Marine ancestors of freshwater sponges had to undergo a series of physiological adaptations to colonize harsh and heterogeneous limnic environments. Besides reduced salinity, river-lake systems also have calcium concentrations far lower than seawater. Cell adhesion in sponges is mediated by calcium-dependent multivalent self-interactions of sulfated polysaccharide components of membrane-bound proteoglycans named aggregation factors. Cells of marine sponges require seawater average calcium concentration (10 mM) to sustain adhesion promoted by aggregation factors. We demonstrate here that the freshwater sponge Spongilla alba can thrive in a calcium-poor aquatic environment and that their cells are able to aggregate and form primmorphs with calcium concentrations 40-fold lower than that required by marine sponges cells. We also find that their gemmules need calcium and other micronutrients to hatch and generate new sponges. The sulfated polysaccharide purified from S. alba has sulfate content and molecular size notably lower than those from marine sponges. Nuclear magnetic resonance analyses indicated that it is composed of a central backbone of non- and 2-sulfated α- and β-glucose units decorated with branches of α-glucose. Assessments with atomic force microscopy/single-molecule force spectroscopy show that S. alba glucan requires 10-fold less calcium than sulfated polysaccharides from marine sponges to self-interact efficiently. Such an ability to retain multicellular morphology with low environmental calcium must have been a crucial evolutionary step for freshwater sponges to successfully colonize inland waters.

中文翻译：

---

由碳水化合物-碳水化合物相互作用介导的淡水海绵细胞的粘附需要低环境钙。

淡水海绵的海洋祖先必须经历一系列生理适应才能在恶劣和异质的湖泊环境中定居。除了降低盐度外，河湖系统的钙浓度也远低于海水。海绵中的细胞粘附是由称为聚集因子的膜结合蛋白聚糖的硫酸化多糖成分的钙依赖性多价自相互作用介导的。海洋海绵细胞需要海水平均钙浓度 (10 mM) 来维持由聚集因子促进的粘附。我们在这里展示了淡水海绵Spongilla alba可以在缺钙的水生环境中茁壮成长，并且它们的细胞能够聚集并形成钙浓度比海洋海绵细胞所需钙浓度低 40 倍的原形。我们还发现它们的宝石需要钙和其他微量营养素来孵化和产生新的海绵。从S. alba纯化的硫酸化多糖的硫酸盐含量和分子大小明显低于海绵。核磁共振分析表明，它由非和 2-硫酸化 α- 和 β- 葡萄糖单元的中心骨架组成，并装饰有 α-葡萄糖分支。用原子力显微镜/单分子力谱进行的评估表明，S. alba葡聚糖需要比来自海绵的硫酸化多糖少 10 倍的钙才能有效地自我相互作用。这种以低环境钙保持多细胞形态的能力一定是淡水海绵成功定居内陆水域的关键进化步骤。