D4Science-II will develop technology to enable interoperation of diverse data e-Infrastructures that are running autonomously, thereby creating e-Infrastructure Ecosystems that can serve a significantly expanded set of communities dealing with multidisciplinary challenges whose solution is currently beyond reach. Furthermore, D4Science-II will bring together several scientific e-Infrastructures established in the areas of biodiversity, fishery resource management, high energy physics, etc., to set up a prototypical instance of such Ecosystem. This will support several critical scientific scenarios that are distinct but also feed into and enrich each other in nontrivial ways. Finally, possibly in collaboration with appropriate international bodies and initiatives, D4Science will take steps to ensure sustainability of the Ecosystem, some of them based on and synchronized with the dissemination, training and standardization activities of the project.Technologically, D4Science-II constitutes a continuation of the DILIGENT and D4Science projects, which have initiated an effort towards using existing network, grid, and repository infrastructures to deploy on top of them a pan-European research e-Infrastructure that will enable unlimited generation and dissemination of scientific knowledge. As the next step towards this goal, D4Science-II will transform the current, operational D4Science e-Infrastructure into the linchpin of an e-Infrastructure Ecosystem, holding together and mediating between all participating e-Infrastructures through programmatically-available interoperability services.The D4Science-II Ecosystem will include among others, the GENESI-DR and DRIVER repository e-Infrastructures, and important thematic repositories maintained by international organizations, e.g., INSPIRE and AquaMaps. The project will create Virtual Research Environments offering significantly enhanced services to scientist without incurring high development and maintenance costs.