Ammonium rhythm in cultures of the cyanobacterium Microcystis firma

Over a period of several days, rhythmic changes in extracellular NH⁺₄ concentration take place in cultures of the cyanobacterium Microcystis firma (Bré et Lenorm.) Schmidle, strain Gromov/St. Petersb. 398, under conditions of restricted CO₂ supply and light/dark alternation. The changes are enhanced by nitrate supply. Among the various processes generating intracellular NH⁺₄ (NH⁴₄ uptake, NO⁻₃ reduction, protein and amino acid degradation, photorespiration), NO⁻₃ reduction appears as the one most important. This can be concluded from experiments with and without nitrate and/or ammonium in the medium. In the presence of saturating CO₂, continuous light, or continuous darkness, rhythmic NH⁺⁴₄ oscillations are not induced. Studies of the incorporation of NH⁺₄ nitrogen by in vivo ¹⁵N-NMR show that if CO₂ is supplied, ¹⁵N is accumulated in several components with the following time course: in the first hour in Gln (δ), in the second hour in the α-amino groups of most nonbranched amino acids, in the third hour in γ-aminobutyric acid (GABA), Orn (δ) and Lys (ε), and in the sixth hour in Ala. Carbon limitation, however, results in accumulation of label in the amide nitrogen of glutamine only.     

Accelerated organic matter decomposition in thermokarst lakes upon carbon and phosphorus inputs

Mineralization of dissolved organic matter (DOM) in thermokarst lakes plays a non-negligible role in the permafrost carbon (C) cycle, but remains poorly understood due to its complex interactions with external C and nutrient inputs (i.e., aquatic priming and nutrient effects). Based on large-scale lake sampling and laboratory incubations, in combination with ¹³C-stable-isotope labeling, optical spectroscopy, and high-throughput sequencing, we examined large-scale patterns and dominant drivers of priming and nutrient effects of DOM biodegradation across 30 thermokarst lakes along a 1100-km transect on the Tibetan Plateau. We observed that labile C and phosphorus (P) rather than nitrogen (N) inputs stimulated DOM biodegradation, with the priming and P effects being 172% and 451% over unamended control, respectively. We also detected significant interactive effects of labile C and nutrient supply on DOM biodegradation, with the combined labile C and nutrient additions inducing stronger microbial mineralization than C or nutrient treatment alone, illustrating that microbial activity in alpine thermokarst lakes is co-limited by both C and nutrients. We further found that the aquatic priming was mainly driven by DOM quality, with the priming intensity increasing with DOM recalcitrance, reflecting the limitation of external C as energy sources for microbial activity. Greater priming intensity was also associated with higher community-level ribosomal RNA gene operon (rrn) copy number and bacterial diversity as well as increased background soluble reactive P concentration. In contrast, the P effect decreased with DOM recalcitrance as well as with background soluble reactive P and ammonium concentrations, revealing the declining importance of P availability in mediating DOM biodegradation with enhanced C limitation but reduced nutrient limitation. Overall, the stimulation of external C and P inputs on DOM biodegradation in thermokarst lakes would amplify C-climate feedback in this alpine permafrost region.     

VARIABILITY IN NITRATE UPTAKE KINETICS IN THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE)1

Short-term (1–9 min) nitrate uptake kinetics were measured in Thalassiosira pseudonana (Hust.) Hasle & Heimdal grown in nitrate-limited, ammonium-limited, and nitrate-sufficient continuous cultures. For all cultures, maximal nitrate uptake rates did not develop until approximately 3 min after nitrate addition; thereafter, nitrate uptake rates remained constant or declined slightly. The K_s and V_max for the nitrate-limited cultures were higher at any growth rate than those for the ammonium-limited or nitrate-sufficient cultures. Thus, much higher nitrate concentrations would be required to saturate nitrate uptake in nitrate-limited Thalassiosira pseudonana than is usually considered necessary. The lack of data for other species grown under a range of environmental conditions makes it difficult to generalize about the effect of preconditioning on nitrate uptake kinetics.     

COMPETITION BETWEEN STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA(CYANOPHYCEAE) UNDER VARYING MODES OF PHOSPHATE SUPPLY1

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. were grown in mixed cultures with various phosphate (P_i) additions. One pulse of P_i each day (semi-continuous cultures) favored M. aeruginosa whereas S. luetkemuellerii was favored when the same quantity of P_i was supplied continuously (chemostats). Both species coexisted under P limitation provided that the nutrient was supplied in an appropriate mode. The ability of each species to compete for P depended on their P_i uptake characteristics and their capability to retain the accumulated P_i. High affinity in uptake at low P_i concentrations contributed considerably to the growth eficiency of S. luetkemuellerii under continuous supply of P_iM. aeruginosa was, however, consistently superior to S. luetkemuellerii in accuniulatiug the newly added P, but had a high rate of P_i release. In both -types of cultures, a net high of P went from M. aeruginosa to S. luetkemuellerii. The kinetic characteristics of the two species were used to simulate the outcome of competition experiments. Simulations agreed with the experimental data f both uptake and P_i release were considered in the model. The zlariable P*(the concentration of P_i at which the net uptake is equal to μ·Q_P is a function of uptake and release of P_i but could not explain the chemostat results. S. luetkemuellerii was the winner in many experiments even if its P*was higher thou that of M. aeruginosa. Thus, in the present case P_c (the concentration at which the net uptake is zero) was a better predictor of the ability to compete for P_i under steady state as well as transient conditions in the P_i supply.     

Restoration of eutrophied shallow softwater lakes based upon carbon and phosphorus limitation

Plant communities from oligotrophic, poorly buffered waters are seriously threatened by both, acidification and eutrophication/alkalinization. Acidification is mainly caused by atmospheric deposition of acidifying substances while eutrophication is often the result of inlet of nutrient enriched, calcareous brook- or groundwater. The plant production in very soft waters is often limited by low levels of inorganic carbon, nitrogen and/or phosphorus. This paper deals with the possibilities for restoration of formerly oligotrophic but now eutrophied and alkalinized softwater systems. Restoration based upon nitrogen limitation is not likely to be successful as the atmospheric deposition of nitrogen in The Netherlands is very high. Phosphorus limitation can also be a problem. One can stop the input of phosphorus and remove the mud layer, but the problem remains that also the deeper mineral sandy sediments are saturated with phosphate. A possible remedy, however, is a combination of carbon- and phosphorus limitation. Many plants from eutrophic environments never occur in very soft waters, probably as a result of carbon limitation. In addition, mobilisation of phosphate is much lower in waters with very low bicarbonate levels. Restoration of a former oligotrophic softwater lake by reducing the inlet of calcareous surface water, in combination with removal of the organic sediment layer, appeared to be very successful. Many endangered plant species such asIsoetes echinospora, Luronium natans, Deschampsia setacea andEchinodorus repens developed spontaneously from the still viable seedbank.     

Mating tactics in external fertilizers when sperm is limited

Among externally fertilizing animals in aquatic habitats, theproportion of a female's egg clutch that is successfully fertilizedoften falls below 100%. In many such species, particularly incoral reef fishes, males spawn daily at high frequencies, oftenwith little or no sperm competition. A major evolutionary problemfor such males is how best to allocate sperm over successivespawns. Females face the problem of ensuring complete fertilizationof their egg clutch. Here we model male and female mating tacticswhen daily sperm production is limited and with various assumptionsconcerning how differences in the number of sperm released duringa mating influence the number of eggs fertilized. The modelsreveal conditions under which males can maximize daily reproductivesuccess, either by releasing a fixed number of sperm duringall successive spawns or by matching sperm numbers to the clutchsize of their mates. These patterns of sperm allocation exertdifferent pressures on females, which may respond evolutionarilyby developing various mating tactics of their own.     

Phosphorus limitation of forest leaf area and net primary production on a highly weathered soil

We tested the hypothesis that P was the nutrient limiting net primary production of a nativeMetrosideros polymorpha forest on a highly weathered montane tropical soil in Hawaii. A factorial experiment used all combinations of three fertilizer treatments: nitrogen (N), phosphorus (P) and a mix of other essential nutrients (OE), consisting primarily of mineral derived cations and excluding N and P. P addition, but not N or OE, increased leaf area index within 12 months, foliar P concentration measured at 18 months, and stem diameter increment within 18 months. Stem growth at 18 months was even greater when trees fertilized with P also received the OE treatment. N and P additions increased leaf litterfall and N and P in combination further increased litterfall. The sequence of responses suggests that increased available P promoted an increase in photosynthetic area which led to increased wood production. P was the essential element most limiting to primary production on old volcanic soil in contrast to the N limitation found on young volcanic soils.     

Production rates of Eurytemora affinis in the Elbe estuary,comparison of field and enclosure production estimates

Production rates of the calanoid copepod Eurytemora affinis were estimated from field studies in the Elbe estuary and from an enclosure experiment. As one basic parameter of production rates, the body length, was compared between both investigations. Most of the copepodid stages in the enclosure experiment reached a significant greater length than the copepodids in the estuary. The differences in length between copepods from the field and the experiment could mainly be explained by a four times higher chlorophyll-a level in the enclosure experiment. The better food supply also results in a higher individual growth rate for all instars in the enclosure experiment. Therefore the population of Eurytemora affinis in the Elbe estuary was regarded as food limited during certain times of the year, especially in late spring and summer.Maximum daily production rate in the enclosure experiment (40 µg dw l^–1 d^–1) was four times higher than in the estuary (12 µg dw l^–1 d^–1). The mean daily P:B ratio in the enclosure was 0.301 d^–1 compared to 0.11 d^–1 in the estuary.     

Ammonium-limited growth and uptake by Oscillatoria agardhii in chemostat cultures

Growth of Oscillatoria agardhil was studied in ammonium-limited chemostat cultures, at various dilution rates (=growth rates, μ). The uptake kinetics for ammonium of nitrogen (ammonium or nitrate)-limited chemostat cultures also was investigated. The kinetics of ammonium-limited growth could be adequately described by both the Monod and Droop equations, and were closely similar to the nitrate-limited growth kinetics of this species. The uptake kinetics for ammonium showed similarities as well as differences with the uptake kinetics for nitrate. The similarities were apparent in the uptake capacity values for ammonium and nitrate , which were identical, high and independent of μ. The differences were to be found in the half-saturation constants for ammonium uptake and nitrate uptake , the former being hardly influenced by μ. A consitutive, high affinity, system is likely to operate in the uptake and assimilation of ammonium by nitrogen-limited O. agardhii. The use of ammonium uptake parameters in studies of growth-limiting factors in nature can provide information as to whether a nitrogen-limitation prevails in natural habitats of this species.