Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Am Nat. 2011 Jul;178(1):98-112. doi: 10.1086/660284.
Mixotrophic organisms combine autotrophic and heterotrophic nutrition and are abundant in both freshwater and marine environments. Recent observations indicate that mixotrophs constitute a large fraction of the biomass, bacterivory, and primary production in oligotrophic environments. While mixotrophy allows greater flexibility in terms of resource acquisition, any advantage must be traded off against an associated increase in metabolic costs, which appear to make mixotrophs uncompetitive relative to obligate autotrophs and heterotrophs. Using an idealized model of cell physiology and community competition, we identify one mechanism by which mixotrophs can effectively outcompete specialists for nutrient elements. At low resource concentrations, when the uptake of nutrients is limited by diffusion toward the cell, the investment in cell membrane transporters can be minimized. In this situation, mixotrophs can acquire limiting elements in both organic and inorganic forms, outcompeting their specialist competitors that can utilize only one of these forms. This advantage can be enough to offset as much as a twofold increase in additional metabolic costs incurred by mixotrophs. This mechanism is particularly relevant for the maintenance of mixotrophic populations and productivity in the highly oligotrophic subtropical oceans.
混养生物结合了自养和异养营养，在淡水和海洋环境中都很丰富。最近的观察表明，在贫营养环境中，混养生物构成了生物量、细菌食性和初级生产力的很大一部分。虽然混养使资源获取更加灵活，但任何优势都必须与相关的代谢成本增加相权衡，这使得混养生物相对于专性自养生物和异养生物没有竞争力。利用细胞生理学和群落竞争的理想化模型，我们确定了一种机制，通过这种机制，混养生物可以有效地与营养元素的专家竞争。在低资源浓度下，当营养物质的吸收受到扩散到细胞的限制时，可以最小化细胞膜转运蛋白的投资。在这种情况下，混养生物可以以有机和无机形式获取限制元素，从而击败只能利用其中一种形式的专门竞争者。这种优势足以抵消混养生物额外代谢成本增加两倍的影响。这种机制对于维持高度贫营养的亚热带海洋中的混养种群和生产力特别重要。
