由浮游植物生态生理学驱动的海洋有机质化学计量的全球模式
Global patterns in marine organic matter stoichiometry driven by phytoplankton ecophysiology
作者：Keisuke Inomura, Curtis Deutsch, Oliver Jahn, Stephanie Dutkiewicz, Michael J. Follows
期刊：Nature Geoscience
The proportion of major elements in marine organic matter links cellular processes to global nutrient, oxygen and carbon cycles. Differences in the C:N:P ratios of organic matter have been observed between ocean biomes, but these patterns have yet to be quantified from the underlying small-scale physiological and ecological processes. Here we use an ecosystem model that includes adaptive resource allocation within and between ecologically distinct plankton size classes to attribute the causes of global patterns in the C:N:P ratios. We find that patterns of N:C variation are largely driven by common physiological adjustment strategies across all phytoplankton, while patterns of N:P are driven by ecological selection for taxonomic groups with different phosphorus storage capacities. Although N:C varies widely due to cellular adjustment to light and nutrients, its latitudinal gradient is modest because of depth-dependent trade-offs between nutrient and light availability. Strong latitudinal variation in N:P reflects an ecological balance favouring small plankton with lower P storage capacity in the subtropics, and larger eukaryotes with a higher cellular P storage capacity in nutrient-rich high latitudes. A weaker N:P difference between southern and northern hemispheres, and between the Atlantic and Pacific oceans, reflects differences in phosphate available for cellular storage. Despite simulating only two phytoplankton size classes, the emergent global variability of elemental ratios resembles that of all measured species, suggesting that the range of growth conditions and ecological selection sustain the observed diversity of stoichiometry among phytoplankton.
Fig. a,b, Variations in elemental ratios in ‘small’ (prokaryotic) phytoplankton (a) and ‘large’ (eukaryotic) phytoplankton (b). Colour shading indicates N:P, computed as the ratio of N:C (x axis) and P:C (y axis). Laboratory data for small prokaryotic cells (white points, a) and eukaryotic cells (white points, b) at a variety of growth rates and light intensities (excluding a few outliers) (see Supplementary Data and references there). Arrows indicate the stoichiometric ratios predicted by the allocation model decomposed into structural and storage components based on average nutrient and light conditions from the surface ocean at 50° S, where N and P nutrients are largely replete. Lilac arrows indicate the modelled contribution from acclimation in the absence of P storage. Observed points fall above those lines due to P storage, the sense of which is indicated by the light blue vectors (modelled P storage). Larger, eukaryotic cells in b are associated with higher storage contributions (longer blue vector) than smaller prokaryotic cells in a. c, Differences in P storage between small and large plankton size classes in the model are based on empirical estimates derived from laboratory studies of prokaryotic20,39 and eukaryotic44 phytoplankton (n = 43 and 28 for prokaryotes and eukaryotes, respectively. A few data with excess growth-limiting nutrients at the steady state39 are not included). The box represents median (centre line) and first and third quartiles (box). The whiskers represent the value range without outliers (those outside the box by 1.5 times the interquartile range). The P storage is estimated based on the differences in P:C under N and P limitations for closest growth rates.
海洋有机物中主要元素的比例将细胞活动过程与全球营养物、氧气以及碳循环联系起来。在海洋生物群落之间已经观测到有机物C:N:P比值（碳氮磷比值）的差异，但在小规模的生理和生态过程中这些差异模式尚未被量化。本文中我们使用了一种生态模型，这个模型中包含了生态学水平上大小截然不同的浮游生物内部和相互之间的适应性资源分配，以确定影响C:N:P比值中全球模式的因素。我们发现N:C比值（氮碳比值）的变化很大程度上是由所有浮游植物共同的生理调节策略所驱动，而N:P比值（氮磷比值）的模式变化则是由不同储磷能力的分类群生态选择所驱动。尽管N:C比值（氮碳比值）因细胞会对光照和营养物质的调整而变化很大，但因为营养物质和光照供应之间的平衡随深度而变化，其纬度梯度变化不大。N:P比值（氮磷比值）在纬度上的大幅度变化反映了这样一种生态平衡，即亚热带地区偏好具有较低储磷能力的小型浮游生物，而营养丰富的高纬度地区则偏好具有较高细胞储磷能力的较大真核生物。南半球和北半球，大西洋和太平洋之间较小的N:P比值（氮磷比值）差异反映了可用于细胞储存的磷酸盐的差异性。尽管只模拟了两种大小等级的浮游植物，但元素比例的全球差异性与所有观测物种相似，研究表明生长条件的范围与生态选择维持了所观测到的浮游植物间化学计量的多样性。
（实习生吕晴编译）
