Unease on the High Seas: The Effect of Climate Change on Marine Microbial Ecology

Since life began, animals and humans alike have been dependant on the ocean for survival. The ocean provides us with food, produces around half of the world’s oxygen¹, and provides a home for thousands of different species of animals and microorganisms. Another, perhaps less well-known, way in which the ocean benefits life is its buffering effect against changes in the Earth’s climate.

The Ocean as a Climate Regulator

As things stand, global temperatures have slowly climbed over the last century as the climate reels from the effect of the fossil fuels burned during the industrial revolution. Without the ocean’s ability to effectively absorb heat², the world would be in turmoil. Recent statistics put global temperature at around 1 ^oC above the average temperature recorded for the 20^th century³. Without the ocean’s buffering capacity, however, it is estimated that the global temperature would have risen by roughly 36 ^oC⁴ compared to the mid-1900s. The depth of the ocean allows absorbed heat to dissipate effectively, but the overall ocean temperature is still rising, putting marine life at risk. As well as the temperature increases, the pH of the ocean is also changing in a process called ocean acidification⁵. As CO₂ levels in the air rise, the amount of CO₂ absorbed by the ocean also rises and forces a decrease in oceanic pH by disrupting the ocean’s natural carbonate chemistry cycle.

The combination of warming and acidification has already been shown to interfere with marine life, with studies proving negative effects on the growth of a huge portion of marine life, from sea urchins⁶ to corals⁷, and threatens to destabilise entire marine ecosystems⁸.

Marine Bacteria and Climate Change

Marine ecosystems are home to billions of microbes in every litre of seawater⁹, and they play an important role in keeping marine life healthy. Microbes are a major component in the cycling of nutrients, such as nitrogen and phosphorus, and pollutants, like carbon dioxide, between the air, sea, and land¹⁰. In fact, it’s thought that marine microbes are responsible for producing around half of all global primary production¹¹, where primary production is defined as the amount of chemical energy as biomass that a primary producer can create. Consequently, any impact of climate change on marine microbes will ripple through the nutrient cycle and create a knock-on effect through the food chain. Establishing ways to forecast the impact of climate change on the marine food web, and mitigate some of its worse effects, has therefore become a hot area of research.

Predicting the Impacts of Climate Change

While there are a whole host of studies that give good and accurate insight into the general effects of climate change, the ability to predict future impacts of these changes on marine life and the marine food web is limited. Early studies aimed to improve this predictive power, but were often over-simplified, only considering global warming without taking into account the effects of ocean acidification¹², or vice-versa¹³.

A new study¹⁴ from Professor Ivan Nagelkerken’s research group at the University of Adelaide uses a longer-term, larger-scale study to overcome these previous over-simplifications. Professor Nagelkerken stresses the social importance of his group’s study, stating, “This study is an important step forward as empirical data is used to build a food-web model that can better predict the impacts of climate change on various trophic levels. Such studies are still scarce and are much needed to inform policy makers and managers about how ocean productivity and fisheries resources might change under a future climate if we do not reduce our greenhouse gas emissions.”

The study uses a series of mesocosms containing marine life across three broad trophic levels, namely, primary producers (commonly microorganisms, such as cyanobacteria and certain types of phytoplankton), herbivores, and carnivorous predators, in order to produce a more accurate model of the effects of climate change on marine life. These effects were quantified by analysing the energy flow between the trophic levels under four sets of differing conditions. These conditions were: elevated CO₂ levels; elevated temperature; elevated CO₂ levels and elevated temperature; and a control group which were subject to the CO₂ and temperature ranges equivalent to those in the present day.

When ocean acidification was simulated, various herbivores and carnivores demonstrated an increased energy flow compared to the control group. On the surface, this finding appears surprising given the detrimental effects of climate change forecasted by other studies. In reality this acidification will be accompanied by rising ocean temperatures, so this must be factored in to give an accurate prediction.

In all scenarios that included warming, it was found that there was an increase in primary producer biomass resulting from an increase in the biomass of primary producer microorganisms, in particular, an expansion in the amount of cyanobacteria present. This would explain a second observed effect under warming: a decrease in energy flow between the primary producers and herbivores, and between herbivores and carnivores.  Cyanobacteria produce chemicals that can be harmful to herbivores¹⁵, rendering the algae inedible to them, and causing a constriction on the energy flow in the food web.

Inland Waters and the Threat of Cyanobacteria

Cyanobacteria bloom in lakes and rivers just as well as they do in deep ocean habitats, and these will also be worsened by climate change. Human and animal exposure to these harmful marine microbes¹⁶ isn’t just possible, it’s likely, and as a result possible ways of controlling these blooms must be found.

One of the methods currently showing promise is the treatment of the toxic cyanobacteria blooms with hydrogen peroxide. Dr Ben Wagstaff and his colleagues at the John Innes Centre developed the method¹⁷ following a major bloom of the cyanobacterium in the nearby Broads National Park. “When cyanobacteria bloom in waterways that are used for fishing or other recreational activities, such as boating or water sports, these areas often have to be closed to the public until the blooms naturally decline; a process that can take weeks. We have found that low doses of hydrogen peroxide can be used to treat harmful algal blooms in a process that takes 1-2 days.” Dr Wagstaff explains, “Importantly, at the concentrations needed to kill algae such as cyanobacteria, other macroinvertebrates and aquatic life such as fish are unaffected.”

The group has reported successful field tests in the Broads National Park and expects that fisheries and water sports centres could benefit greatly from adopting the treatment, but stresses that it may not be practical when scaled up to treat larger bodies of water. Still, with research like this into the treatment of localised cyanobacteria blooms affording results, the hope is that future methods may be able to selectively protect the more vulnerable oceanic habitats, such as coral reefs¹⁸, in the event that the worst climate change forecasts are realised and they are put at risk.

The Outlook

Now, more than ever, there is a focus on our law makers to create and fund projects that will mitigate the effects of climate change, but this cannot be done without the right scientific tools for analysis, prediction and prevention. Scientific studies like those above are incredibly important to rationalise the possible impact that climate change will have on our world, and also to forecast the consequences of any action, or lack thereof, which we take. It is clear that climate change stands to cause great disruption to marine microbes, and hence the entire food web, if no action is taken and conditions worsen as predicted. Local treatments are promising as a way to manage initial effects on human populations, but the widespread changes in ocean habitats can only be halted by equally widespread changes in the societal response to climate change.

References

1 J. Roach, Natl. Geogr. News, https://news.nationalgeographic.com/news/2004/06/0607_040607_phytoplankton.html, accessed, March 2018.
2 S. Levitus, J. I. Antonov, T. P. Boyer, O. K. Baranova, H. E. Garcia, R. A. Locarnini, A. V. Mishonov, J. R. Reagan, D. Seidov, E. S. Yarosh and M. M. Zweng, Geophys. Res. Lett., 2012, 39, n/a-n/a.
3 NOAA National Centers for Environmental Information, State of the Climate: Global Climate Report for March 2017, 2017.
4 Grantham Institute, Ocean heat uptake and the global surface temperature record, 2015.
5 cencoos.org, https://www.cencoos.org/learn/oa/intro, accessed, March 2018.
6 M. Byrne, M. Lamare, D. Winter, S. A. Dworjanyn and S. Uthicke, Philos. Trans. R. Soc. B Biol. Sci., 2013, 368, 20120439–20120439.
7 S. C. Doney, V. J. Fabry, R. A. Feely and J. A. Kleypas, Ann. Rev. Mar. Sci., 2009, 1, 169–192.
8 I. Nagelkerken and S. D. Connell, Proc. Natl. Acad. Sci. U. S. A., 2015, 112, 13272–7.
9 M. A. Moran, Science, 2015, 350.
10 K. R. Arrigo, Nature, 2005, 437, 349–355.
11 D. L. Kirchman, Processes in microbial ecology, Oxford University Press, 2012.
12 M. I. O’Connor, M. F. Piehler, D. M. Leech, A. Anton and J. F. Bruno, PLoS Biol., 2009, 7, e1000178.
13 S. Das and N. Mangwani, Oceanologia, 2015, 57, 349–361.
14 H. Ullah, I. Nagelkerken, S. U. Goldenberg and D. A. Fordham, PLOS Biol., 2018, 16, e2003446.
15 J. M. O’Neil, T. W. Davis, M. A. Burford and C. J. Gobler, Harmful Algae, 2012, 14, 313–334.
16 L. E. Brand, Amyotroph. Lateral Scler., 2009, 10, 85–95.
17 B. A. Wagstaff, E. S. Hems, M. Rejzek, J. Pratscher, E. Brooks, S. Kuhaudomlarp, E. C. O’Neill, M. I. Donaldson, S. Lane, J. Currie, A. M. Hindes, G. Malin, J. C. Murrell and R. A. Field, Biochem. Soc. Trans., 2018, BST20170393.
18 B. S. Guida and F. Garcia-Pichel, Proc. Natl. Acad. Sci. U. S. A., 2016, 113, 5712–7.