A Tale of Non-Obligate Macroalgal Calcifers, Sea Urchins and CO2
Johnson, V.R., Russell, B.D., Fabricius, K.A.E., Brownlee, C. and Hall-Spencer, J.M. 2012. Temperate and tropical brown macroalgae thrive, despite decalcification, along natural CO2 gradients. Global Change Biology 18: 2792-2803.
Against this backdrop 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. So what did they learn?
"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."
In considering their findings, 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)."
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