Dissimilarities in polyphosphate accumulation between Shewanella loihica and the model bacterium Shewanella oneidensis

Polyphosphate plays an important role in the physiological adaptation of bacteria during growth and development, as well as in their response to nutritional and environmental stresses. Moreover, polyphosphate accumulation is of environmental relevance and is linked to the formation of phosphate minerals. In particular, ferrous iron phosphate minerals represent a key step in the recycling of phosphorous in aquatic and terrestrial ecosystems. In this study, we investigated the polyphosphate accumulation potential of two selected species of iron-reducing Shewanella. We studied polyphosphate accumulation in Shewanella loihica by a combination of methods including indirect and direct polyphosphate measurements. During iron reduction in absence of an external phosphate source, we observed a correlation between polyphosphate accumulation, orthophosphate release and production of ferrous phosphate minerals. We also used Shewanella oneidensis, the well-characterized iron-reducing bacterium, as model organism in our study. In contrast to S. loihica, the ability to accumulate polyphosphate appears to be less pronounced in S. oneidensis. These physiological data were confronted with the analysis of the genomes of both species. Genes for phosphate transporters (pit, pst), polyphosphate kinase (ppk) and exopolyphosphatase (ppx) were identified in the genome of S. loihica and S. oneidensis, but the ppk gene is disrupted in S. oneidensis. Expression of these genes during aerobic growth and anaerobic iron reduction are currently under scrutiny by RT-qPCR analysis. We will discuss the results obtained in the context of biogenic iron phosphate mineral formation, the formation of such minerals in natural and manmade ecosystems, as well as their potential use for the recovery and recycling of phosphorous.