Recent studies have suggested that diatom viruses are an important factor affecting diatom population dynamics, which in turn are important in considering marine primary productivity. The marine planktonic diatom Chaetoceros tenuissimus Meunier is a cosmopolitan species and often causes blooms off the western coast of Japan. To date, two viruses, C. tenuissimus DNA virus (CtenDNAV) type I and CtenRNAV type I, have been identified that potentially affect C. tenuissimus population dynamics in the natural environment. In this study, we successfully isolated and characterized two additional novel viruses (CtenDNAV type II and CtenRNAV type II). This paper reports the basic characteristics of these new viruses isolated from surface water or sediment from the Hiroshima Bay, Japan. The physiological and morphological characteristics of the two new viruses were similar to those of the previously isolated viruses. However, the amino acid sequences of the structural proteins of CtenDNAV type II and Cten-RNAV type II were clearly distinct from those of both type I viruses, with identity scores of 38.3% and 27.6%, respectively. Our results suggest that at least four genetically distinct viruses sharing the same diatom host are present in western Japan and affect the population dynamics of C. tenuissimus. Moreover, the result that CtenRNAV type II lysed multiple diatom species indicates that RNA viruses may affect various diatom populations in the natural environment.
Following the first reports of large numbers of virus-like particles (VLP) in seawater, aquatic viruses have been intensively studied (1, 2). Despite their small size, viruses represent a large proportion of the biomass of the oceans (3) and are regarded as an important component of aquatic ecosystems; they are known to regulate carbon cycling, microbial biomass, and the genetic diversity of their host organisms (3,4,5,6). Microalgae sustain a large portion of aquatic primary production, and viruses that infect them are considered to significantly affect their population dynamics (5). To improve our understanding of aquatic ecology, microalgal viruses and their hosts should be investigated in greater detail.Marine diatoms generate as much organic carbon via photosynthesis as the globe's rainforests each year (7,8,9). Recently, several reports have suggested that diatom population dynamics are potentially affected by viruses as well as by diverse physical, chemical, and biological factors (10, 11). The cosmopolitan marine planktonic diatom Chaetoceros tenuissimus Meunier often causes blooms off the coast of western Japan. To date, two distinct viruses that infect it have been isolated and characterized: C. tenuissimus DNA virus (defined as CtenDNAV type I in this study) (12) and C. tenuissimus RNA virus (defined as CtenRNAV type I) (13). The two viruses are morphologically similar; both are icodahedral and 31 to 37 m in diameter. However, their genome structures and infectious features differ. The burst sizes (calculated on the basis of the increase in the infec...