Data from: Niche limits of symbiotic gut microbiota constrain the salinity tolerance of brine shrimp

Symbiosis generally causes an expansion of the niche of each partner along the axis for which a service is mutually provided. However, for other axes, the niche can be restricted to the intersection of each partner’s niche and can thus be constrained rather than expanded by mutualism. We explore this phenomenon using Artemia as a model system. This crustacean is able to survive at very high salinities but not at low salinities, although its hemolymph’s salinity is close to freshwater. We hypothesized that this low-salinity paradox results from poor performance of its associated microbiota at low salinity. We showed that, in sterile conditions, Artemia had low survival at all salinities when algae were the only source of carbon. In contrast, survival was high at all salinities when fed with yeast. We also demonstrated that bacteria isolated from Artemia’s gut reached higher densities at high salinities than at low salinities, including when grown on algae. Taken together, our results show that Artemia can survive at low salinities, but their gut microbiota, which are required for algae digestion, have reduced fitness. Widespread facultative symbiosis may thus be an important determinant of niche limits along axes not specific to the mutualistic interaction.共生作用通常会沿互利服务提供的维度，拓展共生双方各自的生态位（niche）。然而，对于其他维度而言，生态位会被限定在共生双方各自生态位的交集范围内，因此互利共生反而会对其产生限制而非拓展作用。本研究以卤虫（Artemia）作为模式系统，对该现象展开探究。该甲壳动物能够在极高盐度的环境中存活，却无法适应低盐度环境，尽管其血淋巴（hemolymph）的盐度接近淡水。我们提出假说：这一低盐度悖论源于其共生微生物群在低盐环境下的生存能力不佳。实验结果表明，在无菌条件下，若仅以藻类作为唯一碳源，卤虫在各盐度梯度下的存活率均较低。与之相反，当以酵母为食物时，卤虫在所有盐度环境下的存活率均较高。本研究还证实，从卤虫肠道分离得到的细菌，在高盐环境下的种群密度高于低盐环境，包括以藻类为营养基质的培养条件下亦是如此。综合来看，本研究结果显示，卤虫本身可在低盐环境中存活，但负责藻类消化的肠道微生物群的适合度（fitness）出现了下降。由此可见，广泛存在的兼性共生（facultative symbiosis）或许是决定非互利互作专属维度下生态位边界的重要因素。