PHYSICOCHEMICAL CONTROLS ON FORAMINIFERAL SIZE 2500 A Pacific


603 1000 0


2500 B


Atlantic


rate (B) is well predicted by organism biomass and temperature. Here, we modify the relationship presented by Gillooly et al. (2001) to account for the linear scaling of mass to specific metabolic rate (b0) in protists, which differs from the 3/4-power scaling characteristic of metazoans (DeLong et al. 2010):


B=b0MeEi = kT (1.1) Reaction kinetics vary with temperature 1000 0


02468 10 Dissolved oxygen (ml/liter)


FIGURE 4. Range of dissolved oxygen concentration for sites included in our biogeographic data set for the Pacific (A) and Atlantic (B) continental margins of North America, illustrating the absence of low-oxygen waters in the Atlantic basin (Garcia et al. 2010).


according to the Boltzmann factor, e−Ei/kT, where T is the absolute temperature (in degrees K), Ei is the activation energy (i.e., the mini- mum amount of energy that a species must possess in order to metabolize), and k is Boltzmann’s constant. We further modify this equation by substituting the product of biovo- lume (V) and cell density (r) for cell biomass (M) in order to compare to the results of our North American data set:


B=b0 V  r ðÞeEi = kT (1.2) Cell size (i.e., biomass or biovolume) and the Discussion


Effects of Temperature and Dissolved Oxygen on Test Morphology


The results presented herein demonstrate


the importance of temperature and dissolved oxygen concentration in driving morphologi- cal variation across broad geographic scales. Regardless of how the data are parsed (North America vs. Atlantic vs. Pacific), the environ- mental signals remain. These findings are consistent with the effects of temperature and dissolved oxygen concentration on the metabolic demands of individual organisms and also explain the differences observed between the Pacific and Atlantic continental margins. Physiological calculations help to explain


why the association between oxygen availability and test morphology declines in strength at higher dissolved oxygen levels. Gillooly et al. (2001) showed that metabolic


effects of ambient temperature on metabolic rates primarily dictate the oxygen demand of the foraminifera. Oxygen transport across the cell surface is controlled by its surface area (SA) and can be represented as: SA · u · [O2], where u is the cytoplasmic streaming velocity (Payne et al. 2012a). Given that oxygen transport across the cell surface must be at least equal to the oxygen demand, we estimated the maximum theoretical volume–to–surface area ratio (i.e., sphere morphology) of benthic foraminifera given variations in seawater tem- perature and dissolved oxygen concentration:


V: SAmax =u  O2 ½= 3bEi = kT 0 (2) To estimate the shape of this relationship


with respect to dissolved oxygen and temperature given constant values of the other parameters, we assign a mass-specific metabolic rate constant (b0) of 3.4 · 1010W/kg. We estimated this constant by converting the respiration rates (nl O2 cell−1 h−1) of Rotallid foraminiferal species reported by Geslin et al. (2011) to their respective mass-specific metabolic rates. We used the mean b0 for our analyses: b0 for these reported Rotallid


Dissolved oxygen occurrences


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