Editor's Choice

November 2010 (98:6)


Understanding interactions is of prima facie importance in ecology, and several papers in the current issue of Journal of Ecology represent excellent examples of studies addressing this issue. Of these, the paper ‘Effect of explosive shallow hydrothermal vents on δ13C and growth performance in the seagrass Posidonia oceanica’ by Salvatrice Vizzini and colleagues immediately caught my eye, not just because the use of the adjective ‘explosive’ in the title, but because of the habitat—hydrothermal vents.

The harsh environment associated with oceanic hydrothermal vents captures the public’s attention and fascinates scientists, but not normally plant ecologists. Deep-sea hydrothermal vents are associated with an unusual and diverse suite of organisms, and have even been suggested as a possible site for the origin of life on earth. The shallow-water hydrothermal vents forming the focus of Vizzini et al.’s study may be less spectacular, but are perhaps more likely to affect marine ecosystems over a wider area. Associated with tectonically active regions these vents can emit a cocktail of gases including hydrogen sulphide, hydrogen and methane, as well as large amounts of carbon dioxide. Organisms able to tolerate these conditions include chemolithoautotropic bacteria, cyanobacteria, microalgae, macroalgae and seagrasses. The CO2 emissions may provide a photosynthetic substrate for nearby algae and seagrasses, albeit under harsh conditions. Some hydrothermal vents are not hot or contaminated with hydrogen sulphide and other toxic compounds, and so are useful for in situ studies of the effects of elevated CO2 on photosynthetic organisms.
Salvatrice Vizzini and her team from the Università di Palermo in Sicily, Italy, focused on the response of an endemic seagrass, Posidonia oceanica, to a specific hydrothermal event that occurred during November 2002 following a low-magnitude seismic event in the southern Tyrrhenian Sea, Italy. An approximately 280-m2, 10-m deep undersea crater was formed at a depth of 10-15 m, accompanied by an out-gassing of CO2 that raised local sea temperatures to 50 °C for several months. The annual production of vertically growing rhizome segments of the seagrass sampled from three nearby sites were back-dated and measured, which allowed the authors to reconstruct the history of growth over a 14-year period that encompassed the year of vent activity. Measurement of carbon stable isotope ratios (δ13C) from rhizome and rhizome-scale samples allowed determination of photosynthetic carbon sources to be made.

They found low growth rates and decreased tissue δ13C of seagrass during the three years following the hydrothermal event. This result was interpreted to reflect a response to physiological stress caused by light limitation due to associated turbidity and high water temperatures. Low δ13C signatures in the rhizomes and scales reflected increased plant isotopic discrimination during the time of stress-induced low production. The data did not support the possibility of assimilation of an isotopically unusual (δ13C-depleted) source, or a response to an increase in carbon availability during or following the hydrothermal event.

This study demonstrates that Posidonia oceanica is sensitive to hydrothermal vent activity and that changes in this activity can be recorded in the δ13C signature of scales and rhizomes and vertical rhizome growth of the plant. The authors suggest that these results indicate that P. oceanica, and perhaps other seagrasses, could be used to conduct relevant ecological studies on physiological stresses and the role of hydrothermal carbon in shallow water areas affected by hydrothermal vents and provide a record of geological events. This is the first study using isotopic δ13C signatures in seagrasses to document the effect of hydrothermal vents on marine ecosystems. Previous studies include the response of other components of these shallow-water ecosystems such as seagrass epibonts, coralline algae, sea urchins and gastropods. Vizzini et al.’s study is an important part of this story, and is of broad interest due to the implications for the effects of harsh environmental conditions on marine organisms and the carbon cycle.

David Gibson
Editor, Journal of Ecology

References

  • Vizzini,S. , Tomasello, A., Di Maida, G., Pirrotta, M., Mazzola, A., & Calvo. S. (2010) Effect of explosive shallow hydrothermal vents on δ13C and growth performance in the seagrass Posidonia oceanica. Journal of Ecology, 98, 1284-1291.