Abstract
Iron (Fe) is an essential element for marine microbial growth but is present in trace amounts (<0.1 nM) in surface waters of the ocean. In heterotrophic bacteria, Fe-limitation particularly impacts ATP production as Fe is an essential co-factor of enzymes involved in the electron-transport chain as well as the tricarboxylic acid (TCA) cycle. Fe-limitation can therefore drastically reduce both bacterial growth and respiration, consequently affecting the efficiency of organic carbon remineralization. Heterotrophic bacteria possess various strategies to cope with Fe-limitation. In the present study we tested the hypothesis that the induction of the glyoxylate shunt can represent one such strategy. Genetic approaches were used to gain insight into the potential role the glyoxylate shunt may have in alleviating Fe-stress using the gammaproteobacterium Photobacterium angustum S14. A recombinant bioluminescent reporter of P. angustum S14 (icl-luc) revealed a strong and significant increase in icl expression when cells were subjected to strong Fe-limitation. Although both the growth and respiration rates decreased for the wildtype and an isocitrate lyase knockout mutant (∆icl) under strong Fe-limitation, they were ±30% lower for ∆icl as compared to the wildtype. Complementation of ∆icl restored the growth and respiration rates to those observed in the wild type, further confirming the importance of the glyoxylate shunt under strong Fe-limitation. Due to the ubiquitous nature of the glyoxylate shunt within marine bacteria, our results lead us to propose this pathway as an important acclimation strategy for marine heterotrophic bacteria that are subjected to Fe-limitation.
Highlights
Since the initial experiments of Martin and Fitzwater (1988) and the resulting Fe-hypothesis (Martin, 1990), it has been well established that in High Nutrient Low Chlorophyll (HNLC) regions, the constraint in microbial growth is dictated by the low bioavailable Fe-levels found in their surface waters
This was first shown in a study where the cellular Fe:C ratio for several heterotrophic bacterial isolates changed depending on the Fe-concentrations present in the environment (Tortell et al, 1996)
Fe-limitation was obtained through the addition of either 0, 10, 50, 100, and 500 μM of DFOB into Aquil medium already containing 5.4 μM of Fe
Summary
Since the initial experiments of Martin and Fitzwater (1988) and the resulting Fe-hypothesis (Martin, 1990), it has been well established that in High Nutrient Low Chlorophyll (HNLC) regions, the constraint in microbial growth is dictated by the low bioavailable Fe-levels found in their surface waters. The increased expression of isocitrate lyase, a gene involved in the glyoxylate shunt, was further noted in experiments conducted by Smith et al (2010) using various “Candidatus” Pelagibacter ubique strains and in a field study of Beier et al (2015) for members of the SAR11 clade. These observations suggest that the glyoxylate shunt is a possible acclimation strategy for heterotrophic marine bacteria in dealing with Fe-limitation, the strong association of the glyoxylate shunt toward the metabolism of certain carbon compounds makes it difficult to directly ascertain its induction with Fe-limitation. These results sparked interest in whether the induction of the glyoxylate shunt could serve as a metabolic acclimation strategy for bacteria subjected to Fe-limitation in a marine context
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
