Creatures living at the bottom of the sea get much of their food from those that lived above and died. This biological detritus slowly descends to the sea floor. In the language of the trade, "pelagic organisms are a trophic resource for benthic ones." Marine scientists Dell’Anno and Danovaro at the University of Marche in Ancona, Italy, report that a significant component of this food resource is the DNA from dead organisms that lived on higher levels of the ocean. This DNA that rains down like manna amounts to about 107 metric tons per year worldwide. Being rich in phosphate, it supplies the prokaryotes in deep-sea sediment with about 47% of their need for this element (but only 4% of the carbon and 7% of the nitrogen). This is a key nutritional resource, as the authors suggest. By a couple of criteria, direct measurements and nuclease sensitivity, most of this DNA is extracellular. So abundant is this DNA that it amounts to six- to eight-fold times that contained in all the organisms in the top 10 cm of the world’s marine sediments. Not much is known about the rate of descent of the DNA through the vertical water column.
As the authors say: "DNA… is typically viewed only as the genetic material associated with living biomass." Besides food, is there any other role for DNA in the benthos? It seems possible that DNA that escapes degradation by nucleases could be taken up by competent organisms. If so, horizontal gene transfer would have a "vertical" component.