Fig. 1. The structure of marine microbial ecosystems and synthetic microbial communities.

a Schematic representation of marine microbial ecosystems structured by primary producers, primary degraders and ultimate consumers. Primary producers in the euphotic zone absorb solar energy and store it in the form of organic matter. Primary degraders in sediments degrade complex organic compounds into small organic compounds. Ultimate consumers in deep sediments oxidize small organic compounds completely through anaerobic respiration with NO₃^-, Mn⁴⁺, Fe³⁺, SO₄^2- as electron acceptors. During these processes, the electron flow supports microbial growth, metabolism and cycling of elements. b Three synthetic microbial communities, including the two-species (C+S), three-species (C+E+S) and four-species (C+E+S+G) systems, were designed for photoelectric conversion. A sucrose-secreting strain of cyanobacteria was chosen as the primary producer, E. coli was chosen as the primary degrader, while S. oneidensis and G. sulfurreducens were used as the ultimate consumers. Electricity is generated by S. oneidensis and G. sulfurreducens through anaerobic respiration with an electrode as terminal electron acceptor. Among the three synthetic microbial communities, the four-species microbial community is the system completely mimicking the structure of the marine microbial ecosystems and forms the basis of the miniaturized bionic ocean-battery.