Characterization of Siderophores in the Southern Ocean

Abstract

Iron is an essential but limiting nutrient for phytoplankton growth in the marine environment. In some oceanic bodies like the Southern Ocean, the chlorophyll levels are low even if the nutrients are replete in these waters. These areas, otherwise known as high nutrient low chlorophyll (HNLC) regions, have very low Fe levels (Butler 2005). Previous works have shown evidence that dissolved Fe is generally present complexed with organic ligands (Rue and Bruland, 1997; Boye et al., 2001; Vraspir and Butler, 2009). These ligands prevent the Fe from precipitating under aerobic conditions and at near-neutral pH and thereby make them more bioavailable to organisms. The similarities of the stability constants of these ligands with the stability constants of known siderophores, or those that were isolated from cultured organisms, led to the assumption that these ligands may be or include siderophores (Rue and Bruland, 1995; Macrellis et al., 2001; Gledhill et al., 2004). Siderophores are low molecular-weight, organic compounds with a high affinity to Fe3+. Due to contamination problems and the difficulty in sampling, only a small number of marine siderophores have been chemically characterized. However, taking into consideration that these organic compounds could possibly be one of the driving factors that control productivity in the ocean and have influence on Fe biogeochemical cycling, a study focusing on the determination of their chemical nature is very important.

In the present study, both fieldwork and experiments were conducted to determine the chemical nature of the marine siderophores in different water masses around New Zealand. Analytical methods were initially tested by characterizing siderophores biosynthesized by cultured bacteria isolated from thermoalkaphilic terrestrial system and marine bacteria grown under low Fe media.

Catechol and hydroxamate moieties were detected from the thermoalkaphilc Caldalkalibacillus thermarum strain TA2.A1 by chemical assays. However, only a hydroxamate-type siderophore was identified by HPLC-MS analysis. A catechol siderophore was produced and detected from the heterotrophic marine organism Vibrio alginolyticus PWH3a. The enterobactin hydrolysis dimer, dihydroxy benzoylserine ((DHBS)2) was recognized from the MSMS fragmentation of the compound. Furthermore, a carboxylate-type similar to the known siderophore rhizofferin was detected from a culture of the aerobic marine proteobacteria roseobacter Silicibacter pomeroyi DSS-3.

In neritic and Sub-Antarctic waters off the south eastern coast of New Zealand, a total of six hydroxamate type siderophores were detected and identified in surface waters. The Sub-Antarctic water represents a Fe-deplete body of water while neritic waters were considered Fe-replete. A large proportion of strong L1 class Fe-binding ligands were found in surface waters corroborating the detection of siderophores at the same depths and the assumptions that most of the strong ligands will be dominant at the surface. Furthermore, seasonal and water-mass-specific differences were observed for the ratio of L1 to the sum of ligands present in the surface water.

In the mesotrophic and Fe replete subtropical waters off the eastern coast of New Zealand, hydroxamate-type siderophores, which all exhibit ferrioxamine fragmentation patterns, were identified together with their Na adducts in surface waters. Two among the five hydroxamate siderophores were suspected to also contain carboxylate groups, however, due to sample limitation this was not confirmed. The results were strongly supported by chemical assays, estimates of siderophore- producing bacterial abundance using CAS agar plate experiments and electrochemical measurements of the complexing capacity. The strong L1 class was found to dominate the Fe-binding ligand distribution in surface waters. Furthermore, a catechol-type siderophore was likewise qualitatively detected using chemical assay but as with neritic and Sub-Antarctic samples, this was not confirmed or detected by HPLC separation and mass spectrometry measurements.

On board incubation experiments focusing on the origin of the ligands were also conducted during the Fe Cycle II cruise in the sub tropical waters off the eastern coast of New Zealand. The goal was to determine whether siderophores are produced during the bioremineralization of marine particles. Results showed the presence of a known open-chained ferrioxamine B and G siderophore among two more hydroxamate siderophore types found. The chemical assays and CAS agar test experiments strongly agreed with the HPLC-MS results. The electrochemistry analysis, however, failed to detect strong L1 class Fe-binding ligands.

In general, this study has successfully identified the major functional groups of siderophores found in both Fe-deplete and Fe-replete water bodies around New Zealand. It can be concluded that hydroxamate (mainly ferrioxamine types) siderophores are present and potentially dominant in this area. The exact chemical structures of most siderophore were not confirmed. However, two known open- chained ferrioxamines (i.e. B and G) were detected from the particle remineralization experiment conducted during the Fe Cycle II cruise in sub tropical waters.