UNLABELLED: The majority of microorganisms in the environment have yet to be isolated in pure cultures, and the reasons behind this phenomenon remain elusive. In this study, we investigated the possibility of the commonly used gelling agent including agar and gellan gum as a source of carbon and energy in anaerobic roll-tube cultivation from one mangrove sediment sample and two high-temperature oilfield samples. Based on growth tests and genomic evidence, anaerobic gellan degraders were retrieved from genera of IMPORTANCE: Most microbial species inhabiting natural environments have not been isolated in pure cultures using conventional media and laboratory conditions, but the reason behind this is unclear. Here, we provided a new explanation for the phenomenon, in that both the gelling agents, like agar and gellan gum, and reducing agent L-cysteine-HCl in the media provide extra carbon and energy sources to microorganisms and therefore decrease the chance in isolation specifically for the supplemented substrate which is supposed to be the sole source of carbon and energy. This result demonstrated that further improvement in the effectiveness of isolation of targeted microorganisms will be facilitated by subtracting the overlooked organic ingredients in the medium and more innovations.