Background: Marine snow (small amorphous aggregates with colloidal properties) is present in all oceans of the world.
Surface water warming and the consequent increase of water column stability can favour the coalescence of marine snow
into marine mucilage, large marine aggregates representing an ephemeral and extreme habitat. Marine mucilage
characterize aquatic systems with altered environmental conditions.
Methodology/Principal Findings: We investigated, by means of molecular techniques, viruses and prokaryotes within the
mucilage and in surrounding seawater to examine the potential of mucilage to host new microbial diversity and/or spread
marine diseases. We found that marine mucilage contained a large and unexpectedly exclusive microbial biodiversity and
hosted pathogenic species that were absent in surrounding seawater. We also investigated the relationship between
climate change and the frequency of mucilage in the Mediterranean Sea over the last 200 years and found that the number
of mucilage outbreaks increased almost exponentially in the last 20 years. The increasing frequency of mucilage outbreaks is
closely associated with the temperature anomalies.
Conclusions/Significance: We conclude that the spreading of mucilage in the Mediterranean Sea is linked to climate-driven
sea surface warming. The mucilage can act as a controlling factor of microbial diversity across wide oceanic regions and
could have the potential to act as a carrier of specific microorganisms, thereby increasing the spread of pathogenic bacteria.
