TonB-dependent Transporters of Fe-petrobactin in Marinobacter nauticus and Southern Ocean bacteria

TonB-dependent transporters (TBDTs) play a crucial role in acquiring iron through siderophores in marine environments. However, determining the specific substrates for these transporters is a significant challenge in understanding the ecological and mechanistic roles of siderophore production and uptake. In this study, researchers identified two TBDTs, OptA and OptB, in the bacterium Marinobacter nauticus.

Although these transporters share low sequence identity, they both facilitate the uptake of Fe-petrobactin. Despite their functional overlap, they appear to utilize different regulatory mechanisms, implying that their usage varies under specific environmental conditions. Phylogenetic analyses suggest that OptA has co-evolved with the petrobactin biosynthetic gene cluster, while OptB has followed a separate evolutionary path, acquiring the ability to import petrobactin as a xenosiderophore.

In natural communities from the Southern Ocean, resuscitated bacterial communities demonstrated that Fe-petrobactin stimulated prokaryotic growth. Utilizing amino acid similarity, researchers identified OptB-like transporters in metagenomes and MAGs (metagenome-assembled genomes). Taxa containing these transporters, such as Pseudoaltermonas, responded specifically to the addition of Fe-petrobactin.

Moreover, these OptB-like transporters were detected in environmental metagenomes from the Southern Ocean, but the petrobactin biosynthetic gene clusters were not present, indicating that petrobactin primarily functions as a xenosiderophore in natural communities.