Temporal And Spatial Distribution And Molecular Diversity Of Sediment Viruses In The Western Pacific Ocean

Viruses are primitive non-cellular organism that replicate only in the living cells of organisms and can infect all forms of cellular life in the biosphere.Marine viruses are ubiquitous and highly abundant organisms,most of which infect prokaryotes as hosts.Viruses are an important part of the marine micro-food web,and have a significant impact on the earth's carbon and nitrogen cycles.By lysing their host,viruses convert microbial biomass into dissolved organic matter(DOM),making it inaccessible to higher trophic levels.The viral-induced organic matter flows have been termed the‘virus shunt'.In addition,viruses have profound effects on the regulation of prokaryotic community structure,diversity and metabolism,and can act as a medium for the genes horizontal transfer.So far,research on marine sediment viruses is far less than that on planktonic viruses,and there are still many deficiencies.Currently,the most widely used procedure for counting sediment viruses and prokaryotes is epifluorescence microscopy(EFM),where the enumeration efficiency of some steps remained to be evaluated,which would have a direct impact on the accurate quantification of viruses and virus-to-prokaryote ratio(VPR).VPR is a basic index used to describe the virus-host relationship and is important in understanding the role of viruses in the environment.The prevailing studies showed that VPR was approximately 10 in sediments,fluctuating over 6 orders of magnitude within and outside different benthic habitats,usually decreasing with water depth.However,previous studies have shown that centrifugation commonly used in EFM can lead to a significant overestimation of VPR,while it is still commonly used in subsequent studies.Most of the previous studies examined a small number of samples in a studied area,and there is a lack of study on larger spatial and temporal scale.Due to the high heterogeneity of marine sediments and the un-unified investigation methods of some virus parameters,the spatial and temporal variability of sediment viruses have not been well characterized.In addition,research on the diversity of marine sediment viruses is also inadequate.There are many difficulties in studying the virus diversity of marine sediments especially in larger scales,since viruses are difficult to cultivate and have extremely small genomes lacking universal primers,as well as special physical and chemical conditions in benthic habitats.According to these,the present study optimized the main steps the most widely used procedure for counting sediment viruses by fluorescence microscopy(EFM).Using optimized EFM procedure,the present study investigated the temporal and spatial variations of sediment viral abundance(VA),prokaryotic abundance(PA),VPR and virus production(VP)from intertidal zone through continental-shelves to deep-sea plains.In addition,the virus phoH gene was used to explore the diversity and seasonal variation of intertidal sediment viruses.The results of quantitative evaluation showed that the commonly used centrifugation treatment can lead to an overestimation of VPR by 3—4 folds,which affects the cognition of VPR and virus-host relationship in sediments significantly.Based on the collected 135 VPR dataset from 37 publications and the spatio-temporal studied dataset,it was found that the sediment VPR is approximately 2 fluctuating mostly within 10 across both temporal and spatial scales,which was far less than the previous cognition.In addition,sediment VPR is one order of magnitude lower than that in the pelagic habitats and fluctuated to a much less extent,indicating that the virushost relationship and the ecological function of viruses in the two ecosystems may be very different.The sediment VA and PA was highly dynamic throughout the year and was closely related to each other in the intertidal zone of Qingdao Bay,with a temporal pattern significantly related to the surface temperature of the sediment.An insignificant seasonal pattern in the VPR was observed in the intertidal zone,with lower VPR values occurring at high temperatures,indicating the close numerical dependence of viruses on their hosts and the stability of the intertidal virus-host relationship.The sediment Phaeophytin a(Pha)content was most relevant to the spatial pattern of VA,PA and VPR on mesoscale,suggesting that the trophic conditions of the upper water column and the sedimentation of organic matter to the bottom are the key factors affecting the spatial distribution of viral and prokaryotic.The study of viral production showed that the sediment viruses were highly active in the intertidal zone of Qingdao Bay,and the influence of benthic viruses on their host was higher in sandy sediments than that in muddy-sand sediments.Sediment VP,virusinduced prokaryotes mortality(VIPM),and the release of organic carbon by virus lytic infection all showed a downward trend from the intertidal zone to the deep-sea plain,indicating that the impact of the viruses on the prokaryotic community decreases with the decrease of active cells proportion in prokaryotic communities as the sediment nutritional conditions becoming barren.In the intertidal zone of Qingdao Bay,the diversity of sediment viral phoH gene was extremely high,and there are significant differences between sites and seasons.Although the VA of sandy site S was much lower than the muddy-sand site M-S,the richness and evenness of viral phoH gene of the former was significantly higher than that of the latter,and the community structure of the former was more stable between seasons.Some of the of the dominant OTU of virus phoH gene showed restrictive distribution in different samples,while more were widespread in the intertidal sediments.Amino acid phylogenetic analysis showed that the diversity of viral phoH genes in sediments is much greater than that of water bodies,and four new viral phoH gene groups have been discovered.