The viral shunt is a mechanism that prevents marine microbial particulate organic matter (POM) from migrating up trophic levels by recycling them into dissolved organic matter (DOM), which can be readily taken up by microorganisms. The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM. Viruses can easily infect microorganisms in the microbial loop due to their relative abundance compared to microbes. Prokaryotic and eukaryotic mortality contribute to carbon nutrient recycling through cell lysis. There is evidence as well of nitrogen (specifically ammonium) regeneration. This nutrient recycling helps stimulate microbial growth. As much as 25% of the primary production from phytoplankton in the global oceans may be recycled within the microbial loop through the viral shunt.
Discovery and impact
Viral shunt was first described in 1999 by Steven W. Wilhelm and Curtis A. Suttle. Their original paper has since been cited over 1000 times. For his contributions to understanding of viral roles in marine ecosystems, Suttle has received numerous awards, including being named a Fellow of the Royal Society of Canada, receiving the A.G. Huntsman Award for Excellence in Marine Science, and the Timothy R. Parsons Medal for Excellence in Ocean Sciences from the Department of Fisheries and Oceans. Both Suttle and Wilhelm have been elected Fellows of the American Academy of Microbiology as well as the Association for the Sciences of Limnology and Oceanography (ASLO). The field of marine virology has rapidly expanded since the mid-1990s, coinciding with the first publication of viral shunt. During this time, further studies have established the existence of the viral shunt as a "fact" of the field. The recycling of nutrients in the viral shunt has indicated to scientists that viruses are a necessary component in new models of global change. Virologists in soil sciences have begun to investigate the application of viral shunt to explain nutrient recycling in terrestrial systems. More recently, new theories concerning the potential role of viruses in carbon export - grouped under the idea of the "viral shuttle" have emerged. While perhaps at odds on the surface, these theories are not mutually exclusive.
Bacterial growth efficiency There is evidence to suggest that the viral shunt system can directly control bacterial growth efficiency (BGE) in pelagic regions. Carbon flow models indicated that decreased BGE could be largely explained by the viral shunt, which caused the conversion of bacterial biomass to DOM. The biodiversity in these pelagic environments are so tightly coupled that the production of viruses depends on their bacterial host metabolisms, so any factors that limit bacterial growth also limit viral growth. Enrichment of nitrogen has been observed to allow for an increase in viral production (up to 3-fold) but not bacterial biomass. Through the viral shunt a high viral-induced mortality relative to bacterial growth resulted in the effective generation of DOC/DOM that is available for microbial re-consumption and offers an effective way to recycle key nutrients within the microbial food web. Data extracted from other aquatic regions such as the Western-North Pacific displayed large variability, which may be a result of methodologies and environmental conditions. Nonetheless, a common trend appeared to be a reduced BGE with an increasing viral shunt pathway. From carbon flow models it is clear that viral shunting allows bacterial biomass to be converted to DOC/DOM that are ultimately recycled such that the bacteria may consume the DOC/DOM, indicating that the viral shunt pathway is a major regulator of BGE in marine pelagic waters.
Links to microbial processes
Microbial loop The microbial loop acts as a pathway and connection between different relationships in an ecosystem. The microbial loop connects the pool of DOM to the rest of the food web, specifically various microorganisms in the water column. This allows for constant cycling of this dissolved organic matter. Stratification of the water column due to the pycnocline affects the amount of dissolved carbon in the upper mixing layer, and the mechanisms shows seasonal variation. The microbial loop is based on micro-interactions between different trophic levels of microbes and organisms. When nutrients enter the microbial loop, they tend to remain in the photic zone longer, due to the location, and slow sinking rates of microbes. Eventually, through varying processes, DOM, through use of available nutrients, is produced by phytoplankton, as well as consumed by bacteria. Bacteria utilize this DOM, yet are then preyed on by larger microbes, such as microflagellates, which helps to regenerate nutrients. Viruses tend to outnumber bacterial abundance by about ten times and phytoplankton by about a hundred times in the upper mixing layer and bacterial abundance tends to decrease with depth, while phytoplankton remain closer to shallow depths. The effects of the viral shunt are more pronounced in the upper mixing layer. Viruses found can be non-specific (broad host range), abundant, and can infect all forms of microbes. Viruses are a magnitude higher in abundance in almost all aquatic locations compared to their microbial hosts, allowing high rates/levels of microbial infection. The impact of the viral shunt varies seasonally as many microbes show seasonal abundance maximums in temperate and oligotrophic waters during different times of the year, meaning viral abundance also varies with season.
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