Committee on Evolutionary Biology, University of Chicago, Chicago, Illinois, USA.
Department of Ecology and Evolution, University of Chicago, Chicago, Illinois, USA.
Ecology. 2021 Feb;102(2):e03221. doi: 10.1002/ecy.3221. Epub 2021 Jan 6.
Canopy-forming kelps are foundational species in coastal ecosystems, fixing tremendous amounts of carbon, yet we know little about the ecological and physiological determinants of dissolved organic carbon (DOC) release by kelps. We examined DOC release by the bull kelp, Nereocystis luetkeana, in relation to carbon fixation, nutrient uptake, tissue nitrogen content, and light availability. DOC release was approximately 3.5 times greater during the day than at night. During the day, N. luetkeana blades released an average of 16.2% of fixed carbon as DOC. Carbon fixation increased with light availability but DOC release did not, leading to a lower proportion of fixed carbon released as DOC at high light levels. We found no relationship between carbon fixation and DOC release rates measured concurrently. Rather, DOC release by N. luetkeana blades declined with marginal significance as blade tissue nitrogen content increased and with experimental nitrate addition, supporting the role of stoichiometric relationships in DOC release. Using a stable isotope ( C) tracer method, we demonstrated that inorganic carbon is rapidly fixed and released by N. luetkeana blades as DOC, within hours. However, recently fixed carbon ( DOC) comprised less than 20% of the total DOC released, indicating that isotope studies that rely on tracer production alone may underestimate total DOC release, as it is decoupled from recent kelp productivity. Comparing carbon and nitrogen assimilation dynamics of the annual kelp N. luetkeana with the perennial kelp Macrocystis pyrifera revealed that N. luetkeana had significantly higher carbon fixation, DOC production and nitrogen uptake rates per unit dry mass. Both kelp species were able to perform light-independent carbon fixation at night. Carbon fixation by the annual kelp N. luetkeana is as high as 2.35 kg C·m ·yr , but an average of 16% of this carbon (376 g C·m ·yr ) is released as DOC. As kelp forests are increasingly viewed as vehicles for carbon sequestration, it is important to consider the fate of this substantial quantity of DOC released by canopy-forming kelps.
形成树冠的巨藻是沿海生态系统的基础物种，它们固定了大量的碳，但我们对巨藻释放溶解有机碳（DOC）的生态和生理决定因素知之甚少。我们研究了与碳固定、养分吸收、组织氮含量和光可用性有关的巨藻释放 DOC 的情况。巨藻的 DOC 释放量大约是白天的 3.5 倍，比夜间多。在白天，Nereocystis luetkeana 叶片平均释放 16.2%的固定碳作为 DOC。碳固定随着光可用性的增加而增加，但 DOC 释放并没有增加，导致高光水平下释放的固定碳中作为 DOC 的比例较低。我们没有发现碳固定和同时测量的 DOC 释放率之间存在关系。相反，随着叶片组织氮含量的增加和实验添加硝酸盐，Nereocystis luetkeana 叶片的 DOC 释放量略有下降，这支持了化学计量关系在 DOC 释放中的作用。使用稳定同位素（ C）示踪法，我们证明了无机碳被 N. luetkeana 叶片迅速固定并释放为 DOC，在几小时内。然而，最近固定的碳（ DOC）不到总 DOC 释放量的 20%，这表明仅依靠示踪剂生产的同位素研究可能会低估总 DOC 释放量，因为它与最近的巨藻生产力解耦。比较一年生巨藻 N. luetkeana 和多年生巨藻 Macrocystis pyrifera 的碳和氮同化动力学表明，N. luetkeana 的碳固定、DOC 产生和单位干质量氮吸收速率都显著更高。两种巨藻都能够在夜间进行光独立的碳固定。一年生巨藻 N. luetkeana 的碳固定量高达 2.35 公斤 C·m -2·yr -1，但其中 16%的碳（376 克 C·m -2·yr -1）以 DOC 的形式释放。由于巨藻林越来越被视为碳封存的工具，因此考虑到这些由树冠形成的巨藻释放的大量 DOC 的命运是很重要的。
