|Marine unicellular cyanobacteria of the Synechococcus group occupy an important position at the base of the marine food web: they are abundant in the world's oceans and as a result are major primary producers on a global scale and one of the most numerous genomes on earth. Members of this group are adapted to life in the ocean; they are obligately marine, having elevated growth requirements not only for Na+, but also for Cl-, Mg2+, and Ca2+; they have the ability to acquire major nutrients and trace metals at the submicromolar concentrations found in the oligotrophic open seas, and their light-harvesting apparatus is uniquely adapted to the spectral quality of light in the ocean. Furthermore, a third of the open ocean isolates of Synechococcus possess a unique type of swimming motility not seen in any other type of microorganism: they propel themselves through seawater at speeds of up to 25 µm/sec in the absence of any demonstrable external organelle. The Department of Energy/Joint Genome Institute (DOE/JGI) have completely sequenced the genome of open ocean Synechococcus WH8102, and we (and other colleauges) are in the process of annotating the genome.|
Fe acquisition in marine Synechococcus
To date, a complete high affinity
Fe scavenging system (i.e., outer membrane ferric siderophore receptor protein,
periplasmic iron binding protein, inner-membrane channel, and ATPase) has
yet to be characterized/demonstrated in any oceanic cyanobacterium.
Siderophore production, on the other hand, has been investigated in freshwater,
coastal, and some marine cyanobacteria. A clear trend has emerged; many
freshwater cyanobacteria make detectable siderophores, while in open oceanSynechococcus
spp. and Trichodesmium spp. siderophores have yet to be
detected. This observation is quite surprising because it has been
shown that nitrogen fixing Trichodesmium cells have high cellular
Fe requirements in the field, and open ocean Synechococcusspp.
are numerous in regions where the Fe concentrations are very low and suspected
to be limiting.
| Detailed searches
of the Synechococcus and Prochlorococcus genomes have not clearly
defined the molecular mechanism of Fe acquisition employed by these organisms.
However, three proteins predicted to be involved in Fe scavenging have been
identified, based on similarity, in the genome of Synechococcus WH8102:
an Fe binding protein (45% identity to IdiA from freshwater Synechococcus
PCC6301), inner membrane channel protein (35% identity to HitB from Haemophilus
influenza), and an ATPase (42% identity with SfuC from Serratia
marcescens). Similar homologues were identified in the
genomes of Prochlorococcus(MED4
Together these three proteins could compose a complete periplasmic binding
protein dependent ABC transporter for Fe. In addition, both Prochlorococcus
spp. and Synechococcus WH8102 appear to be lacking any
defined outer membrane receptor proteins for Fe:siderophore complexes, siderophore
biosynthetic genes, and the proteins TonB, ExbB, and ExbD (as shown in figure
to the right). In contrast, homologues to these proteins were easily
identifiable through searches of the freshwater cyanobacterial genomes(Anabaena
PCC7120, Synechocystis PCC6803, and Nostocpunctiforme).
Taken together these field and genomic observations would argue for the existence
of a high affinity Fe scavenging system in these oceanic cyanobacteria that
may be partially or completely novel.
The final goal of this work is to develop a paradigm for Fe acquisition for oceanic cyanobacteria using genomic, proteomic, genetic, and physiological techniques. This knowledge will be invaluable in predicting how small changes in Fe concentrations (i.e., Fe seeding experiments) will affect primary productivity at the bacterial level. As a first step to elucidating the Fe scavenging mechanism employed by open ocean cyanobacteria, I have proposed to delineate the cellular role of the IdiA protein.
Development of quantitative stress assays to probe the in situ nutrient physiology of open ocean Synechococcus WH8102
Open ocean cyanobacteria (e.g.
Synechococcus) are prominent components of the marine biosphere that
contribute significantly to global primary production. In many regions
of the ocean, the bioavailability of nutrients and trace metals (e.g. P and
Fe) are suspected to be limiting the growth and productivity of these organisms.
Due to the complex geochemistries of these compounds, demonstrating limitation
of phytoplankton productivity by direct chemical measurements is not trivial.
Although there are many valuable techniques to assess phytoplankton growth
limitation (ranging from simple bottle enrichments to immunological techniques),
they are generally unable to quantitatively resolve how nutrient limitation
impacts specific phytoplankton species or functional groups (e.g. Synechococcus).
Conversely, molecular stress assays where gene expression is monitored in
situ are highly anticipated techniques that could add quantitative parameters
to these analyses. This project is designed to test the applicability
of quantitative-reverse transcriptase polymerase chain reaction (Q-RT PCR)
gene expression assays to probe the nutrient status of open ocean Synechococcus