Photoresponsive modulation of a freshwater phytoplankton community by bacterial lipopeptides

Photoreactive lipopeptide siderophores are intriguing compounds that are well-known from the marine environment. Once they have complexed Fe3+, these compounds undergo photooxidative cleavage, thereby releasing Fe2+ into the environment. The solubilized iron is then not only available to the siderophore producer, but to the whole microbial community. This can result in mutualistic interactions, with bacteria providing iron to phytoplankton and obtaining photosynthetically fixed carbon in exchange (carbon for iron mutualism). Photoreactive lipopeptide siderophores have recently also been found in freshwater and soil. In this study, variochelins from Variovorax boronicumulans are presented as further example, as well as seven further putative producers discovered by genome mining. These findings corroborate the assumption that photoreactive lipopeptide siderophores are also widely distributed in non-marine environments, raising the interest for their ecological role. In freshwater, mutualistic iron sharing is well-conceivable. This is addressed in the second part of the study. Here, the cupriachelin-based interaction between the bacterium Cupriavidus necator H16 and the diatom Navicula pelliculosa is investigated. Since iron usually represents a limiting factor for algal growth, diatoms are expected to strongly benefit from a carbon for iron mutualism. If so, they could further maximize their advantage by manipulating siderophore biosynthesis by the bacterium. Effectively, culture supernatants of N. pelliculosa were found to induce cupriachelin transcription levels in C. necator H16. Planktic interactions are well-documented on a substrate exchange basis, but this is one of the few studies reporting crosstalk on a transcriptional level. In conclusion, photoreactive siderophores play an important role in iron cycling in aquatic environments. They form a basis for interspecies interactions and thereby have the potential to shape planktic communities.