Viriobenthos in freshwater and marine sediments: a review

1. Viruses are the most abundant biological entities on the planet, and sediments provide a highly suitable environment for them. This review presents the first comparative synthesis of information on the fresh water and marine viriobenthos and explores differences and similarities to the better known virioplankton. We present methods for studying life cycles of the viriobenthos, data on viral distribution and diversity, interactions with host microbes, and information on the role of viruses in benthic food webs and biogeochemical cycles. 2. Most approaches developed for the virioplankton are also applicable to viriobenthos, although methods for analysing benthic viruses may differ in important details. 3. Benthic viruses are very abundant in both marine and freshwater sediments, where 10(7)-10(10) can occur in 1 g of dry sediment. Although information on viral production (VP) and decay rates in freshwater sediments is very limited, the data suggest that VP and decay could also be high. These data highlight the potential ecological importance of benthic viruses, suggesting that they could play a key role in prokaryotic mortality and in biogeochemical cycles. 4. There is clear indirect evidence for the importance of viriobenthos in marine and freshwater ecosystems. However, large numbers of visibly infected cells have not been observed, suggesting limited effects on prokaryote population and community dynamics. The apparent paradox between high viral abundance and low impact is currently unresolved, while several aspects of viral life cycles in sediments (e.g. chronic infection) are almost completely unknown. 5. Studies on viriobenthic diversity and community structure are at a pioneering stage. First results from a few studies using pulsed-field gel electrophoresis and especially from metagenomic analyses indicate, however, that viriobenthic assemblages are both highly diverse and distinct from the virioplankton. 6. Estimates of global viral abundance in the top 1 m of fresh water and marine sediments are 0.5 and 28.7 x 10(28) viruses respectively. Similar rough estimates of production are 0.6 and 34.4 x 10(28) viruses day(-1), suggesting an average turnover time of 20 h.