Viruses are the most common biological entities in the marine environment. There has not been a global survey of these viruses, and consequently, it is not known what types of viruses are in Earth's oceans or how they are distributed. Metagenomic analyses of 184 viral assemblages collected over a decade and representing 68 sites in four major oceanic regions showed that most of the viral sequences were not similar to those in the current databases. There was a distinct “marine-ness” quality to the viral assemblages. Global diversity was very high, presumably several hundred thousand of species, and regional richness varied on a North-South latitudinal gradient. The marine regions had different assemblages of viruses. Cyanophages and a newly discovered clade of single-stranded DNA phages dominated the Sargasso Sea sample, whereas prophage-like sequences were most common in the Arctic. However most viral species were found to be widespread. With a majority of shared species between oceanic regions, most of the differences between viral assemblages seemed to be explained by variation in the occurrence of the most common viral species and not by exclusion of different viral genomes. These results support the idea that viruses are widely dispersed and that local environmental conditions enrich for certain viral types through selective pressure.

A variant of the cauliflower mosaic virus 35S promoter with transcriptional activity approximately tenfold higher than that of the natural promoter was constructed by tandem duplication of 250 base pairs of upstream sequences. The duplicated region also acted as a strong enhancer of heterologous promoters, increasing the activity of an adjacent and divergently transcribed transferred DNA gene several hundredfold, and to a lesser extent, that of another transferred DNA gene from a remote downstream position. This optimized enhancer element should be very useful for obtaining high levels of expression of foreign genes in transgenic plants.

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Eight different phycoerythrin-and phycocyanin-containing strains of Synechococcus spp. and 1 strain of Anacystis marina were screened against 29 natural virus communities taken from 3 locations in south Texas (USA) coastal waters, at different times of the year. In add~tion, 1 sample was screened from Peconic Bay, New York, USA. Cyanophages were detected in all samples, but the frequency with which they were detected and their abundance depended upon the strain of Synechococcus sp. that was screened. Viruses that infected red Synechococcus spp. strains (DC2, SYN 48) were particularly common and in some instances were in excess of 105 ml-l The abundances of cyanophages were weakly correlated with temperature (r2= 0.53 to 0.70), although they occurred at all of the temperatures (12 to 30.4 "C) and salinities (18 to 70 ppt) that were screened. The 7 cyanophages that were cloned belonged to the same 3 families of viruses that have been observed to infect freshwater cyanobacteria, namely the Siphoviridae (formerly Styloviridae), Myoviridae and Podoviridae. The cyanophage clones varied in host-specificity. For example, 1 clone infected a single Synechococcus sp. strain of 12 that were tested, whereas, another infected 4 of 9 strains tested. Growth characteristics of 1 of the virus clones was determined for a single host (BCCI). Photosynthesis in BCCI was not affected until near the onset of cell lysis and the virus burst cycle was complete ca 17 h post-infection. The burst size was approximately 250 infective particles. The high abundance of cyanophages in the natural environment provides further evidence that viruses are probably important regulators of phytoplankton dynamics in marine systems.

Cyanophages infecting marine Synechococcus cells were frequently very abundant and were found in every seawater sample along a transect in the western Gulf of Mexico and during a 28-month period in Aransas Pass, Tex. In Aransas Pass their abundance varied seasonally, with the lowest concentrations coincident with cooler water and lower salinity. Along the transect, viruses infecting Synechococcus strains DC2 and SYN48 ranged in concentration from a few hundred per milliliter at 97 m deep and 83 km offshore to ca. 4 x 105 ml-' near the surface at stations within 18 km of the coast. The highest concentrations occurred at the surface, where salinity decreased from ca. 35.5 to 34 ppt and Synechococcus concentrations were greatest. Viruses infecting strains SNC1, SNC2, and 838BG were distributed in a similar manner but were much less abundant (<10 to >5 x 103 ml-'). When Synechococcus concentrations exceeded ca. 103 mnl-, cyanophage concentrations increased markedly (ca. 102 to >105 ml-'), suggesting that a minimum host density was required for efficient viral propagation. Data on the decay rate of viral infectivity d (per day), as a function of solar irradiance I (millimoles of quanta per square meter per second), were used to develop a relationship (d = 0.26101-0.00718; r2 = 0.69) for conservatively estimating the destruction of infectious viruses in the mixed layer of two offshore stations. Assuming that virus production balances losses and that the burst size is 250, ca. 5 to 7% of Synechococcus cells would be infected daily by viruses. Calculations based on contact rates between Synechococcus cells and infectious viruses produce similar results (5 to 14%). Moreover, balancing estimates of viral production with contact rates for the farthest offshore station required that most Synechococcus cells be susceptible to infection, that most contacts result in infection, and that the burst size be about 324 viruses per lytic event. In contrast, in nearshore waters, where ca. 80%o of Synechococcus cells would be contacted daily by infectious cyanophages, only ca. 1% of the contacts would have to result in infection to balance the estimated virus removal rates. These results indicate that cyanophages are an abundant and dynamic component of marine planktonic communities and are probably responsible for lysing a small but significant portion of the Synechococcus population on a daily basis.