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

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.

Who will benefit from this research?
(a) Aquatic microbial and molecular ecologists
(b) Marine ecologists and biogeochemists studying the marine phosphorus cycle
(c) Ecosystem modelers
(d) Public and school students interested in science and technology

How will they benefit from this research?
(a) Aquatic microbial and molecular ecologists: We will produce a novel set of data on the presence and expression of 5 classes of phosphorus-acquisition genes present in 3 ecologically important and numerically abundant species along a gradient in phosphorus-availability in the Atlantic Ocean. The dataset and associated outcomes will add to the ongoing discussions on how microbes survive under nutrient limited conditions. More so, we propose to use multiple species to gain a more holistic view (albeit not complete) of how the microbial ecosystem operates in subtropical regions, rather than the single species approach conventionally used. Thus, we envisage that marine microbial and molecular ecologists will be interested in our study and its outcomes.
(b) Marine ecologists and biogeochemists studying the phosphorus cycle: We will design and validate PCR primer sets to target genes in the specific organisms of interest. The gene sequences will be verified and published, and therefore available to other scientists studying the marine phosphorus cycle.
(c) Ecosystem modelers: We believe that ecosystem modelers will be interested in our data set as it will provide some insight into the potential for competition or interaction in nutrient acquisition at the species and community level. There are a large number of marine ecosystem models that include representative Prochlorococcus, Synechococcus and Trichodesmium populations (e.g. Darwin Model developed by Mick Follows at MIT) and thus the species targeted in this study are of direct relevance to these models and therefore will impact to the marine modeling community.
(d) The popularity of television programmes such as 'CSI: Crime Scene Investigation' have revolutionized the public's view of scientific laboratories, and chemical and molecular analysis. The public is now able to watch samples being put through intricate machines and have become familiar with terms such as 'DNA', 'fingerprinting', 'mass spectrometry' and 'chromatography'. We believe that such programmes have enhanced the public's interest in tools used by scientists to uncover evidence and make new discoveries.