Host-dependent differences in abundance, composition and host range of cyanophages from the Red Sea.

Cyanobacteria coexist in the oceans with a wealth of phages that infect them. While numerous studies have investigated Synechococcus phages, much less data are available for Prochlorococcus phages. Furthermore, little is known about cyanophage composition. Here, we examined the abundance and relative composition of cyanophages on six cyanobacterial hosts in samples collected during spring and summer from the Red Sea. Maximal abundances found on Synechococcus of 35 000 phages/ml are within ranges found previously, whereas the 24 000 phages/ml found on Prochlorococcus are approximately 10-fold higher than previous findings. T7-like, T4-like and 'unknown' phages were isolated on all hosts, including many T4-like phages on high-light adapted Prochlorococcus strains, whereas TIM5-like phages were found only on Synechococcus. Large differences in cyanophage abundance and composition were found for different hosts on the same sampling date, as well as for the same host on different dates, with few predictable patterns discerned. Host range analyses showed that T7-like and TIM5-like phages were quite host-specific, whereas the breadth of hosts for T4-like phages was related to host type: those isolated on high-light adapted Prochlorococcus were considerably more host-specific than those on low-light adapted Prochlorococcus or Synechococcus. These host-related differences likely contribute to the complexity of host-phage interactions in the oceans.