Layout 1 INTRODUCTION Marine viruses are a relevant source of mortality for a wide range of organisms, and by killing their hosts they in- fluence the functioning of the marine food webs and nutri- ent cycling in the oceans of the world (Wommack and Colwell, 2000; Weinbauer, 2004; Suttle, 2007; Danovaro et al., 2016). Viral lysis, indeed, diverts cell biomass away from higher trophic levels, enhancing the supply of Dis- solved (DOM) and Particulate Organic Matter (POM) pools (i.e., a concept also defined as viral shunt; Wommack and Colwell, 2000; Figure 1), thus modulating food webs and the functioning of the marine ecosystems (Suttle, 2007). Due to their pervasive role, viruses have been proposed as integral components of global models of Carbon (C) cy- cling and nutrient regeneration (Suttle, 2007; Danovaro et al., 2008b; Weitz et al., 2015), and of current and future cli- mate models (Danovaro et al., 2011). Deep-sea sediments (>200 m depth) represent over ca. 65% of the Earth’s surface and have key roles in biomass production and biogeochemical cycles (Tyler, 2003; Danovaro et al., 2014; Corinaldesi, 2015). In these systems, nearly all the prokaryotic C production is transformed into organic detritus by viral lysis, thus representing an addi- tional important trophic resource for benthic metabolism (Danovaro et al., 2008b). The key role of viruses in benthic trophodynamics and biogeochemical cycles is not only modulated by the extent to which they infect and lyse their hosts (with the conse- quent release of the cell contents) but also by the extent to which they decay if they find no suitable hosts to infect or lose infectivity (Corinaldesi et al., 2010; Dell’Anno et al., 2015; Figure 1). Viral decay is indeed the process concerning the decom- position of organic material of viral origin (e.g., largely pro- teins and nucleic acids), which being very labile and promptly usable, is recycled and re-channel in the food web by benthic prokaryotes (Dell’Anno et al., 2015). Being deep-sea benthic systems characterized by very low inputs and availability of trophic resources (Corinaldesi et al., 2010), it has been reported that viral decay can represent a relevant process in oligotrophic environments such as the deep-sea floor (Dell’Anno et al., 2015). The term ‘viral decay’ can be ambiguous since it might indicate either the loss of infectivity (due to damage of nu- cleic acids or viral receptors on the capsid) or the complete degradation of viral particles (Danovaro et al., 2008a). So far the most reliable estimates of viral decay have been ob- tained by using a sediment dilution-based technique fol- lowed by viral nucleic-acid staining and epifluorescence microscopy (Dell’Anno et al., 2009; Dell’Anno et al., 2015). However, this approach does not allow us to dis- criminate between infective and non-infective viruses (without contextually calculating the virus-killed cells with additional procedures) because both remain equally visible under the microscope. Therefore, the term “viral decay” is generally used to indicate degraded or no longer detectable viruses under a microscope, whose decrease rate can be de- termined during time-course experiments (Corinaldesi et al., 2010). This note will present a brief overview of the available information on viral decay in deep-sea sediments, showing that while long-standing questions about the role of viruses in these ecosystems have been answered in recent decades, many more questions will arise as we get closer to under- standing how, paradoxically, these biological entities help sustain life in our oceans. Quantitative relevance of viral decay rates in deep-sea sediments Pioneer studies in aquatic ecosystems have supposed that viral production is balanced by viral decay based on the assumption that viral dynamics are in a steady-state condition (Thingstad, 2000; Wommack and Colwell, 2000). However, further investigations in different benthic systems documented that viruses, decay at lower rates than they are produced (Glud and Middelboe, 2004; Corinaldesi et al., 2007), possibly because viral production and viral decay are controlled by different factors (Parada et al., 2007). Fur- ther investigations on deep-sea sediments on a global scale SHORT NOTE Deep-sea benthic ecosystems waste nothing and recycle everything, even viruses Cinzia Corinaldesi Department of Materials, Environmental Sciences and Urban Planning, Polytechnic University of Marche, Ancona, Italy ABSTRACT Viruses are the most abundant biological entities of the global ocean and have a pervasive role in marine ecosystems because, being a major cause of mortality, they module the functioning of food webs, and biogeochemical cycling. This role is due not only to their ability to infect and lyse marine organisms but also to the decomposition of their particles (viral decay). The organic matter of viral origin, indeed, can be recycled by benthic organisms thus representing an additional important food source for their me- tabolism, especially in deep-sea sediments, characterized by very low availability of trophic resources. This short note will present an overview of the available information on viral decay in deep-sea benthic ecosystems. Non -co mmerc ial us e o nly C. Corinaldesi68 (spanning the Arctic Ocean, northeast Atlantic Ocean, and the Mediterranean Sea) and different habitats from the shelves to the abyssal planes (i.e., continental slopes and margins, deep-sea anoxic basins, and seamounts) revealed that viral decay estimates range from ca. 7 to 163 x 1011 viruses m-2 d-1 (Corinaldesi et al., 2007; Danovaro et al., 2009; Corinaldesi et al., 2010; Corinaldesi et al., 2012; Corinaldesi et al. 2014; Dell’Anno et al., 2015; Table 1). Such viral decay estimates account, on average, for ca. 30% of the gross viral production (i.e., total viral production after lysis of infected cells; Corinaldesi et al., 2010), thus viral decay is less than half of the net viral production (i.e., viral production net of viral decay). This can explain the high in- fection rates, especially under 1000 m depth, where viruses are responsible for the abatement of a very large fraction of the prokaryotic heterotrophic production (Danovaro et al., 2008b). Factors influencing viral decay rates While viral production depends on the abundance, metabolic activity, and burst size (i.e., number of viruses released per infected cell) of prokaryotic hosts (Glud and Middelboe, 2004; Parada et al., 2007), viral decay is in- fluenced by a complex interaction of physical, chemical and biological variables (Danovaro et al., 2008a). In par- ticular, in aquatic systems, viral decay has been related to solar radiation, temperature, pH, organic matter, salts, Figure 1. Infection and lysis of prokaryotic cells (in green) by viruses (in red) and viral decay in deep-sea benthic ecosystems. a) Viral decay: viruses decompose releasing genetic material and other components (e.g., proteins), b) viral lysis and infection: prokaryotic cells are infected and lysed, with the replication of new viruses, and c) viral shunt: release of organic matter and nucleic acids by lysed cells. Image courtesy of Michael Tangherlini. Non -co mmerc ial us e o nly Deep-sea benthic ecosystems waste nothing and recycle everything, even viruses 69 heavy metals, protozoan grazing, and enzymes (e.g., Danovaro et al., 2008a and references inside). Experimen- tal investigations indicated a key role of the prokaryotic extracellular enzymes, such as DNases and proteases, in viral decomposition (Corinaldesi et al., 2010) as the viral decay rates increased in the surface sediments after en- zyme addition, even if this was not the case of the sub- surface sediments. Further studies, in different oceanic regions, revealed that viral decay rates are controlled pri- marily by the extracellular enzymatic activities that hy- drolyze the proteins of the viral capsids, confirming the key role of proteolytic activities in the viral decomposition (Dell’Anno et al., 2015). Conversely, numerous studies have documented that the sedimentary matrix (especially clay minerals) can offer protection to viral particles by de- laying the loss of infectivity of viruses and reducing their exposure to substances that cause their decay (e.g., Gerba, 1984; Sakoda et al., 1997, Danovaro et al., 2008a and ref- erences inside). In this regard, some investigations pro- posed that viral decay rates in sub-superficial deep-sea sediments are lower than in the surface layers (Cai et al. 2019) and that they could persist in undisturbed sediments for hundreds of thousands of years (Middelboe et al. 2011). However, the implications of virus preservation in the subsurface have not yet been elucidated. Contribution of viral decay to biogeochemical processes Available estimations indicate that the decomposition of deep-sea benthic viruses releases�37–50 megatons of C per year, representing an important source of labile organic compounds. In particular, organic material deriving from decomposed deep-sea viruses accounts for 3±1%, 6±2%, and 12±3% to C, N, and P of the material photosynthetically produced, which is supplied to the bathyal/abyssal sedi- ments through its sinking (Dell’Anno et al. 2015). Existing literature also indicates that benthic viral decay is signifi- cantly related to prokaryotic heterotrophic C production, which in turn is related to C released from viral decompo- sition. This suggests a tight interaction between virus de- composition and prokaryotic metabolism (Corinaldesi et al., 2010) as demonstrated by experimental assays, which con- firmed that the addition of purified and inactivated virus concentrates significantly increases the growth rates of prokaryotes (Dell’Anno et al. 2015). Since organic compounds released by the decomposi- tion of virus particles (i.e. proteins and nucleic acids) in deep-sea surface sediments have much faster rates of degra- dation than most organic matter sinking to the ocean floor (Middelburg and Meysman, 2007), the use of labile C from decomposed viruses by benthic prokaryotes can contribute to rapid N and P cycling (Dell’Anno et al. 2015), thus sup- porting the functioning and biogeochemical processes of the largest biome of the biosphere. Virus decomposition together with virus-induced cell lysis can also stimulate ammonia-dependent archaeal chemoautotrophic production, thus contributing to the major primary production processes occurring in deep-sea sediments (Danovaro et al. 2017). CONCLUSIONS There is no doubt that viruses play a key role in the life and death of all marine organisms, in controlling their bio- diversity and biogeochemical cycles of the global ocean. Deep-sea viral ecology also shows how even the smallest particles, enemies of marine organisms, are recycled with- out waste. In the last decade, viral ecology has been driven by metagenomic analyses, which have expanded the cata- log of viral genomes, most of which are still of unknown origin, and with functions that remain putative. This short note aims not only to provide some insights into the impor- tance of viral decay in deep-sea sediments but also to un- derline the need to continue to explore their role in the global ocean to better elucidate how they influence ecosys- tem functioning, especially in light of current and future scenarios of global change. Table 1. Estimates of Viral Decay rates (VD) and their contribution to Viral Production (VP) in surface sediments of different deep-sea benthic habitats (>200 m depth). Basin Habitat Depth VD VD/VP* Bibliographic sources m 1011 m–2 d–1 % Eastern Mediterranean Deep-sea anoxic basins 3575 42.2±27.5§ 32 Corinaldesi et al. 2007 Western Mediterranean Seamounts 3430-3581 36.7-75.5 35 Danovaro et al. 2009 Central and Western Mediterranean Continental slopes and margins 300-990 11.5-63.4 37 Corinaldesi et al. 2010 Western Mediterranean Continental slopes 217-222 7.1-26.3 29 Corinaldesi et al. 2012 Eastern Mediterranean Deep-sea anoxic basins 2952-3352 134.0-163.0 16 Corinaldesi et al. 2014 Eastern Mediterranean Continental slope 2793 34.9±27.3§ 29 Corinaldesi et al. 2014 Arctic, Mediterranean, Atlantic From continental shelves to abyssal plains 200-5600 8.0-128.0 25 Dell’Anno et al. 2015 *VP is the gross viral production; §Standard deviation. Non -co mmerc ial us e o nly C. Corinaldesi70 Corresponding author: Cinzia Corinaldesi, Department of Materials, Environmental Sciences and Urban Planning, Polytechnic University of Marche, via Brecce Bianche 12, Ancona, Italy. E-mail: c.corinaldesi@univpm.it Conflict of interest: The author declares no potential conflict of in- terest. Funding: None. Availability of data and materials: All data generated or analyzed during this study are included in this published article. Key words: Marine viruses, benthic deep-sea ecosystems, viral decay, viral infection. Acknowledgments: This paper is a contribution to the projects PRIN GLIDE 2017 and PRIN Viride 2017. Received: 30 November 2022. Accepted: 16 December 2022. Publisher’s note: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. 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