…and the answer is: Holtzbrinck.
Holtzbrinck also holds a majority share in SpringerNature, as Roger Schonfeld detailed for The Scholarly Kitchen in October 2017 together with an analysis of a possible scholarly publishing “duoply”:
Any kind of acquisition of Digital Science would leave Springer Nature and Elsevier with many of the pieces of a research workflow business in combination with publishing operations and platforms. There are a number of differences between the two workflow businesses and publishing operations, but the fundamentals would be that the two largest scientific publishers, and the two largest open access publishers, would also be the two largest scientific workflow providers. This would reinforce the emerging marketplace dynamics as one leading towards duopoly.
Watch this space.
Schonfeld, Roger (2017). Who Owns Digital Science? The Scholarly Kitchen. https://scholarlykitchen.sspnet.org/2017/10/23/ownership-digital-science/