Shen Shang, Shimizu Yoshihisa
Research Center for Environmental Quality Management, Kyoto University, Kyoto, Japan.
Front Microbiol. 2021 May 11;12:624980. doi: 10.3389/fmicb.2021.624980. eCollection 2021.
As viruses regulate prokaryotic abundance and the carbon cycle by infecting and lysing their prokaryotic hosts, the volume of infected prokaryotes is an important parameter for understanding the impact of viruses on aquatic environments. However, literature regarding the seasonal and spatial variations in the cell volume of infected prokaryotes is limited, despite the volume of the prokaryotic community varying dynamically with season and water column depth. Here, we conducted a field survey for two annual cycles in a large and deep freshwater lake (Lake Biwa, Japan), where large prokaryotes inhabit the deeper layer during the stratified period. We used transmission electron microscopy to reveal the seasonal and spatial variation in the frequency of viral infection and cell volume of infected prokaryotes. We found that the viral infection rate in the surface layer increased when estimated contact rates increased during the middle of the stratified period, whereas the infection rate in the deeper layer increased despite low estimated contact rates during the end of the stratified period. In addition, in the deeper layer, the fraction of large prokaryotes in the total and infected prokaryotic communities increased progressively while the number of intracellular viral particles increased. We suggest different ways in which the viral abundance is maintained in the two water layers. In the surface layer, it is speculated that viral abundance is supported by the high viral infection rate because of the high activity of prokaryotes, whereas in the deeper layer, it might be supported by the larger number of intracellular viral particles released from large prokaryotes. Moreover, large prokaryotes could contribute as important sources of organic substrates viral lysis in the deeper layer, where labile dissolved organic matter is depleted.
由于病毒通过感染和裂解原核宿主来调节原核生物的丰度和碳循环，感染的原核生物的体积是理解病毒对水生环境影响的一个重要参数。然而，尽管原核生物群落的体积随季节和水柱深度动态变化，但关于感染的原核生物细胞体积的季节和空间变化的文献却很有限。在这里，我们在一个大型深水淡水湖（日本琵琶湖）进行了为期两个年度周期的实地调查，在分层期，大型原核生物栖息在较深的水层。我们使用透射电子显微镜来揭示病毒感染频率和感染的原核生物细胞体积的季节和空间变化。我们发现，在分层期中期，当估计的接触率增加时，表层的病毒感染率会增加，而在分层期末期，尽管估计的接触率较低，但深层的感染率却增加了。此外，在深层，大型原核生物在总原核生物群落和感染的原核生物群落中的比例逐渐增加，而细胞内病毒颗粒的数量也增加了。我们提出了在两个水层中维持病毒丰度的不同方式。在表层，推测病毒丰度是由原核生物的高活性导致的高病毒感染率所支持的，而在深层，它可能是由大型原核生物释放的大量细胞内病毒颗粒所支持的。此外，在深层，不稳定的溶解有机物被耗尽，大型原核生物可能作为有机底物的重要来源，促进病毒裂解。
