You might think that seawater is too salty for viruses and that they would never survive in such a hostile environment. Alternatively you might think that if viruses can survive on surfaces like plastic, metal and cardboard then why not also in the sea. But do we know?
Well back in 1990 Scientists confirmed that viruses can indeed be found in seawater; while looking at seawater with transmission electron microscopes (TEM) they saw “something unusual” but they didn’t actually know at that time what it was… however they could see them! So they started to study them. Remember, TEMs can “see” much smaller objects than normal light microscopes; the limit of magnification of a light microscope is about 1,000x, not enough to see a virus, whereas TEMs can magnify 10,000,000x! OK I know you youngsters think 1990 seems a long time ago but in scientific terms it’s not that long, it’s only since these discoveries in the 1990s that they actually confirmed viruses were there, even though “infection” from contaminated seawater was suspected.
Further studies have shown that in fact there are lots and lots and lots of viruses in seawater….
Using TEMs Scientists estimate that there are 10,000,000 viruses per ml of seawater, but this varies depending on where in the sea you sample. Higher numbers of viruses are found in marine sediment close to the shore and lower levels further from shore or in deep water layers.
Based on estimates of seawater volume this means there are about 4x1030 viruses in the World’s seas, making viruses the most abundant entity in this environment! Apparently someone has worked out that if these viruses were lined up end-to-end they would reach 10 million light years… and that’s a very long way!!!
What are these viruses doing in the sea?
The viruses present in the sea are not just floating around having a holiday, they are actively infecting things. Argh, this is just unnecessary “news” for the present state of affairs!! Remember, viruses cannot reproduce without hijacking the reproductive mechanism of a host’s cell. There would be no selective advantage to the viruses if all they were doing was floating around, so there must be something for them to infect.
For the most part these viruses are actually infecting bacteria, fungi and algae but they can also infect higher life forms like crustaceans and even whales. Some viruses are specific to what they can infect whereas others are more promiscuous, some examples include:
- Phycodnaviridae and Marnaviridae infect algae
- Mimiviridae (which have a massive genome of over 1 million bases compare that to SARS Cov2 which has 30,000 bases) infect protists (a loose group of eukaryotes that are neither animal, plant nor fungus… actually science is unsure exactly what they are!)
- Novel RNA viruses with no name yet that infect fungi (maybe there needs to be a competition to name that one?)
- White Spot Syndrome Virus (love that name!) which infects shrimps, of all things, (so maybe don’t eat spotty shrimps!)
There are two methods of viral reproduction; one being the lytic life cycle, the other being the lysogenic cycle. Some of the viruses e.g. myoviruses have lytic life cycles whereby they infect bacteria, reproduce and then lyse the bacterial cell to release more virus. Other viruses e.g. siphoviruses incorporate their genes into the bacterium and then as the bacterium reproduces it passes the viral genes onto its daughter cells. Whatever mechanism of reproduction the viruses use, they are very effective and very infective.
Do different viruses occur in different places?
This is still not completely understood but Scientists have found nearly identical viral genetic sequences all around the World, from areas as widespread as the Gulf of Mexico, the Southern Ocean and meltwater from the Arctic. This suggests that indistinguishable viruses are everywhere within the marine environment and therefore all over the World.
Shouldn’t we be killing them off?!? Where’s the bleach!!!???
Whoa hold on, remember the benefits of “our” normal flora, well these viruses might have a purpose too, if not for us directly, for the environment as a whole… human interference (put down the bleach!) can unbalance our delicate World! Leave these particular viruses alone…
How do viruses in the sea effect the environment?
It appears that viruses in the sea are very good at helping nutrients get recycled within the marine environment. The viruses break down living organisms and release nutrients in dissolved forms back into seawater where they can be used by other microorganisms. Hooray!
However, this recycling has the additional effect of releasing carbon in a dissolved form into surface water where in can release into the atmosphere; usually large particles containing carbon sink into deeper water forming a carbon sink. This would suggest that if seawater virus levels increased more carbon would be released to the atmosphere which in turn would affect global warming… so maybe there is an ideal balance of viruses in the sea to prevent this happening.
What don’t we know about viruses in the sea?
Well, it appears we pretty much don’t know any more about the viruses found in the sea. Scientists have “identified” a small number of viruses but while discovering these they have seen other fragments of viral RNA and DNA showing that there are so many more they haven’t yet discovered. Work on the known viruses has found many viral genes but what these genes do is also unknown. It’s all a big mystery!
What does this mean for clinical microbiology?
Okay, so I’m a Clinical Microbiologist and whilst this is fascinating what does it mean to humans?
The basic answer is I have absolutely no idea… I just loved the topic!
What! Investigate then!!! Can these marine viruses infect humans?
Well at present this is unknown. In order to find out, studies would have to be done to look for evidence of infection by these known marine viruses in humans, probably starting with those who work in marine environments such as in the fishing industry, marine biologists, professional divers etc. Of course we’d need some good tests to do this, especially antibody tests (testing for immunity or evidence of past infection), and we all now know how hard these are to create!
The first indication of infection from marine origin (caliciviruses) can be traced to 1932 in a large herd of pigs in the USA who were being fed food scraps collected from restaurants, the disease was known as Vesicular Exanthema of Swine (VES) but by 1956 the USA stated “they” had eradicated it!! In fact it’s now known to be difficult or impossible to contain and eradicate as pathogenic caliciviruses continue to emerge from the sea in unexpected forms, at unexpected times, in unexpected places! The potential exposure to marine caliciviruses from the sea is substantial, e.g. a 35-ton Gray whale, can eliminate a quantity of faeces containing an estimated 1013 caliciviruses daily and those marine caliciviruses can remain viable for more than 14 days in 15°C seawater.
What is clear from veterinary science is that marine viruses have crossed the intertidal zone to infect terrestrial species such as aquatic carnivorous mammals (seals, whales, walrus etc.) as well as rabbits, pigs, foxes, donkeys, sheep, cats, dogs and sea birds. Scientists have also managed to transfer infection between marine and land species in the laboratory setting, including isolates originally from seals that replicated readily in primate and human cell lines. Other studies suggest neutralizing antibodies to specific marine viruses have been present in human patients as well as in researchers working with marine caliciviruses!
The conclusion being drawn is that fish, and perhaps other ocean products, provide a vehicle for transmission of these marine caliciviruses to terrestrial animals, including us! In reality this is an area we just don’t know enough about yet.
Maybe, with all of the interest currently being shown in animal viruses as a result of SARS Cov2 there will be more funding and future opportunities to study this neglected viral ecosystem. In time we might have a better idea of what all of this means for us humans. If you’re wondering if there is SARS Cov2 in seawater the science currently says no, although fragments of RNA (not live virus) have been found in sewerage nothing has been found in the sea; but a sewerage outlet is never going to be a good place to swim! NB drinking water is safe as it is treated.
In the meantime this might be another reason my swimming trunks are unlikely to get much use this year… maybe next year?
- Viruses in the sea. Curtis Suttle, Nature 2005, Vol. 437, 15 Sept; 356-361
- Marine Caliciviruses and Diseases of Pisces, Pigs, Pinnipeds and People. Alvin W. Smith Douglas E. Skilling, Steven E. Poet, International Association for Aquatic Animal Medicine archive - 1989
- Calicivirus Emergence from Ocean Reservoirs: Zoonotic and Interspecies Movements. Alvin W. Smith, Douglas E. Skilling, Neil Cherry, Jay H. Mead, David O. Matson, EID Journal 1998, Volume 4, Number 1, March