Strzepek Robert F, Harrison Paul J
Department of Botany, University of British Columbia, Vancouver BC Canada V6T 1Z4.
Nature. 2004 Oct 7;431(7009):689-92. doi: 10.1038/nature02954.
Diatoms are a key taxon of eukaryotic phytoplankton and a major contributor to global carbon fixation. They are ubiquitous in the marine ecosystem despite marked gradients in environmental properties, such as dissolved iron concentrations, between coastal and oceanic waters. Previous studies have shown that offshore species of diatoms and other eukaryotic algae have evolved lower iron requirements to subsist in iron-poor oceanic waters, but the biochemical mechanisms responsible for their decreased iron demand are unknown. Here we show, using laboratory-cultured model species, a fundamental difference between a coastal and an oceanic diatom in their photosynthetic architecture. Specifically, the oceanic diatom had up to fivefold lower photosystem I and up to sevenfold lower cytochrome b6f complex concentrations than a coastal diatom. These changes to the photosynthetic apparatus markedly decrease the cellular iron requirements of the oceanic diatom but not its photosynthetic rates. However, oceanic diatoms might have also sacrificed their ability to acclimate to rapid fluctuations in light intensity--a characteristic of dynamic and turbid coastal waters. We suggest that diatoms, and probably other eukaryotic algal taxa, exploited this difference in the underwater light climate between oceanic and coastal waters, enabling them to decrease their iron requirements without compromising photosynthetic capacity. This adaptation probably facilitated the colonization of the open ocean by diatoms, and contributes to their persistence in this iron-impoverished environment.
硅藻是真核浮游植物的关键分类群，也是全球碳固定的主要贡献者。尽管沿海和大洋水域的环境特性（如溶解铁浓度）存在明显梯度，但它们在海洋生态系统中无处不在。先前的研究表明，硅藻和其他真核藻类的近海物种已经进化出较低的铁需求，以便在缺铁的大洋水域中生存，但其铁需求降低的生化机制尚不清楚。在这里，我们使用实验室培养的模式物种表明，一种沿海硅藻和一种大洋硅藻在光合结构上存在根本差异。具体而言，大洋硅藻的光系统I浓度比沿海硅藻低多达五倍，细胞色素b6f复合体浓度比沿海硅藻低多达七倍。光合装置的这些变化显著降低了大洋硅藻的细胞铁需求，但并未降低其光合速率。然而，大洋硅藻可能也牺牲了其适应光照强度快速波动的能力，而这是动态且浑浊的沿海水域的一个特征。我们认为，硅藻以及可能其他真核藻类分类群利用了大洋和沿海水域水下光气候的这种差异，使其能够在不影响光合能力的情况下降低铁需求。这种适应可能促进了硅藻对开阔海洋的定殖，并有助于它们在这种缺铁环境中持续存在。
