Half-saturation Constants For Growth Research Articles

An Integrated Response of Trichodesmium erythraeum IMS101 Growth and Photo-Physiology to Iron, CO2, and Light Intensity.

We have assessed how varying CO2 (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m−2 s−1) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe′) concentrations between 20 and 9,600 pM. We found significant iron dependencies of growth rate and the initial slope and maximal relative PSII electron transport rates (rPm). Under iron-limiting concentrations, high-light increased growth rates and rPm; possibly indicating a lower allocation of resources to iron-containing photosynthetic proteins. Higher CO2 increased growth rates across all iron concentrations, enabled growth to occur at lower Fe′ concentrations, increased rPm and lowered the iron half saturation constants for growth (Km). We attribute these CO2 responses to the operation of the CCM and the ATP spent/saved for CO2 uptake and transport at low and high CO2, respectively. It seems reasonable to conclude that T. erythraeum IMS101 can exhibit a high degree of phenotypic plasticity in response to CO2, light intensity and iron-limitation. These results are important given predictions of increased dissolved CO2 and water column stratification (i.e., higher light exposures) over the coming decades.

Parameter Estimation Using Sliding Mode Observers: Application to the Monod Kinetic Model

In this paper a technique borrowed from modern control theory, called sliding mode observers, is used as the vehicle for nonlinear parameter estimation. The observer is a dynamic system derived from the original state equations that guides, in real time, parameter values from initial estimates to optimal ones. This is done by driving errors between the estimated and "data" states to zero. Observers are developed and demonstrated for the Monod model for biokinetics, which has been used widely in bioprocess design for both aerobic and anaerobic bio-processes under both toxic and non-toxic conditions. The Monod model has four parameters; often, two of the parameters (y, yield and b, decay coefficient) are assumed or calculated from other sources such as from thermodynamic considerations. The other two parameters (k, maximum specific uptake rate of the substrate and ks, the half saturation constant for growth) are determined from experimental data. The determination of these parameters is often performed using nonlinear regression or related methods and it has been demonstrated that these are particularly difficult parameter estimation problems. Examples are presented where only k is unknown and when both k and ks are to be determined. One case where the data are corrupted with noise is considered.The primary objective of the paper is to introduce the methodology of sliding mode observers for nonlinear parameter estimation to the chemical and process engineering communities.

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.

Physiological responses of three species of marine pico-phytoplankton to ammonium, phosphate, iron and light limitation

Experiments were conducted with three species of marine pico-phytoplankton: Synechococcus sp. (CCMP 839), Pelagomonas calceolata (CCMP 1756) and Prasinomonas capsulatus (CCMP 1617) in order to collect physiological parameters for pico-phytoplankton to be utilised in Ocean Biogeochemical Climate Models. The main parameters to follow the effects of ammonium, phosphate, iron and light limitation were cell growth rates (μ), half saturation constants for growth (K m), N, P and Fe quota (per cell or per mol C), and photochemical quantum efficiency (F v/F m). The nitrate and phosphate limitation experiments demonstrated that the small phytoplankton species could grow at low N and P concentrations. K m values were in the micro-molar (NH 4 +) and sub-micro-molar (PO 4 3−) range. N and P quota were in the femto-molar range per cell and varied from nutrient-deplete to nutrient-replete conditions. F v/F m values were only adversely affected at the lowest N and P concentrations in these experiments. In the Fe limitation experiments, it was shown that all three species were adversely affected only at extremely low Fe concentrations. Iron chelating agents had to be added to force the species in Fe limitation till ultimately growth stopped. K m values with respect to dissolved Fe were in the femto-molar range. Fe quota were in the low zepto-molar (10 −21 M) range per cell, and varied considerably from Fe limiting to Fe replete growth conditions. F v/F m values diminished only at the lowest iron concentrations. In the light limitation experiments, growth rates and photochemical quantum efficiencies were adversely affected only at irradiance levels below 10 μmol photons m −2 s −1. These results indicate that the pico-phytoplankton species will hardly ever be completely stopped in their growth by NH 4 +, PO 4 3−, Fe or light (separately) under natural conditions.

HOW CRITICAL IS THE CRITICAL N:P RATIO?1

The value of cellular N:P that corresponds to co‐limitation by N and P, the critical (Rcrit) or optimum ratio, has been used to infer the competitive advantage of phytoplankton growing in P‐impoverished systems. Using a revised quota model, with a normalized quota function and capable of simulating surge transport, the interactions between the minimum P‐quota (PCo), the shape of the P‐quota–cell growth relationship (affected by constantKQP), and transport kinetics in affecting the utility ofRcritare considered. For a lowPCoto endow an organism with a highRcritover a wide range of growth rates, the P‐quota curve must be more hyperbolic (KQPlow) rather than linear (KQPhigh).PCoandKQPalso affect the half saturation constant for growth,KgP ; this and the capacity to transport nutrients at rates above those required to sustain steady‐state growth endows a competitive advantage. However, the kinetics of transport into the organism have a greater potential for affectingKgPthan changing the kinetics of internal P usage. Thus, the value ofRcritis not a critical factor affecting competition except in extreme oligotrophic conditions. For competition between species, nutrient transport, accumulation, and resource utilization are all important. However, the efficiency of internal resource utilization is of lesser importance, and certainly not of greater importance, than resource acquisition. Multinutrient models intended to describe competition need to recognize these interactions; the traditional quota model is poorly equipped to do so.

Quantification of algal iron requirements in the Subantarctic Southern Ocean (Indian sector)

Abstract Shipboard iron-addition incubation experiments were carried out in the Indian sector of the Subantarctic Southern Ocean during the Antares -IV campaign in late January–February 1999. The aim of these experiments was to estimate the dissolved iron requirements of the native phytoplankton community in this oceanic region, in order to improve the parameterisation of iron as a limiting nutrient in a coupled 1D physical–biogeochemical ocean model. The experiments were conducted with plankton collected from the upper water column (∼20 m depth) at three sites in the Crozet Basin between 43–46°S and 61–65°E: (1) the Polar Front Zone (PFZ, dissolved Fe=0.33 nM), (2) the confluence of the Subantarctic and Subtropical Fronts (SAF/STF, dissolved Fe=0.29 nM), and (3) the southern Subtropical Zone (STZ, dissolved Fe=0.09 nM). Experimental results from each site indicate that algal community growth rates varied as a function of added iron concentration. Monod saturation functions fitted to the experimental data yield estimates for the community half-saturation constant for growth with respect to iron ( K μ ) of 0.41–0.45 nM Fe (PFZ), 0.055–0.086 nM Fe (SAF/STF) and 0.092–0.093 nM Fe (STZ, with macronutrients added), each of which has an estimated uncertainty of ±20%. The K μ estimate for the SAF/STF site reflects the mixed algal assemblage (diatoms+nanoplankton+dinoflagellates) that grew in the experimental incubations, whereas the K μ estimates for the PFZ and STZ sites probably reflect the Fe requirements of the small pennate diatoms such as Pseudo-nitzschia spp., which dominated the algal biomass produced in these experiments. The fact that there are significant differences between the K μ estimates for the PFZ and STZ sites suggests that similar diatom assemblages may have quite different iron requirements, perhaps due to differences in environmental conditions (e.g., light, macronutrient levels). We also examine the sensitivity of a one-dimensional coupled physical–biogeochemical model to the choice of K μ for iron, using time-series observations from the KERFIX station close to the Polar Front. The model was best able to simulate the KERFIX observations using a diatom K μ value of ∼0.1 nM Fe, which is considerably less than our experimental estimate of ∼0.4 nM Fe for the PFZ. This discrepancy probably reflects differences in the iron requirements of diatom populations immediately north and south of the Polar Front in the Kerguelen region, due to differences in diatom species composition, availability of light and silicic acid, or the environmental and physiological histories of the diatom populations.

Survey of selenium requirements in marine phytoplankton

A survey of 27 species of marine phytoplankters, representing 4 algal classes, was conducted to determine if selenium was required for growth. The species were grown in natural and artificial seawater media to which 1 0 ~ M Na,SeO, was added (controls) and in artifical seawater with no Se addition. Fifteen out of 20 species of diatoms tested required Se for growth. Eight out of 9 oceanlc diatoms showed an apparent Se requirement. Only 1 (Scrippsiella trochoidea) of the 4 dinoflagellates tested exhibited a similar SE requirement. Two Chrysochromulina spp. and the one Synecl~ococcus p. grew normally in the artifical seawater medium without added Se. For the species that did not require a Se addition to the artificial seawater medium, it was not possible to determine whether these species have a very low Se requirement that is met by the background Se contamination in the artificial seawater or they actually have no Se requirement. The effect of various Se concentrations (10-l0 to 10-' M) on growth rates of 4 diatom species (3 coastal and 1 ocednic) were examined. The half saturation constant for growth (K,,) was approximately 10-' M Se for all 4 species.

Ecotypic differentiation of Laminaria longicruris in relation to seawater nitrate concentration

Genetic differentiation of Laminaria longicruris populations has been demonstrated in nitrate-poor (St. Margaret's Bay) and nitrate-rich (Bay of Fundy) regions of Nova Scotia, Canada. At 9°C half-saturation constants for growth (Ks) were smaller (1.1 μM NO 3 - ) for St. Margaret's Bay plants than for plants from the Bay of Fundy (2.3 μM NO 3 - ). Maximum specific growth rate (μmax) and maximum uptake rate (Vmax) were higher for St. Margaret's Bay than for Fundy plants. However half-saturation constants for nitrate uptake (Km) were similar for plants from the two populations. The capacity to accumulate nitrate in an internal tissue pool was larger in St. Margaret's Bay plants than in Fundy plants. The differences among the populations are thought to be adaptive responses to the local nitrate environments.

Phosphate and Silicate Kinetics for the Lake Michigan Diatom Diatoma Elongatum

Growth rates and uptake rates by a clone of Diatoma elongatum isolated from Lake Michigan were measured under silicate and phosphate limitation using batch and semicontinuous methods. Under phosphate limited batch growth conditions, the half saturation constant for growth (K) was 0.02 μM PO 4-P and the maximum growth rate (μ m) was 0.7 doublings day -1; under silicate limited conditions, K was 1.51 μM SiO 2-Si, and μ m was 1.2 doublings day -1. The results from these direct measurements of growth under nutrient limitation agreed well with the steady state kinetic information obtained for a two-step (uptake and utilization) growth model Batch uptake experiments and steady state growth experiments provided kinetic information used to estimate K, the calculated half saturation constant for growth. K was 0.03 μM PO 4-P under phosphate limitation, and 1.18 μM SiO 2-Si under silicate limitation. We observed a dependence of the coefficient of luxury consumption on the steady state growth rate. D. elongatum can have up to 28 times more phosphate stores than needed, and up to 4.4 times more silicate.