Inherent tendency of <i>Synechococcus</i> and heterotrophic bacteria for mutualism on long-term coexistence despite environmental interference.

Abstract

Mutualism between <i>Synechococcus</i> and heterotrophic bacteria has been found to support their prolonged survival in nutrient-depleted conditions. However, environmental interference on the fate of their mutualism is not understood. Here, we show that exogenous nutrients disrupt their established mutualism. Once the exogenous nutrients were exhausted, <i>Synechococcus</i> and heterotrophic bacteria gradually reestablished their metabolic mutualism during 450 days of culture, which revived unhealthy <i>Synechococcus</i> cells. Using metagenomics, metatranscriptomics, and the ¹⁵N tracer method, we reveal that the associated bacterial nitrogen fixation triggered the reestablishment of the mutualism and revival of <i>Synechococcus</i> health. During this process, bacterial community structure and functions underwent tremendous adjustments to achieve the driving effect, and a cogeneration of nitrogen, phosphorus, iron, and vitamin by the heterotrophic bacteria sustained <i>Synechococcus</i>'s prolonged healthy growth. Our findings suggest that <i>Synechococcus</i> and heterotrophic bacteria may have an inherent tendency toward mutualism despite environmental interference. This may exhibit their coevolutionary adaptations in nutrient-deficient environments.