Data from: Experimental adaptation to marine conditions by a freshwater alga

The marine-freshwater boundary has been suggested as one of the most difficult to cross for organisms. Salt is a major ecological factor and provides an unequalled range of ecological opportunity because marine habitats are much more extensive than freshwater habitats, and because salt strongly affects the structure of microbial communities. We exposed experimental populations of the freshwater alga Chlamydomonas reinhardtii to steadily increasing concentrations of salt. About 98% of the lines went extinct. The ones that survived now thrive in growth medium with 36 gL−1 NaCl, and in seawater. Our results indicate that adaptation to marine conditions proceeded first through genetic assimilation of an inducible response to relatively low salt concentrations that was present in the ancestors, and subsequently by the evolution of an enhanced inducible response to high salt concentrations. These changes appear to have evolved through reversible and irreversible modifications respectively. The evolution of marine from freshwater lineages is an example that clearly indicates the possibility of studying certain aspects of major ecological transitions in the laboratory.

海水-淡水交界带被认为是生物最难跨越的生态屏障之一。盐度是核心生态因子，其所带来的生态机遇无与伦比——这一方面源于海洋生境总面积远大于淡水生境，另一方面盐度对微生物群落结构具有极强的塑造作用。我们将淡水藻类莱茵衣藻（Chlamydomonas reinhardtii）的实验种群置于盐浓度逐步升高的环境中培养，约98%的实验株系最终灭绝。存活下来的株系现已可在含36 g·L⁻¹氯化钠的培养基以及天然海水中良好生长。研究结果表明，种群对海洋环境的适应分为两个阶段：首先通过遗传同化祖先种群中固有存在的、针对低浓度盐的诱导型响应实现初步适应，随后进化出针对高浓度盐的增强型诱导响应。上述两类适应性变化分别通过可逆与不可逆的遗传修饰演化而来。淡水谱系向海洋生境的进化实例，清晰证实了在实验室中研究重大生态转型的部分关键环节具有可行性。