Dose-dependent transcriptional responses of marine prokaryotes to Fe-rich colloids of glacial and non-glacial origin in the Kerguelen region (Southern Ocean)

The trace metal iron (Fe) plays a key role in regulating microbial carbon metabolism through complex processes. In ocean regions where Fe is potentially limiting, new Fe sources can significantly influence prokaryotic activity and growth, stimulating the microbially-mediated cycling of carbon. The accelerated melting of glaciers delivers large quantities of Fe-rich material to the ocean, but the bioavailability of the diverse glacial Fe species remains poorly understood. Here, we investigated the response of marine prokaryotes to Fe contained in the colloidal fraction (20 - 200 nm size fraction) originating from the Cook Ice Cap on Kerguelen Island (Southern Ocean). We assessed growth and gene transcriptional responses to dose-additions of Fe-rich colloids from two sources, glacial meltwater and a non-glacial origin. The two colloid types resulted both in dose-dependent, but contrasting growth responses. While non-glacial colloids stimulated prokaryotic growth, the opposite pattern was observed with glacial colloids. The transcriptional profiles revealed an increasing trend in Fe-limitation, as evidenced by transcripts of the glyoxylate shunt, as a function of the addition of either colloid type. However, only non-glacial colloids induced a dose-dependent increase in transcripts of aromatic compound degradation, suggesting that these Fe-rich colloids could also provide C. These results point to the key role of C regulating the prokaryotic growth in response to Fe-rich colloids. The clear trends in the expression of genes in response to dose addition allowed to map transcript abundances to nutritional stress and specific metabolic pathways. Dose additions of limiting nutrients in incubation experiments induce quantitative transcriptional responses, thereby offering insights to the nutritional status of prokaryotes and to the mechanisms leading to Fe and C co-limitation.