Microbial co-cultures promise the development of more efficient bioproductions. However, the design of obligate mutualisms is complicated when using organisms that possess differing growth rates or incompatible media requirements. In this work, we investigate sucrose production by cscB Synechococcus elongatus PCC 7942 within a polyacrylate hydrogel matrix. This system secretes sucrose only when the hydrogel is spatially constrained, demonstrating a new utilization of hydrogel swelling pressure to control the osmotic strength of a microbial microenvironment. The sucrose produced via the constrained microbial hydrogel is used to grow the diazotrophic organism, Azotobacter vinelandii, in a mutually dependent fashion. The growth of this hydrogel-based coculture has several advantages over batch cultures, including better growth over a longer period of time and decreased salt stress on A. vinelandii. Biotechnol. Bioeng. 2017;114: 1195-1200. (c) 2016 Wiley Periodicals, Inc.