School of Biochemistry, University of Bristol, Bristol, UK.
Department of Biology and Marine Biology, University of North Carolina Wilmington, Wilmington, North Carolina, USA.
Environ Microbiol. 2023 Feb;25(2):315-330. doi: 10.1111/1462-2920.16280. Epub 2022 Nov 22.
Coccolithophores are an important group of calcifying marine phytoplankton. Although coccolithophores are not silicified, some species exhibit a requirement for Si in the calcification process. These species also possess a novel protein (SITL) that resembles the SIT family of Si transporters found in diatoms. However, the nature of Si transport in coccolithophores is not yet known, making it difficult to determine the wider role of Si in coccolithophore biology. Here, we show that coccolithophore SITLs act as Na -coupled Si transporters when expressed in heterologous systems and exhibit similar characteristics to diatom SITs. We find that CbSITL from Coccolithus braarudii is transcriptionally regulated by Si availability and is expressed in environmental coccolithophore populations. However, the Si requirement of C. braarudii and other coccolithophores is very low, with transport rates of exogenous Si below the level of detection in sensitive assays of Si transport. As coccoliths contain only low levels of Si, we propose that Si acts to support the calcification process, rather than forming a structural component of the coccolith itself. Si is therefore acting as a micronutrient in coccolithophores and natural populations are only likely to experience Si limitation in circumstances where dissolved silicon (DSi) is depleted to extreme levels.
颗石藻是一类重要的钙化海洋浮游植物。尽管颗石藻不会硅化，但有些物种在钙化过程中确实需要硅。这些物种还拥有一种新型蛋白（SITL），类似于硅藻中 SIT 家族的硅转运蛋白。然而，目前尚不清楚颗石藻中硅的运输方式，这使得确定硅在颗石藻生物学中的更广泛作用变得困难。在这里，我们表明，当在异源系统中表达时，颗石藻 SITLs 充当 Na 偶联的硅转运蛋白，并且表现出与硅藻 SITs 相似的特征。我们发现，来自 Coccolithus braarudii 的 CbSITL 受硅可用性的转录调控，并在环境颗石藻种群中表达。然而，C. braarudii 和其他颗石藻对硅的需求非常低，在对硅转运的敏感测定中，外源性硅的转运率低于检测水平。由于颗石中只含有低水平的硅，我们提出硅的作用是支持钙化过程，而不是形成颗石本身的结构成分。因此，硅在颗石藻中充当微量元素，只有在溶解硅（DSi）耗尽到极端水平的情况下，自然种群才可能经历硅限制。
