Do viruses protect phytoplankton from UV stress?

As recent evidence suggests, ultraviolet radiation (and most significantly ultraviolet radiation B or UVB) is on the increase in north and south temperate latitudes. The detrimental effects of this have been emphasised in many previous studies regarding mutagenesis, primary production rates, loss of pigmentation and many others. All of this is in spite of defensive strategies employed by many systems (avoidance, repair etc.).

Marine viruses are the most abundant biological particles in the sea, and they are known for playing a significant role in many essential marine cycles. Ultraviolet radiation (UVR) is considered responsible for the loss of infectivity and destruction in bacteriophages and cyanophages, though some viruses have been shown to possess a repair gene for such eventualities. The effects of UVR on the interactions between viruses and phytoplankton are currently not investigated due to the lack of available cultured samples.

In this investigation, 5 important marine phytoplankton-virus systems were cultured onto F2 medium and kept in water at a constant temperature (10-15^o C depending on their place of origin). These were regularly mixed and exposed to 14 hours of light to 10 hours of dark, with UVR lamps (UVA and UVB) on for 4 hours of each light period. These conditions were to mimic normal surface water conditions, with normal recorded levels of UV radiation.

3 experiments were conducted. The first contained only phytoplankton and was exposed to photosynthetic active radiation (PAR) (normal light) only, PAR+UVA and PAR+UVA+UVB. The second consisted of the same tests, but with the addition of viruses to fresh cultures of the same strain. These phytoplankton were previously infected by the virus, allowed to recover to ensure immunity against it and then co-cultured with the same virus. The third involved isolating the viruses from the second experiment and adding them to more fresh culture to test their ability to re-infect phytoplankton after exposure to UVR.

The results showed that there was no difference at all in the cultures when exposed to PAR or UVA. However, there was varied change in response to the addition of UVB – in experiment 1 some species showed no change while others showed intermediate responses and some even died. In experiment 2 some of the cultures seemed to be less sensitive to UVB than they were in experiment 1. In experiment 3, viruses exposed to UVB tended to take longer to infect the phytoplankton cells, suggesting that infectivity was decreased.

It seems that in this experiment, the viruses indirectly protected the phytoplankton from UVB damage. It is possible that during infection, the viruses actually transferred protective genes to their host. Though this theory is yet to be proven, it could demonstrate an ecological advantage for previously infected phytoplankton, as they are not only resistant to the virus, but also seemingly less sensitive to UVB. There are however further implications for these results – during periods of high levels of UVB, infectivity of viruses could decrease which could lead to long lasting algal blooms, having a knock-on effect on the wider marine ecosystem.

Reference: Jacquet, S. and Bratbak, G. (2003) Effects of ultraviolet radiation on marine virus-phytoplankton interactions, FEMS Microbiology Ecology, 44, 279-289