Low Half-saturation Constants Research Articles

Phytoplankton Dynamics and Biogeochemistry: Model Studies

The seasonal dynamics of the NE Black Sea phytoplankton follow the following pattern: small diatoms (spring) → coccolithophorid Emiliania huxleyi (late spring–early summer) → large diatoms (summer). Our hypothesis states that nitrogen and phosphorus concentrations regulate the seasonal phytoplankton dynamics. A minimum number of parameters is enough to understand the mechanisms of dominant species change. Based on the concept of intracellular regulation, the following parameters were evaluated: the minimum nitrogen and phosphorus quotas; half-saturation constants for nitrogen and phosphorus uptake; the maximum specific growth rate of the dominant phytoplankton species. Computational experiments on the model show the following: (1) in spring, a species with a high maximum specific growth rate becomes dominant; (2) in late spring and early summer, a species with a low minimum nitrogen quota and a low half-saturation constant for nitrogen uptake is observed; (3) in summer, a low minimum phosphorus quota and a low half-saturation constant for phosphorus uptake allow the species to become dominant.

Reduced nitrite accumulation at the primary nitrite maximum in the cyclonic eddies in the western North Pacific subtropical gyre.

Nitrite, an intermediate product of the oxidation of ammonia to nitrate (nitrification), accumulates in upper oceans, forming the primary nitrite maximum (PNM). Nitrite concentrations in the PNM are relatively low in the western North Pacific subtropical gyre (wNPSG), where eddies are frequent and intense. To explain these low nitrite concentrations, we investigated nitrification in cyclonic eddies in the wNPSG. We detected relatively low half-saturation constants (i.e., high substrate affinities) for ammonia and nitrite oxidation at 150 to 200 meter water depth. Eddy-induced displacement of high-affinity nitrifiers and increased substrate supply enhanced ammonia and nitrite oxidation, depleting ambient substrate concentrations in the euphotic zone. Nitrite oxidation is more strongly enhanced by the cyclonic eddies than ammonia oxidation, reducing concentrations and accelerating the turnover of nitrite in the PNM. These findings demonstrate a spatial decoupling of the two steps of nitrification in response to mesoscale processes and provide insights into physical-ecological controls on the PNM.

Evaluation of the key factors to dominate aerobic ammonia-oxidizing archaea in wastewater treatment plant

Wastewater treatment plants (WWTPs) face great challenges globally, especially in China, for increasingly strict discharge standard. Abundance of AOA was found dramatically higher in a SBR WWTP in South China (ZSH) recently, which achieved extremely low ammonium concentration in effluents. Moreover, AOA equips lower half-saturation constant (Km) for dissolved oxygen (DO) and ammonia in comparison with AOB, which indicate AOA can oxidize ammonia to nitrite more thoroughly than AOB. Thus, AOA may play an important role for nitrogen removal in SBR WWTP. To understand the key factors for AOA dominance in SBR WWTP is of importance for designing and operating WWTP in future. In this study, a Nitrosocosmicus sp. AOA was enriched from the ZSH SBR WWTP. Growth, pH, temperature, ammonium and oxygen tolerance of AOA enrichments were explored. Moreover, ammonia-oxidizing pathway was investigated with presence of PTIO as inhibitor, and the production of middle products (NH2OH and N2O) during ammonia oxidation were monitored over time. Growth of Nitrosocosmicus sp. AOA was observed with a doubling time of 4.5 days, which was active under a wide pH range (6.0–9.0), with optimal pH of 7.5. Activity of Nitrosocosmicus sp. AOA increased with increasing of ammonium concentration below 40 mM, which was suppressed above 40 mM ammonium. Moreover, Km for ammonia and oxygen were 0.14 μM and 4.1 μM, respectively, which suggested the enriched AOA could thoroughly oxidize ammonia under low dissolved oxygen conditions. Activity of Nitrosocosmicus sp. AOA was completely inhibited by PTIO. Emission of N2O by AOA (0.05%) was lower than most reported AOB and Comammox bacteria. Our results demonstrated that the long sludge retention time, intermittent aeration, high ammonium and pH resistance were most probably the controlling factors for AOA dominance in WWTPs, which benefitted to lowing ammonium in effluents, saving energy and reducing greenhouse gas in future.

Alkaline phosphatase activity during a phosphate replete dinoflagellate bloom caused by Prorocentrum obtusidens

Prorocentrum obtusidens Schiller (formerly P. donghaiense Lu), a harmful algal species common in the East China Sea (ECS), often thrives with the depletion of phosphate. Three cruises in the spring of 2013 sampled an entire P. obtusidens bloom process to investigate the dynamics of alkaline phosphatase activity (APA) and phosphorus (P) status of the bloom species using both bulk and cell-specific assays. Unlike previous studies, the bloom of P. obtusidens occurred in a phosphate replete environment. Very high APA, with an average of 76.62 ± 90.24 nmol L–1 h–1, was observed during the early-bloom phase, a value comparable to that in low phosphate environments. The alkaline phosphatase (AP) hydrolytic kinetics also suggested a more efficient AP system with a lower half-saturation constant (Ks), but higher maximum potential hydrolytic velocity (Vmax) in this period. The APA decreased significantly with an average of 24.98 ± 30.98 nmol L–1 h–1 when the bloom reached its peak. The lack of a correlation between dissolved inorganic phosphate (DIP) or dissolved organic phosphate (DOP) concentration and APA suggested that the APA was regulated by the internal P growth demand, rather than the external P availability during the phosphate replete P. obtusidens bloom. These findings facilitate an understanding of the P. obtusidens acclimation strategy with respect to P variations in terms of AP expression during blooms in the ECS.

Properties of main phytoplankton species of the Black Sea, determining the change in their dominance in annual dynamics

Using mathematical model, we tested three hypotheses of bloom regulation mechanisms for dominant phytoplankton species in the northeastern Black Sea. The model is based on the concept of intracellular regulation using the Droop equation, and values of the model constants that satisfy the conditions of domination are found. The bloom of the small-cell diatom Pseudo-nitzschia pseudodelicatissima is due to higher specific growth rates than other species in early spring. The half-saturation constants and minimum quotas for mineral nutrition elements (nitrogen and phosphorus) are also relatively high. In late spring and early summer, the bloom of coccolithophore Emiliania huxleyi is usually recorded. Prerequisites are a low half-saturation constant for nitrogen uptake and minimum quota for this nutrient. The third mechanism gains strength in the summer with the deepening of seasonal thermocline and the dominance of large-cell diatoms. It was shown that for the supremacy of large diatom Proboscia alata, it is necessary to have low half-saturation constants for the phosphorus uptake and minimum quotas for this nutrient.

Combined Effects of Experimental Warming and Eutrophication on Phytoplankton Dynamics and Nitrogen Uptake

Shallow lakes are highly vulnerable to damages caused by human activities and warming trends. To assess whether and how community structures of phytoplankton and nitrogen uptake respond to the combined effects of elevated temperature and eutrophication, we performed a mesocosm experiment in field by combining a 4.5 °C increase in temperature and the addition of phosphorus. Our results demonstrated that the combination of rising temperatures and phosphorus loading stimulated the maximum biomass built up by the phytoplankton community, and changed the phytoplankton community by significantly increasing the number of Chlorophyta and Cyanophyta, and decreasing that of Cryptophyta. We also examined the effects of climate warming and eutrophication on phytoplankton nitrogen uptake and dynamics using 15N tracer techniques. The addition of phosphorus slightly increased the phytoplankton nitrate uptake velocity and relative preference index, but decreased the nitrate uptake turnover time. Warming relatively increased the ammonium uptake velocity and the relative preference index, but decreased the ammonium turnover time. In kinetic studies, NH4+ exhibited a higher maximum uptake rate (Vmax) and a lower half-saturation constant (Ks) than NO3− substrates due to temperature elevation and the addition of phosphorus. Hence, warming and eutrophication increased the capacity of phytoplankton for NH4+ uptake and their affinity at low substrate concentrations. Thus, the combined effects of climate warming and phosphorus nutrient availability may increase the prevalence of Chlorophyta and Cyanophyta, and change the nitrogen cycling of aquatic ecosystems.

Understanding the nitrogen uptake and assimilation of the Chinese strain of Aureococcus anophagefferens (Pelagophyceae)

Brown tides caused by Aureococcus anophagefferens have occurred annually and severely impacted on the local ecology and economy in the coastal waters of Qinhuangdao, China since 2009. The uptake preference, pigment composition and growth responses of the Chinese strain of A. anophagefferens to different nitrogen substrates were studied in the laboratory to better understand its nitrogen (N) strategies and compare the findings with those from the other strains in the U.S.A. Our results indicated that A. anophagefferens had a high absolute uptake rate (ρmax) for NH4+ and low half-saturation constants (Ks) for NO3− and urea. It grew faster on the organic substrates (urea and glutamic acid) than that on the inorganic substrates (NO3− and NH4+). A. anophagefferens had higher contents of cellular pigments for both photosynthesis (chlorophyll a, chlorophyll c2, 19′-butanoyloxyfucoxanthin and fucoxanthin) and photoprotection (β-Carotene and diadinoxanthin) on urea than did on NO3− and NH4+, suggesting a stronger capability for photosynthesis on urea. A. anophagefferens had higher contents of cellular carbon, N and proteins on urea than did on NO3− and NH4+, suggesting a stronger capability for the acclimation and storage of C and N on urea. A. anophagefferens produced more β-carotene rather than chlorophyll a on glutamic acid and thus inhibited photosynthesis and carbon fixation. We found in this study that urea was the best N substrate for the growth of A. anophagefferens. A. anophagefferens might have a competitive advantage over other phytoplankton in waters enriched with DON, particularly with urea.

Phytoplankton traits from long-term oceanographic time-series

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 576:11-25 (2017) - DOI: https://doi.org/10.3354/meps12220 Phytoplankton traits from long-term oceanographic time-series Crispin M. Mutshinda1, Zoe V. Finkel2, Claire E. Widdicombe3, Andrew J. Irwin1,* 1Mathematics & Computer Science, Mount Allison University, Sackville, NB E4L 1E6, Canada 2Environmental Science, Mount Allison University, Sackville, NB E4L 1A7, Canada 3Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, UK *Corresponding author: airwin@mta.ca ABSTRACT: Trait values are usually extracted from laboratory studies of single phytoplankton species, which presents challenges for understanding the immense diversity of phytoplankton species and the wide range of dynamic ocean environments. Here we use a Bayesian approach and a trait-based model to extract trait values for 4 functional types and 10 diatom species from field data collected at Station L4 in the Western Channel Observatory, English Channel. We find differences in maximum net growth rate, temperature optimum and sensitivity, half-saturation constants for light and nitrogen, and density-dependent loss terms across the functional types. We find evidence of very high linear loss rates, suggesting that grazing may be even more important than commonly assumed and differences in density-dependent loss rates across functional types, indicating the presence of strong niche differentiation among functional types. Low half-saturation constants for nitrogen at the functional type level may indicate widespread mixotrophy. At the species level, we find a wide range of density-dependent effects, which may be a signal of diversity in grazing susceptibility or biotic interactions. This approach may be a way to obtain more realistic and better-constrained trait values for functional types to be used in ecosystem modeling. KEY WORDS: Phytoplankton · Time series · Traits · Growth rate · Grazing rate · English Channel Full text in pdf format Supplementary material PreviousNextCite this article as: Mutshinda CM, Finkel ZV, Widdicombe CE, Irwin AJ (2017) Phytoplankton traits from long-term oceanographic time-series. Mar Ecol Prog Ser 576:11-25. https://doi.org/10.3354/meps12220 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 576. Online publication date: August 03, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Variations in carbon-to-phosphorus ratios of two Australian strains of Cylindrospermopsis raciborskii

The toxic cyanobacterium Cylindrospermopsis raciborskii can form large blooms in freshwater systems, causing water quality problems. The availability of the essential macronutrient phosphorus (P), has a big impact on bloom formation but the variation in physiological response of different strains of C. raciborskii to available P has not previously been examined. This study investigated the carbon:phosphorus (C:P) ratio of two toxic Australian strains of C. raciborskii, AWT205 and NPD, under a range of P concentrations in batch and continuous cultures. P was added as a single dose to batch cultures and in continuous cultures at P concentrations of 0.032, 0.16, 0.64 and 16 μmol P l−1. Cellular carbon and phosphorus content of both strains increased under P-limited conditions (0 μmol P l−1 addition) with zero growth. Strain NPD had a lower C:P ratio (34:1) than AWT205 (150:1) indicating higher P storage capacity, and strain NPD survived P-limited conditions for longer. There was no significant difference in exponential growth rates (0.2 d−1, P ≥ 0.5) under all P concentrations for both strains, with the exception of no P, demonstrating non-P-limited growth even at the lowest concentration (0.032 µmol P l−1) and no increase in growth rate with additional P. 33P uptake measurements were used to show that these strains both have very low half saturation constants (Ks = 0.02 μmol P l−1) compared with other phytoplankton and strains of C. raciborskii. This is indicative of high uptake affinities and suggests that these strains are highly adapted to a low P supply. Overall the results of this study are consistent with the P strategy of storage prioritization over growth rate, and demonstrate differences between the strains in the C:P ratio under P-limitation, indicating variation in P storage.

Low temperature reduces the energetic requirement for the CO2 concentrating mechanism in diatoms.

The goal of this study is to investigate the CO2 concentrating mechanism (CCM) of the dominant phytoplankton species during the growing season at Palmer station in the Western Antarctic Peninsula. Key CCM parameters (cellular half-saturation constants for CO2 fixation, carbonic anhydrase activity, CO2 /HCO3 (-) uptake, δ(13) Corg ) in natural phytoplankton assemblages were determined. Those results, together with additional measurements on CO2 membrane permeability from Fragilariopsis cylindrus laboratory cultures, were used to develop a numerical model of the CCM of cold water diatoms. The field data demonstrate that the dominant species throughout the season possess an effective CCM, which achieves near saturation of CO2 for fixation. The model provides a means to examine the role of eCA activity and HCO3 (-) /CO2 uptake in the functioning of the CCM. According to the model, the increase in δ(13) Corg during the bloom results chiefly from decreasing ambient CO2 concentration (which reduces the gross diffusive flux across the membrane) rather than a shift in inorganic carbon uptake from CO2 to HCO3 (-) . The CCM of diatoms in the Western Antarctic Peninsula functions with a relatively small expenditure of energy, resulting chiefly from the low half-saturation constant for Rubisco at cold temperatures.

Growth and Nitrogen Uptake Kinetics in Cultured Prorocentrum donghaiense

We compared growth kinetics of Prorocentrum donghaiense cultures on different nitrogen (N) compounds including nitrate (NO3 −), ammonium (NH4 +), urea, glutamic acid (glu), dialanine (diala) and cyanate. P. donghaiense exhibited standard Monod-type growth kinetics over a range of N concentraions (0.5–500 μmol N L−1 for NO3 − and NH4 +, 0.5–50 μmol N L−1 for urea, 0.5–100 μmol N L−1 for glu and cyanate, and 0.5–200 μmol N L−1 for diala) for all of the N compounds tested. Cultures grown on glu and urea had the highest maximum growth rates (μm, 1.51±0.06 d−1 and 1.50±0.05 d−1, respectively). However, cultures grown on cyanate, NO3 −, and NH4 + had lower half saturation constants (Kμ, 0.28–0.51 μmol N L−1). N uptake kinetics were measured in NO3 −-deplete and -replete batch cultures of P. donghaiense. In NO3 −-deplete batch cultures, P. donghaiense exhibited Michaelis-Menten type uptake kinetics for NO3 −, NH4 +, urea and algal amino acids; uptake was saturated at or below 50 μmol N L−1. In NO3 −-replete batch cultures, NH4 +, urea, and algal amino acid uptake kinetics were similar to those measured in NO3 −-deplete batch cultures. Together, our results demonstrate that P. donghaiense can grow well on a variety of N sources, and exhibits similar uptake kinetics under both nutrient replete and deplete conditions. This may be an important factor facilitating their growth during bloom initiation and development in N-enriched estuaries where many algae compete for bioavailable N and the nutrient environment changes as a result of algal growth.

Indirect bottom‐up control of consumer‐resource dynamics: Resource‐driven algal quality alters grazer numerical response

The nature of the growth‐rate response of grazing zooplankton to algal concentration depends critically upon food quality, which is often a plastic function of environmental conditions. Phytoplankton quality depends upon nutrient and light availability, which together affect cell C : P and C : N, so that the links from external environment to phytoplankton and then to zooplankton can vary in ways that influence consumer‐resource interaction strengths, and ultimately, food web structure. We measured the effects of resource‐limitation on the population growth rate of rotifers, Brachionus calyciflorus, feeding on the alga Chlamydomonas reinhardtii, and grown in nitrogen‐limiting, phosphorus‐limiting, or nutrient‐sufficient conditions. Consumption of nutrient‐sufficient algae produced a Type II numerical response with a low half saturation constant (Ks) and high asymptotic growth rate (λmax) consistent with high food quality; P‐limited algae also yielded a Type II response but with high Ks and low λmax, meaning poor food quality. In contrast, N‐limited Chlamydomonas resulted in a novel Type IV numerical response in which rotifer growth rate rose to an intermediate λmax and then declined at the highest algal densities, suggesting the production of toxic compounds whose negative effect on growth increases as a function of cell density. Experiments with mixed diets confirmed that N‐limited food was indeed toxic, whereas P‐limited food was simply nutritionally deficient compared with light‐limited algae. Additional experiments showed that even the filtrate of N‐limited algae can be detrimental to rotifer growth.

Isolation and characterization of bacterial strains that have high ability to degrade 1,4-dioxane as a sole carbon and energy source

Four novel metabolic 1,4-dioxane degrading bacteria possessing high ability to degrade 1,4-dioxane (designated strains D1, D6, D11 and D17) were isolated from soil in the drainage area of a chemical factory. Strains D6, D11 and D17 were allocated to Gram-positive actinomycetes, similar to previously reported metabolic 1,4-dioxane degrading bacteria, whereas strain D1 was allocated to Gram-negative Afipia sp. The isolated strains could utilize a variety of carbon sources, including cyclic ethers, especially those with carbons at position 2 that were modified with methyl- or carbonyl-groups. The cell yields on 1,4-dioxane were relatively low (0.179-0.223mg-protein (mg-1,4-dioxane)(-1)), which was likely due to requiring energy for C-O bond fission. The isolated strains showed 2.6-13 times higher specific 1,4-dioxane degradation rates (0.052-0.263mg-1,4-dioxane (mg-protein)(-1) h(-1)) and 2.3-7.8 fold lower half saturation constants (20.6-69.8mgL(-1)) than the most effective 1,4-dioxane degrading bacterium reported to date, Pseudonocardia dioxanivorans CB1190, suggesting high activity and affinity toward 1,4-dioxane degradation. Strains D1 and D6 possessed inducible 1,4-dioxane degrading enzymes, whereas strains D11 and D17 possessed constitutive ones. 1,4-Dioxane degradation (100mgL(-1)) by Afipia sp. D1 was not affected by the co-existence of up to 3,000mgL(-1) of ethylene glycol. The effects of initial pH, incubation temperature and NaCl concentration on 1,4-dioxane degradation by the four strains revealed that they could degrade 1,4-dioxane under a relatively wide range of conditions, suggesting that they have a certain adaptability and applicability for industrial wastewater treatment.

Substrate Kinetics of DMSP-lyases in Various Cultured Symbiodinium Strains

In marine algae, the conversion of the multifunctional metabolite dimethylsulfoniopropionate (DMSP) to dimethylsulfide (DMS) occurs via DMSP-lyase enzymes. Apparent substrate kinetic properties of DMSP-lyases in algae have revealed high levels of variability. Some algal dinoflagellates, including symbiotic zooxanthellae (genus Symbiodinium) produce high intracellular levels of DMSP and DMSP-lyase capabilities vary among specific Symbiodinium strains. To examine the enzyme kinetic properties of DMSP-lyases in zooxanthellae, we used five cultured Symbiodinium microadriaticum (fruedenthal) strains (ccMP 421, 828, 829, 830, 1633) to optimize in vivo DMSP-lyase assay parameters. Optimal activity occurred at ?30 °c with higher enzyme activity at pH 8 compared to pH 7. Further studies were carried out to examine the enzyme kinetic parameters Vmax and Km in two zooxanthellae strains, ccMP 829 and 1633, that differ in their DMSP-lyase basal activity (high and low activity, respectively). Both zooxanthellae strains showed lower half saturation constants for DMSP (range of Km = 0.008-0.124 mM) compared to most other marine phytoplankton tested to date. Vmax values (?mol DMS Lcell volume-1 min-1) increased slightly or by 2.7-fold in strains 1633 and 829, respectively, in conjunction with decreasing growth rates in batch cultures and parallel increases in intracellular DMSP concentrations in strain 829.

Influence of growth manner on nitrifying bacterial communities and nitrification kinetics in three lab-scale bioreactors

The effects of growth type, including attached growth, suspended growth, and combined growth, on the characteristics of communities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were studied in three lab-scale Anaerobic/Anoxic(m)-Oxic(n) (AmOn) systems. These systems amplified activated sludge, biofilms, and a mixture of activated sludge and biofilm (AS-BF). Identical inocula were adopted to analyze the selective effects of mixed growth patterns on nitrifying bacteria. Fluctuations in the concentration of nitrifying bacteria over the 120days of system operation were analyzed, as was the composition of nitrifying bacterial community in the stabilized stage. Analysis was conducted using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and real-time PCR. According to the DGGE patterns, the primary AOB lineages were Nitrosomonas europaea (six sequences), Nitrosomonas oligotropha (two sequences), and Nitrosospira (one sequence). The primary subclass of NOB community was Nitrospira, in which all identified sequences belonged to Nitrospira moscoviensis (14 sequences). Nitrobacter consisted of two lineages, namely Nitrobacter vulgaris (three sequences) and Nitrobacter alkalicus (two sequences). Under identical operating conditions, the composition of nitrifying bacterial communities in the AS-BF system demonstrated significant differences from those in the activated sludge system and those in the biofilm system. Major varieties included several new, dominant bacterial sequences in the AS-BF system, such as N. europaea and Nitrosospira and a higher concentration of AOB relative to the activated sludge system. However, no similar differences were discovered for the concentration of the NOB population. A kinetic study of nitrification demonstrated a higher maximum specific growth rate of mixed sludge and a lower half-saturation constant of mixed biofilm, indicating that the AS-BF system maintained relatively good nitrifying ability.

Eco-physiological responses of nitrogen-fixing cyanobacteria to light

The eco-physiological responses of three nitrogen-fixing cyanobacteria (N-fixing cyanobacteria), Aphanizomenon gracile, Anabaena minderi, and Ana. torques-reginae, to light were assessed under nutrient saturation. The N-fixing cyanobacteria were isolated into monocultures from a natural bloom in a shallow colored lake and their growth irradiance parameters and pigment composition were assessed. The different ecological traits related to light use (μmax, α, I k) suggest that these N-fixing cyanobacteria are well adapted to low light conditions at sufficient nutrients, yet interspecific differences were observed. Aphanizomenon gracile and Anabaena minderi had high relative growth rates at low irradiances (ca. 70% of those in high light), low half saturation constant for light-limited growth (I k < 9.09 μmol photon m−2 s−1) and high efficiency (α < 0.11 day−1 μmol photon−1 m2 s). Conversely, Ana. torques-reginae showed poorer light competitiveness: low relative growth rates at low irradiances (ca. 40% of those in high light), low α (0.009 day−1 μmol photon−1 m2 s) and higher I k (35.5 μmol photon m−2 s−1). Final densities in Aphanizomenon gracile and Anabaena minderi reached bloom densities at irradiances above 30 μmol photon m−2 s−1 with different hierarchy depending on irradiance, whereas Ana. torques-reginae never achieved bloom densities. All species had very low densities at irradiances ≤17 μmol photon m−2 s−1, thus no N-fixing blooms would be expected at these irradiances. Also, under prolonged darkness and at lowest irradiance (0 and 3 μmol photon m−2 s−1) akinetes were degraded, suggesting that in ecosystems with permanently dark sediments, the prevalence of N-fixing cyanobacteria should not be favored. All species displayed peaks of phycocyanin, but no phycoeritrin, probably due to the prevailing red light in the ecosystem from which they were isolated.

Nutrient kinetics modeled from time series of substrate depletion and growth: dissolved silicate uptake of Baltic Sea spring diatoms

We estimated silicate uptake kinetics for 8 spring diatom species using a model based on time series measurements of the depletion of dissolved silicate (DSi) and increases in biomass. Furthermore, the carbon: nitrogen: silicate stoichiometric relationships and maximum growth rates were determined. Differences in DSi uptake kinetics and maximum growth rate were distinct among the species. All the most common diatom species (Chaetoceros wighamii, Pauliella taeniata, Skeletonema costatum and Thalassiosira baltica) were relatively lightly silicified and had variable but relatively low half-saturation constants (Ks), indicating that they are well adapted to low DSi concentrations. The less common Diatoma tenuis and Nitzschia frigida had higher Ks values, suggesting that they are more vulnerable to DSi limitation. The much used nitrogen:silicate ratio of 1 for marine diatom biomass was too low for most of the examined species, while a ratio of 2–3 seems to be more appropriate for these Baltic Sea species.

Determination of ultra‐low oxygen concentrations in oxygen minimum zones by the STOX sensor

The methods used until now have not been able to reliably resolve O2 concentrations in oceanic oxygen minimum zones below 1–2 µmol L−1. We present a new amperometric sensor for the determination of ultra‐low O2 concentrations under in situ conditions. The electrochemical STOX O2 sensor contains a primary sensing cathode and a secondary cathode that, when polarized, prevents entry of O2 into the sensor. This arrangement enables frequent in situ zero calibration and confers the sensor with a detection limit of 1‐10 nmol L−1 O2, even during application on a Conductivity‐Temperature‐Depth (CTD) profiler at great water depths. The sensor was used during the Galathea 3 Expedition to demonstrate that the core of the oxygen minimum zone (OMZ) off Peru contained &lt; 2 nM O2. Application in a reactor on board demonstrated that changes in O2 concentrations in OMZ water containing &lt; 200 nmol L−1 O2 could be monitored over periods of hours to days. The linear decrease in O2 concentration in the reactor indicated very low (&lt; 20 nmol L−1) half saturation constants for the O2 respiring microbial community.

Phosphate and ATP uptake and growth kinetics in axenic cultures of the cyanobacterium Synechococcus CCMP 1334

Phosphorus potentially limits the growth and productivity of the unicellular cyanobacterium Synechococcus in many oligotrophic oceans. Both dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP) could be important P sources in these regimes. We compared DIP and adenosine tri-phosphate (ATP) uptake and growth kinetics using axenic semi-continuous cultures of Synechococcus CCMP 1334 grown on both P substrates, under both P-limited and P-replete conditions. Molar C:total P ratios and N:total P ratios were 417–431 and 65–68, respectively, under severely P-limited growth conditions for both P sources, but were near Redfield values at >1.0 µM P as either DIP or ATP, and were inversely proportional to growth rates. Synechococcus grown on ATP as the sole P-source have the same maximum growth rates, lower half-saturation constants for growth, and similar cellular P quotas compared with cultures grown on DIP. DI33P and 33P-ATP uptake experiments confirmed that both can be used by Synechococcus, with relative uptake rates depending on prior growth history. DIP-limited cultures assimilated ATP faster than DIP, while DIP-replete cultures took up DIP more rapidly. In contrast, for Synechococcus grown under ATP-limited conditions, there is no priority for the utilization of ATP or DIP, but ATP-replete cultures have a greater ability to use DIP. Like other phytoplankton, kinetic constants for Synechococcus DIP uptake and cellular P quotas, C:P and N:P ratios of DIP-grown cells differ when total cellular and intracellular P pools are compared, suggesting a need to re-evaluate these parameters to account for abiotic phosphate scavenging onto cell surfaces. DIP half-saturation constants for growth of this Synechococcus strain would not support observed natural growth rates at the nanomolar DIP concentrations found in most oligotrophic regimes, and hence populations in these areas probably rely primarily on DOP.

Daisyworld inhabited with daisies incorporating a seed size/number trade-off: the mechanism of negative feedback on selection from a standpoint of the competition theory

We reexamined a Daisyworld model from the traditional view of competition theory. Unlike the original model, white and black daisies in our model incorporate a seeding/germination trade-off against bare ground area without assuming the local temperature reward. As a result, the planetary temperature is automatically regulated by two species if the following conditions are met: (i) the species react equally to an environmental condition, but one can alter the environmental condition in the opposite direction to the other. (ii) that one of the two cannot have both a higher maximal growth rate ( μ max ) and lower half-saturation constant ( K) than those of the other. In other words, a pair of phenotypes incorporates a trade-off between quality and number of seeds. We found that the homeostatic regulation can also be reconciled with the adaptive evolution of optimal temperature. The results of simulation imply that biotic environmental feedback can also be maintained when the emergence of polymorphisms (black and white daisies) is closely linked to such a trade-off.