N-123. Identification of a Novel Phosphonate Utilization Pathway in Marine Microorganisms by Functional Metagenomic Analysis

A. Martinez, E. F. DeLong;
Massachusetts Inst. of Technology, Cambridge, MA.

Phosphorus is an essential element for all living organisms. Since dissolved inorganic phosphate can be a limiting nutrient in ocean surface waters, there is considerable interest in understanding the mechanisms, extent and variability of dissolved organic phosphorous (DOP) utilization by marine microorganisms. . About one third of marine DOP consists of phosphonates (Phn), compounds that containing a C-P bond instead of the more common C-O-P bond found in phosphate esters. Aminoethylphosphonate (AEPhn), found in phosphonolipids of marine invertebrates, is believed to be the most abundant phosphonate in the ocean. The ability of some microorganisms (such as E. coli) to utilize Phn as a P source has been recognized for many years, providing detailed information on their Phn utilization pathways. Known C-P hydrolases include the C-P lyase, a multienzyme complex that can release phosphate from a variety of Phns including alkyl-phosphonates, and Phn-compound specific pathways, like the phosphonatase pathway that acts exclusively on AEPhn. Little is known about Phn utilization in marine bacteria. Recently, it has been shown that a complete C-P lyase operon is present in the marine cyanobacterium Trichodesmium erythraeumthat is expressed in phosphorus-depleted cultures, strongly suggesting that members of this genus can use Phns as an alternative source of phosphorus. To characterize Phn utilization pathways in marine microorganisms we screened clones in large insert metagenomic libraries for their ability to complement an E. coli Phn- strain. Using this functional approach we identified clones in a planktonic fosmid library that allowed the E. coli host to grow on AEPhn as the P source. The complementing cloned DNAs contained homologs of some known Phn utilization genes, as well as a completely novel pathway for AEPhn utilization that is present in some marine microorganisms. Our results demonstrate that AEPhn can be an alternative P source in marine bacterioplankton and that functional screening approaches are useful for assigning environmentally relevant functions to hypothetical genes detected in metagenomic surveys.