Effects of sunlight on bacteriophage viability and structure

Area of Science:

• Environmental microbiology
• Aquatic virology
• Microbial ecology

Background:

• Current viral abundance estimates in natural waters use direct counts of virus-like particles (VLPs).
• Direct VLP counts do not distinguish between infectious and non-infectious viral particles.
• Bacteriophage viability can decrease rapidly in environmental conditions, but particle degradation is less understood.

Purpose of the Study:

• To investigate whether bacteriophage particles degrade concurrently with a loss of infectivity.
• To compare direct and viable counts of bacteriophages under varying environmental conditions.
• To determine the impact of sunlight and water depth on bacteriophage abundance and viability.

Main Methods:

• Experimental microcosms were used to incubate two Chesapeake Bay bacteriophages (CB 38 phi and CB 7 phi).
• Microcosms were incubated for 56 hours at two depths in the York River estuary, under sunlight and in darkness.
• Both direct counts of VLPs and viable (infective) counts were performed.

Main Results:

• In surface microcosms exposed to sunlight, bacteriophage decay rates were 0.11 h-1 (CB 38 phi) and 0.06 h-1 (CB 7 phi).
• In dark microcosms and at 1m depth, decay rates averaged 0.052 h-1 (CB 38 phi) and 0.037 h-1 (CB 7 phi), similar to controls.
• Direct VLP counts decreased slowly (0.028 h-1) and were independent of treatment, indicating particle integrity does not directly correlate with infectivity.

Conclusions:

• The destruction of bacteriophage particles is a process distinct from the loss of infectivity.
• Strong sunlight significantly impacts bacteriophage viability in surface waters.
• Direct VLP counts overestimate infectious bacteriophages in sunlit surface waters but may be accurate in deeper, shaded waters.