Accounting for Intrapopulation Variability in Biogeochemical Models Using Agent-Based Methods

Hellweger, Ferdi L.; Kianirad, Ehsan

doi:10.1021/es062046j

Cited by 22 publications

(44 citation statements)

References 33 publications

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“…The original formulation proposed by Droop (4) uses the P quota (q and q 0 in units of P/cell), but it is also commonly applied with internal P content (q and q 0 in units of mol P/mol C) (9,23). If q drops below q 0 , then the cell does not perform photosynthesis.…”

Section: Methodsmentioning

confidence: 99%

“…Cell division is based on the size state variable (m) and a specified minimum cell size parameter (m 0 ). When a cell grows to twice the minimum, it divides and creates two daughter cells with approximately half the size (9,22). Diatoms have a silicate outer cell wall, called a frustule, which provides protection from predators (24).…”

Section: Methodsmentioning

confidence: 99%

“…The difference in growth rate can be explained by Jensen's inequality, which has a larger effect when heterogeneity is larger. Models of field conditions where additional mechanisms (e.g., microscale patchiness) lead to heterogeneity have higher ratios (1.47 for Bucci et al [10] and 1.22 for Hellweger and Kianirad [9]). …”

Section: Figmentioning

confidence: 99%

“…This is a case of Jensen's inequality, which states that for a concave function, the expected value of the function is less than the function of the expected value (7). If the population's average growth rate is determined based on the population's average cell quota (i.e., average the quota then determine the growth rate for the population), then it will be higher than if the growth rate is determined for each individual and then averaged to get the population-averaged growth rate (8,9).…”

mentioning

confidence: 99%

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Use of Agent-Based Modeling To Explore the Mechanisms of Intracellular Phosphorus Heterogeneity in Cultured Phytoplankton

Fredrick

Berges

Twining

et al. 2013

Appl Environ Microbiol

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There can be significant intraspecific individual-level heterogeneity in the intracellular P of phytoplankton, which can affect the population-level growth rate. Several mechanisms can create this heterogeneity, including phenotypic variability in various physiological functions (e.g., nutrient uptake rate). Here, we use modeling to explore the contribution of various mechanisms to the heterogeneity in phytoplankton grown in a laboratory culture. An agent-based model simulates individual cells and their intracellular P. Heterogeneity is introduced by randomizing parameters (e.g., maximum uptake rate) of daughter cells at division. The model was calibrated to observations of the P quota of individual cells of the centric diatom Thalassiosira pseudonana, which were obtained using synchrotron X-ray fluorescence (SXRF). A number of simulations, with individual mechanisms of heterogeneity turned off, then were performed. Comparison of the coefficient of variation (CV) of these and the baseline simulation (i.e., all mechanisms turned on) provides an estimate of the relative contribution of these mechanisms. The results show that the mechanism with the largest contribution to variability is the parameter characterizing the maximum intracellular P, which, when removed, results in a CV of 0.21 compared to a CV of 0.37 with all mechanisms turned on. This suggests that nutrient/element storage capabilities/mechanisms are important determinants of intrapopulation heterogeneity. Phytoplankton play an important role in the ecology and biogeochemistry of freshwater and marine systems. Thus, understanding their composition and physiology (e.g., carbon fixation rate) is important. The elemental composition of phytoplankton was originally considered to be relatively uniform (1). However, subsequent research has shown that it can vary between species (2) or clones (3) and within a species under different conditions (4, 5). The composition can also vary within a species under the same conditions. Two measurements of intracellular P are commonly made: cell-based P (mol P/cell), which we refer to as P quota, and biomass-normalized P (mol P/mol C), which we call internal P content. While P quota is generally easier to measure on a cellspecific basis, it is the internal P content that is important for many physiological processes. Recent observations of intracellular P in individual cells have shown significant intraspecific heterogeneity in laboratory and field populations, with coefficients of variations (CV) ranging from 0.08 to 2.11 for P quota and 0.10 to 2.10 for internal P content (Table 1).Heterogeneity in intracellular P may have important consequences because it can result in heterogeneity in physiology (e.g., photosynthesis rate), which can have significant effects on community ecology (6). Specifically, for phytoplankton the specific growth rate (biomass accumulation) is generally considered to depend on the cell quota of the limiting nutrient according to Droop's function (4). Because the function is nonlinear, heterogenei...

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Figmentioning

confidence: 99%

mentioning

confidence: 99%

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Use of Agent-Based Modeling To Explore the Mechanisms of Intracellular Phosphorus Heterogeneity in Cultured Phytoplankton

Fredrick

Berges

Twining

et al. 2013

Appl Environ Microbiol

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“…Differences in cell behavior can then lead to internal nutrient heterogeneity. In the field, macroscale mixing causes the population at any given time and location to be composed of individuals with different life histories and consequently different nutrient contents (Hellweger and Kianirad 2007). Microscale patchiness in extracellular nutrient concentration (e.g., resulting from zooplankton excretion) and the chance of encountering such a patch can lead to heterogeneity (Lehman and Scavia 1982).…”

Section: Mechanisms Of Heterogeneitymentioning

confidence: 99%

Microscale patchiness leads to large and important intraspecific internal nutrient heterogeneity in phytoplankton

et al. 2011

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Phytoplankton stoichiometry or nutrient content has been shown to vary in a number of dimensions (species, condition, time, space), but the heterogeneity within a species at a given time and location, and the underlying mechanisms and importance have not been explored. There are a number of mechanisms that can create intraspecific heterogeneity, and theory suggests it can affect the population growth rate. We studied heterogeneity in P content of the freshwater diatom Cyclotella meneghiniana in the Charles River in Boston. Single-cell observations using synchrotron-based X-ray fluorescence show that the nutrient status varies from P-starved to P-replete. We simulate individual cells using an agent-based model that accounts for a number of mechanisms that can create heterogeneity, including surface area-based uptake, mortality differentiation, stochastic biological variability in states and behavior, macroscale mixing, and microscale nutrient patch encounter. By performing a number of simulations with various mechanisms turned on/off and comparing to data, we conclude that the heterogeneity is mostly due to microscale patchiness (85%). We explore the importance of accounting for heterogeneity in models by performing a simulation with the growth rate based on the population-average internal nutrient, as is done in conventional populationlevel models. This shows that ignoring heterogeneity increases the population growth rate by a factor of 1.47. To account for different heterogeneity in the laboratory and field, population-level ecosystem models should reduce maximum growth rates. The magnitude of this correction depends on local conditions, and in our case, it is a factor of 0.72.

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Modeling Adaptive Mutation of Enteric Bacteria in Surface Water Using Agent-Based Methods

Bucci

Hoover²,

Hellweger³

2011

Water Air Soil Pollut

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Accounting for Intrapopulation Variability in Biogeochemical Models Using Agent-Based Methods

Cited by 22 publications

References 33 publications

Use of Agent-Based Modeling To Explore the Mechanisms of Intracellular Phosphorus Heterogeneity in Cultured Phytoplankton

Use of Agent-Based Modeling To Explore the Mechanisms of Intracellular Phosphorus Heterogeneity in Cultured Phytoplankton

Microscale patchiness leads to large and important intraspecific internal nutrient heterogeneity in phytoplankton

Modeling Adaptive Mutation of Enteric Bacteria in Surface Water Using Agent-Based Methods

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