Abstract:
An increased understanding of heterotrophic bacterial strategies for acquiring nutrients
and trace elements is critical for elucidating their impact on biogeochemical cycling in
the ocean. It is estimated that iron is a limiting nutrient for phytoplankton growth in over
30% of the open ocean, but still little is known about bacterial strategies for iron
acquisition. Siderophore (Fe ligand) production by bacteria may play a major role in
influencing the bioavailability of iron in the ocean. Despite the importance of
siderophores in the environment, only limited information from a select group of bacteria
is available. On a cruise through the Costa Rica Dome (CRD) upwelling region in July
2005, a library of 867 isolates from five depth profiles inside and outside of the dome
was obtained and screened for siderophore production using the Chrome Azurol-S (CAS)
assay. Phylogenetic affiliation of 134 isolates was determined by sequencing the 16s
rDNA gene, and determined that gamma proteobacteria such as Alteromonas,
Pseudoalteromonas, Halomonas, and Marinobacter dominated the collection, while
alpha-proteobacteria such as Roseobacter were also represented. The isolates obtained
from stations in the CRD showed greater siderophore-producing capabilities between
55m and 100m while strains isolated from outside the CRD had shallower peak (~8-35m)
production. Functional group determination showed that hydroxamate production
dominated from 50-150m, while hydroxamate and catechol production is roughly equal
in shallower waters. By characterizing the siderophores produced by these isolates and
determining the genetic make-up of the population, these findings further our
understanding of how heterotrophic microbes affect biogeochemical processes and the
competitive nature of nutrient acquisition.
