Climate change, Marine mucilage and Microbial Pathogens

A Review of: Roberto Danovaro, Serena Fonda Umani and Antonio Pusceddu (2009). Climate Change and the Potential Spreading of Marine Mucilage and Microbial Pathogens in the Mediterranean Sea. PLoS ONE 4(9).

Marine snow is ubiquitous in the oceans of the world. During summer, water column stratification can promote the coalescence of small sized aggregates into large massive sheets, thin layers, flocs and clouds, know collectively as marine mucilage. Mucilage is a gelatinous evolving stage of marine snow which can reach huge dimensions and cover areas of hundreds of kilometres of coastline.

There are many theories as to what triggers marine mucilage formation and this phenomenon has created increasing concern in coastal areas due to its socio-economical consequences. The Mediterranean basin is one of the most severely affected areas by marine mucilage. Once the mucilage sinks and settles on the sediment it causes yet more problems in it subsequent suffocation of benthic organisms, including bottom dwelling nekton, provoking serious economical damage.

Mucilage is able to entrap high abundances of a wide range of organisms from small phytoplankton to large zooplankton. It is also a major repository for prokaryotes and viruses. Fingerprint techniques such as ARISA provided evidence that mucilage aggregates not only entrap prokaryotes present in the water column but also contained bacterial species which were absent in the surrounding seawater - The study found that more than 90% of the bacterial taxa encountered in the mucilage were not found in the surrounding seawater. It also appears to be somewhat of a bacterial hotspot as the number of ribotypes in the mucilage was ca 65% higher than in the water.

FISH analysis revealed that mucilage contained large numbers of pathogenic bacteria such as Vibrio harveyi that has the potential to infect a wide range of organisms. Although the mechanisms by which the mucilage hosts large numbers of pathogens is not clear, the authors hypothesised that the complex organic matrix of the mucilage offers micro-niches to pathogens with favourable conditions for colonisation and survival.

Analysis of historical reports indicates that the frequency of mucilage has increased almost exponentially in the last two decades in the Mediterranean although these outbreaks do not result from increased primary production of the system.

The authors have evidence to support that with an increase in temperature there is also an increase in mucilage incidents but that temperature alone cannot be used to predict mucilage occurrences at a basin scale because several other factors may also promote mucilage formation and/ or increases the magnitude of this phenomenon including hydrodynamic regime (current speed and water mass turnover), oxygen availability and other factors.

Coastal areas repeatedly affected by mucilage outbreaks share many common environmental problems such as a long history of human exploitation, including over-fishing which can be responsible for the alteration of the benthic biogeochemical cycles, and the presence of macro- and micro-pollutants, which can have a strong influence on microbial-loop functioning and cause an increase in viral infection.

This study highlights the growing importance of marine mucilage. In conclusion the authors point out that if the frequency and persistence of marine mucilage continues to increase, an increased frequency and extension of some marine diseases may result in potential consequences to human health. Their effect on other marine organisms is huge not only when it settles on the sediment but floating in or on the surface of the water column also. More research is needed to fully understand the damaging potential of marine mucilage and also to work out how we can prevent occurrences, especially now that there is evidence that climate change may be a prominent controlling factor. I think this work is very important especially now that it is affecting more and more organisms and because we just don’t know how much potential marine mucilage has to change marine ecosystems.