If the ocean depths are populated by fishes and other marine creatures, the merit is also of the viruses that, by continuously infecting microbial plankton, release nutrients that can be essential to support the marine food chain. These are the conclusions of a study published on Science Advances and conducted by an Italian-Spanish research team involving the Institute of Marine Science of the National Research Council (Ismar-Cnr) and the Spanish counterpart Institute of Marine Sciences of the Consejo Superior de Investigaciones Científicas (Icm-Csic).“The research is based on the analysis of over a thousand samples of seawater collected from the ocean surface to the depth of 4,000 meters along the Atlantic, Pacific and Indian oceans, during a scientific expedition conducted in 2010 and funded by Csic called “Malaspina Expedition” that resembles the homonym circumnavigation expedition led by Alessandro Malaspina in the late 1700s” explains Gian Marco Luna, Ismar-Cnr researcher from Ancona and co-author of the study. “We have demonstrated that viruses in the deep sea, which were so far only little investigated, are able to predate microbial plankton much more actively than believed so far. It is estimated that, in the global ocean, these viruses infect, every second, trillions and trillions of planktonic microbes. The viruses destroy the infected cells and recirculate, in the surrounding seawater, their precious cellular content, made of high quality biomolecules. An important fraction of this organic matter becomes later nourishment for other microorganisms, according to the effect known as 'viral priming', thus feeding the entire food web up to larger organisms, such as fishes”.The researchers also demonstrated that viral infection, particularly in the deep ocean, is responsible for the regeneration of an enormous amount of dissolved organic carbon. "Using flow cytometry, a laser technique used in biology that allows detection and counting of cells and viruses in oceanic water samples, we have shown that viral infection is responsible for the annual release of 140 gigatonnes of carbon, thus contributing to the global oceanic carbon cycle” continues the first author of the research Elena Lara, a Spanish researcher currently in force at Ismar-Cnr in Venice and associated with Icm-Csic in Barcelona. “By breaking the cells of living microbes, therefore, fresh organic carbon is produced, made of more digestible and potentially more usable biomolecules along the trophic web compared to the vast amount of recalcitrant, less usable dissolved organic carbon.” One of the open questions of the study is about the role of this dissolved carbon that is regenerated by the viral infection. “At present, it is not possible to determine whether, once it is respired by the plankton and converted into carbon dioxide, it may have consequences on the ability of the oceans to absorb this gas” Luna concludes. “However, our findings will certainly help us to improve the estimates of the global carbon cycle at the planetary scale, useful also for a better understanding of the future effects of climate change”.
Caption: A CTD-rosette system equipped with a CTD probe used to measure the main physical-chemical properties of the water column (among which temperature and salinity) and with a set of Niskin bottles used to collect seawater samples from the ocean’s depths. (Credits: Gian Marco Luna).
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