Microbial co-occurrence networks as a biomonitoring tool for aquatic environments: a review

Aquatic microbial ecosystems are increasingly under threat from human activities, highlighting the need to for the development and application of biomonitoring tools that can identify anthropogenically induced stress across a wide range of environments. To date, microbial biomonitoring has generally focussed on community composition and univariate endpoints, which do not provide discrete information about how species both interact with each other and as a collective. To address this, co-occurrence networks are being increasingly used to complement traditional community metrics. Co-occurrence network analysis is a quantitative analytical tool that examines the interactions between nodes (e.g. taxa) and their strengths. This information can be integrated and visualised as a network, whose characteristics and topological structures can be quantified. To date, co-occurrence network analysis has rarely been applied to aquatic systems. Here we explore the potential of co-occurrence networks as a biomonitoring tool in aquatic environments, demonstrating its capacity to provide a more comprehensive view of how microbial, notably bacterial, communities may be altered by human activities. We examine the key attributes of networks and providence evidence of how these may change as a response to disturbances while also highlighting some of the challenges associated with making the approach routine.