that are transcriptionally activated at different times after nitrogen stepdown have been identified, and should permit analysis of mechanisms that underlie the progression of heterocyst differentiation.

Model of gas exchange and diffusion in legume nodules : I. Calculation of gas exchange rates and the energy cost of N2 fixation

A mathematical model is described which allows the estimation of rates of O2, CO2, N2, and H2 exchange from legume nodules under steady state conditions of N2 fixation. Calculated rates of gas exchange under defined conditions of nodule size, relative growth rate (RGR), specific total nitrogenase activity (TNA), nitrogenase electron allocation coefficient (EAC), uptake-hydrogenase activity (HUP) and nature of the N export product compared favorably with experimentally-obtained rates reported in the literature. Therefore the model was used to predict the effects of varying each of these nodule characteristics on the rates of gas exchange, and on the apparent respiratory cost (CO2/NH3) and sucrose cost (sucrose consumed/NH3) of N2 fixation.The model predicted that, all other characters being equal, ureide-producing nodules would consume 8% less sucrose per N fixed than asparagine-producing nodules, but would display an apparent respiratory cost which would be 5% higher than that in asparagine-producing nodules. In both ureide-producing and asparagine-producing nodules, the major factor affecting the apparent respiratory cost of N2 fixation was predicted to be EAC, followed by TNA, nodule RGR and nodule size. The relative importance of HUP in improving the apparent respiratory cost of N2 fixation was predicted to be largely dependent upon its potential role in the regulation of EAC.

Effect of N Source on the Steady State Growth and N Assimilation of P-limited Anabaena flos-aquae

Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH(3), NO(3) (-), or N(2). Expressed as a function of relative growth rate, steady state cellular P contents and PO(4) assimilation rates did not vary with N-source. However, N-source did alter the maximal PO(4)-limited growth rate achieved by the cultures: the NO(3) (-) and N(2) cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH(3) grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO(3) (-)-grown cells were the smallest (627 +/- 34 micromoles C . 10(-9) cells), while NH(3)-grown cells were largest (900 +/- 44 micromoles C . 10(-9) cells) and N(2)-fixing cells were intermediate (726 +/- 48 micromoles C . 10(-9) cells) in size. In the NO(3) (-)-and N(2)-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH(3)-grown cells. Heterocysts were absent in NH(3)-grown cultures but were present in both the N(2) and NO(3) (-) cultures. In the NO(3) (-)-grown cultures C(2)H(2) reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N(2)-fixing cultures the acetylene:N(2) ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal.Compared with NH(3), the assimilation of NO(3) (-) and N(2) resulted either in a decrease in cellular C (NO(3) (-) and N(2) cultures) or in a lower maximal growth rate (N(2) culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO(3) (-) or N(2) compared with NH(3).