Strains of microalgae vary in traits between species and populations due to adaptation or stochastic processes. Traits of individual strains may also vary depending on the acclimatization state and external forces, such as abiotic stress. In this study we tested how metal tolerance differs among marine diatoms at three organizational levels: species, populations, and strains. At the species level we compared two pelagic Baltic Sea diatoms (Skeletonema marinoi and Thalassiosira baltica). We found that the between-species differences in tolerance (EC50) to the biologically active metals (Cu, Co, Ni, and Zn) was similar to that within-species. In contrast, the two species differed significantly in tolerance towards the non-essential metals, Ag (three-fold higher in T. baltica), Pb and Cd (two and three-fold higher in S. marinoi). At the population level, we found evidence that increased tolerance against Cu and Co (17 and 41 % higher EC50 on average, respectively) had evolved in a S. marinoi population subjected to historical mining activity. On a strain level we demonstrate how the growth phase of cultures (i.e., cellular densities above exponential growth) modulated dose-response relationships to Ag, Cd, Co, Cu, and Zn. Specifically, the EC50's were reduced by 10–60 % in non-exponentially growing S. marinoi (strain RO5AC), depending on metal. For the essential metals these differences were often larger than the average differences between the two species and populations. Consequently, without careful experimental design, interactions between nutrient limitation and metal stress may interfere with detection of small, but evolutionary and ecologically important, differences in tolerance between microalgae. To avoid such artifacts, we outline a semi-continuous cultivation approach that maintains, and empirically tests, that exponential growth is achieved. We argue that such an approach is essential to enable comparison of population or strain differences in tolerance using dose-response tests on cultures of microalgae.

由于适应或随机过程，微藻菌株在物种和种群之间的性状有所不同。各个菌株的性状也可能根据适应状态和外力（例如非生物胁迫）而变化。在这项研究中，我们测试了海洋硅藻在三个组织级别上的金属耐受性如何不同：物种，种群和品系。在物种水平上，我们比较了两种中上层波罗的海硅藻（Skeletonema marinoi和Thalassiosira baltica）。我们发现，对生物活性金属（铜，钴，镍和锌）的耐受性（EC50）的种间差异与种内相似。相比之下，这两个物种对非必需金属Ag的耐受性差异显着（其中三价T. baltica），Pb和Cd（在S. marinoi中高2到3倍）。在种群水平上，我们发现有证据表明，经历过历史采矿活动的S. marinoi种群对Cu和Co的耐受性增强（平均EC50分别提高了17％和41％）。在菌株水平上，我们证明了培养物的生长阶段（即，指数生长以上的细胞密度）如何调节与Ag，Cd，Co，Cu和Zn的剂量反应关系。具体来说，在非指数生长的S. marinoi中，EC50降低了10–60％。（应变RO5AC），具体取决于金属。对于必需金属，这些差异通常大于两个物种和种群之间的平均差异。因此，如果没有仔细的实验​​设计，养分限制和金属胁迫之间的相互作用可能会干扰微藻之间小的，但具有进化和生态学意义的耐受性差异的检测。为避免此类伪影，我们概述了一种半连续培养方法，该方法可以维持并通过经验测试可以实现指数增长。我们认为，这种方法对于使用微藻培养物的剂量反应测试能够比较种群或菌株的耐受性差异至关重要。