Background
The authors write that "CO2 gradients in natural settings, where whole ecosystems have been exposed to elevated levels of CO2, allow us to investigate changes in the interactions, competition, predation and/or herbivory that involve long-lived metazoan species in benthic marine ecosystems." And they note, in this regard, that "volcanic CO2 gradients are beginning to reveal the ecological shifts that can be expected to occur with globally increasing atmospheric CO2 in both temperate (Hall-Spencer et al., 2008) and tropical ecosystems (Fabricius et al., 2011)."

What was done
Johnson et al. assessed the abundance of herbivores (sea urchins) and the response of brown macroalgae (Padina spp.) to increasing levels of CO2 in two natural settings. One of the sites of their research was a set of shallow, volcanic CO2 seeps on the island of Vulcano, NE Sicily, where P. pavonica was studied; while the other site, where P. australis was studied, took place at comparable seeps in the D'Entrecasteaux Island group, Papua New Guinea.

What was learned
"Along both temperate and tropical rocky shores," in the words of the five scientists, "there was a reduction in sea urchin abundances alongside a proliferation of Padina spp., as CO2 levels increased." In the case of sea urchins, in fact, they discovered that the predators were actually absent in locations having the highest CO2 levels (lowest pH); while in the case of the Padina spp., they found that "even in the lowest pH conditions, P. pavonica and P. australis were still able to calcify, seemingly from the enhancement of photosynthesis under high levels of CO2."

What it means
Johnson et al. opine that the absence of sea urchins in the CO2-enriched areas "may be one explanation for the proliferation of Padina spp., as it becomes released from the top-down control by these keystone grazers," noting that "this effect of sea urchin removal has been observed in other Padina sp. populations (Sammarco et al., 1974) and across other Phaeophyte assemblages (Leinaas and Christie, 1996; Ling et al.,2010)," while in regard to the increase they observed in the photosynthetic capacity of the Padina species under conditions of higher CO2, they note that "increased photosynthetic activity at high CO2 has also been observed in other calcified macroalgae (Reiskind et al., 1988; Semesi et al., 2009)," as well as in "non-calcified macroalgae (Kubler et al., 1999; Connell and Russell, 2010; Russell et al., 2011)."

References
Connell, S.D. and Russell, B.D. 2010. The direct effects of increasing CO2 and temperature on non-calcifying organisms: increasing the potential for phase shifts in kelp forests. Proceedings of the Royal Society of London B 277: 1409-1415.

Fabricius, K.E., Langdon, C., Uthicke, S., Humphrey, C., Noonan, S., De'ath, G., Okazaki, R., Muehllehner, N., Glas, M.S. and Lough, J.M. 2011. Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. Nature Climate Change 1: 165-169.

Hall-Spencer, J.M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S.M., Rowley, S.J., Tedesco, D. and Buia, M.-C. 2008. Volcanic carbon dioxide vents show ecosystem effects of ocean acidification. Nature 454: 96-99.

Kubler, J.E., Johnston, A.M. and Raven, J.A. 1999. The effects of reduced and elevated CO2 and O2 on the seaweed Lomentaria articulata. Plant, Cell and the Environment 22: 1303-1310.

Leinaas, H.P. and Christie, H. 1996. Effects of removing sea urchins (Strongylocentrotus droebachiensis): stability of the barren state and succession of kelp forest recovery in the east Atlantic. Oecologia 105: 524-536.

Ling, S.D., Ibbott, S. and Sanderson, J.C. 2010. Recovery of canopy-forming macroalgae following removal of the enigmatic grazing sea urchin Heliocidaris erythrogramma. Journal of Experimental Marine Biology and Ecology 395: 135-146.

Reiskind, J.B., Seamon, P.T. and Bowes, G. 1988. Alternative methods of photosynthetic carbon assimilation in marine macroalgae. Plant Physiology 87: 686-692.

Russell, B.D., Passarelli, C.A. and Connell, S.D. 2011. Forecasted CO2 modifies the influence of light in shaping subtidal habitat. Journal of Phycology 47: 744-752.

Sammarco, P.W., Levington, J.S. and Ogden, J.C. 1974. Grazing and control of coral reef community structure by Diadema antillarum Philippi (Echinodermata: Echinoidea): a preliminary study. Journal of Marine Research 32: 47-53.

Semesi, I.S., Kangwe, J. and Bjork, M. 2009. Alterations in seawater pH and CO2 affect calcification and photosynthesis in the tropical coralline alga, Hydrolithon sp. (Rhodophyta). Estuarine and Coastal Shelf Science 84: 337-341.