Microbial assimilation of phosphorus in the subtropical Atlantic Ocean: a molecular approach

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Phosphorus (P) is an essential element for all living matter on earth, irrespective of size or habitat. Microbes are microscopic organisms that require P to synthesize building blocks for DNA, build cell envelopes and create energy transfer molecules. In the ocean, P exists in three forms; phosphate (PO4), dissolved organic P (DOP) and particulate P (PP). PO4 is the most readily used by marine microbes for growth. In coastal or subpolar regions, PO4 concentrations are sufficiently high to support microbial growth. However, in regions of the ocean called subtropical gyres, surface PO4 concentrations are extremely low and limit microbial growth. Conversely, DOP concentrations are up to 100 times higher than PO4. A diverse array of microbes live in these PO4-limited but DOP-plentiful regions but how do they cope with P-stress? Do microbes compete for the same small pool of PO4 or can they access the complex DOP pool? How do they co-exist? The research proposed here will begin to answer these questions. During a research cruise in the Atlantic Ocean in 2011, we collected samples along a gradient of PO4 and DOP concentration and availability. We propose to examine the presence and expression of genes contained within microbes that encode for the production of proteins that allow microbes to acquire P, i.e. 'P-acquisition genes'. Molecular studies have shown that some genes can produce proteins that bind PO4 at very low concentrations or enzymes that can cleave P bound to organic phosphorus. For example, the PhoA gene encodes for the production of alkaline phosphatase, an enzyme that cleaves P bound to organic molecules called phosphomonoesters that make up 20 to 75% of the DOP pool. Microbes that possess PhoA can therefore access a large part of the DOP pool. We propose to determine the relative distribution of five P-acquisition genes. However, it is possible for a microbe to possess a gene, but for that gene to be switched off. Therefore, we will not only determine the presence of the genes, but also if they are switched on or are being 'expressed'. We will target 3 ecologically important species living in the ocean: Prochlorococcus and Synechococcus, which are microscopic cyanobacterium that are important for cycling of carbon in the ocean, and Trichodesmium, a colony forming cyanobacteria that are visible to the naked eye and play an important role in the marine nitrogen cycle. In summary, we propose to use molecular techniques to determine the presence and expression of 5 P-acquiring genes in 3 ecologically important species along a gradient in P-availability. Why is this important? Subtropical gyres represent 70% of the world's ocean. Observations from the Atlantic and Pacific Oceans show that these vast regions are showing signs of warming in response to climate change through changes in water column stability and microbial community structure. Importantly, it is predicated that climate change will enhance P-limitation in subtropical gyres and thus it is vital that we understand the P-acquisition genes of ecologically important microbes in order to identify the winners and losers in a changing ocean environment.