Non-Redfield, nutrient synergy, and flexible internal elemental stoichiometry in a marine bacterium

<span class="paragraphSection"><div class="boxTitle">Abstract</div>The stoichiometric constraints of algal growth are well understood, whereas there is less knowledge for heterotrophic bacterioplankton. Growth of the marine bacterium <span style="font-style:italic;">Phaeobacter inhibens</span> DSM 17395, belonging to the globally distributed <span style="font-style:italic;">Roseobacter</span> group, was studied across a wide concentration range of NH₄⁺ and PO₄³⁻. The unique dataset covers 415 different concentration pairs, corresponding to 207 different molar N: P ratios (from 10⁻² to 10⁵). Maximal growth (by growth rate and biomass yield) was observed within a restricted concentration range at N: P ratios (∼50 − 120) markedly above Redfield. Experimentally determined growth parameters deviated to a large part from model predictions based on Liebig's law of the minimum, thus implicating synergistic co-limitation due to biochemical dependence of resources. Internal elemental ratios of <span style="font-style:italic;">P. inhibens</span> varied with external nutrient supply within physiological constraints, thus adding to the growing evidence that aquatic bacteria can be flexible in their internal elemental composition. Taken together, the findings reported here revealed that <span style="font-style:italic;">P. inhibens</span> is well adapted to fluctuating availability of inorganic N and P, expected to occur in its natural habitat (e.g. colonized algae, coastal areas). Moreover, the present study suggests that elemental variability in bacterioplankton needs to be considered in the ecological stoichiometry of the oceans.</span>

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