Oceanic communities are sources or sinks of CO2, depending on the balance between primary production and community respiration. The prediction of how global climate change will modify this metabolic balance of the oceans is limited by the lack of a comprehensive underlying theory. Here, we show that the balance between production and respiration is profoundly affected by environmental temperature. We extend the general metabolic theory of ecology to the production and respiration of oceanic communities and show that ecosystem rates can be reliably scaled from theoretical knowledge of organism physiology and measurement of population abundance. Our theory predicts that the differential temperature-dependence of respiration and photosynthesis at the organism level determines the response of the metabolic balance of the epipelagic ocean to changes in ambient temperature, a prediction that we support with empirical data over the global ocean. Furthermore, our model predicts that there will be a negative feedback of ocean communities to climate warming because they will capture less CO 2 with a future increase in ocean temperature. This feedback of marine biota will further aggravate the anthropogenic effects on global warming.global change ͉ metabolic theory ͉ oceanic carbon cycle T he role of the oceans in the CO 2 budget of the biosphere depends largely on the balance between the uptake of carbon by phytoplankton photosynthesis and its remineralization by the respiration of the whole planktonic community (1). For large areas of the epipelagic ocean, planktonic community respiration (CR) exceeds gross primary production (GPP), resulting in net heterotrophy and a source of CO 2 (2-4). The solution of the contentious debate over the extent of such heterotrophic areas (5-7) is hindered by the limited spatiotemporal coverage achievable by traditional incubation methods (8, 9). Here, we tackle this question from a different perspective based on the metabolic theory of ecology (MTE) (10). The flux rates within an ecosystem are the result of the sum of the individual rates of all its constituent organisms (11,12), which, in turn, are governed by the combined effects of body size and temperature (13-15). Although MTE suggests a universal scaling of metabolic rate as the 3͞4 power of body size, it predicts a differential temperaturedependence of heterotrophic processes (driven by ATP synthesis) and autotrophic rates (controlled by Rubisco carboxylation) (12). Following the MTE, the respiration of a heterotrophic planktonic organism B i can be estimated if we know its body size M i and the ambient absolute temperature T:where b 0 is a normalization constant independent of body size and temperature, e ϪEh͞kT is Boltzmann's factor, where E h is the average activation energy for heterotroph respiration (13), and k is Boltzmann's constant (8.62⅐10 Ϫ5 ⅐eV⅐K Ϫ1 ), and ␣ h is the allometric scaling exponent for body size (14,15). For the metabolic rates of marine autotrophs, things are complicated by the dependence of photosyntheti...

The macroecological relationships among marine phytoplankton total cell density, community size structure and temperature have lacked a theoretical explanation. The tiniest members of this planktonic group comprise cyanobacteria and eukaryotic algae smaller than 2 lm in diameter, collectively known as picophytoplankton. We combine here two ecological rules, the temperature-size relationship with the allometric sizescaling of population abundance to explain a remarkably consistent pattern of increasing picophytoplankton biomass with temperature over the À0.6 to 22 1C range in a merged dataset obtained in the eastern and western temperate North Atlantic Ocean across a diverse range of environmental conditions. Our results show that temperature alone was able to explain 73% of the variance in the relative contribution of small cells to total phytoplankton biomass regardless of differences in trophic status or inorganic nutrient loading. Our analysis predicts a gradual shift toward smaller primary producers in a warmer ocean. Because the fate of photosynthesized organic carbon largely depends on phytoplankton size, we anticipate future alterations in the functioning of oceanic ecosystems.

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Our view of the effects of temperature on bacterial carbon fluxes in the ocean has been confounded by the interplay of resource availability. Using an extensive compilation of cell-specific bacterial respiration (BRi) and production (BPi), we show that both physiological rates respond to changing temperature in a similar manner and follow the predictions of the metabolic theory of ecology. Their apparently different temperature dependence under warm, oligotrophic conditions is due to strong resource limitation of BP, but not of BRi. Thus, and despite previous preconception, bacterial growth efficiency (BGE = BPi/[BPi + BRi]) is not directly regulated by temperature, but by the availability of substrates for growth. We develop simple equations that can be used for the estimation of bacterial community metabolism from temperature, chlorophyll concentration, and bacterial abundance. Since bacteria are the greatest living planktonic biomass, our results challenge current understanding of how warming and shifts in ecosystem trophic state will modify oceanic carbon cycle feedbacks to climate change.

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