Elevated Colonization of Microborers at a Volcanically Acidified Coral Reef

Abstract

Experiments have demonstrated that ocean acidification (OA) conditions projected to occur by the end of the century will slow the calcification of numerous coral species and accelerate the biological erosion of reef habitats (bioerosion). Microborers, which bore holes less than 100 μm diameter, are one of the most pervasive agents of bioerosion and are present throughout all calcium carbonate substrates within the reef environment. The response of diverse reef functional groups to OA is known from real-world ecosystems, but to date our understanding of the relationship between ocean pH and carbonate dissolution by microborers is limited to controlled laboratory experiments. Here we examine the settlement of microborers to pure mineral calcium carbonate substrates (calcite) along a natural pH gradient at a volcanically acidified reef at Maug, Commonwealth of the Northern Mariana Islands (CNMI). Colonization of pioneer microborers was higher in the lower pH waters near the vent field. Depth of microborer penetration was highly variable both among and within sites (4.2–195.5 μm) over the short duration of the study (3 mo.) and no clear relationship to increasing CO2 was observed. Calculated rates of biogenic dissolution, however, were highest at the two sites closer to the vent and were not significantly different from each other. These data represent the first evidence of OA-enhancement of microboring flora colonization in newly available substrates and provide further evidence that microborers, especially bioeroding chlorophytes, respond positively to low pH. The accelerated breakdown and dissolution of reef framework structures with OA will likely lead to declines in structural complexity and integrity, as well as possible loss of essential habitat.

Limited Carbonate Dissolution by Boring Microflora at Two Volcanically Acidified Temperate Sites: Ischia (Italy, Mediterranean Sea) and Faial (Azores, NE Atlantic Ocean)

“…Measured depths of penetration in all substrates at the CO 2 venting areas in Ischia and Faial were particularly low (<30 μm after 6 months at Faial) and always lower than those found at ambient pH. Those depths, due mainly to Phaeophila sp., were similar to those reported by several authors for various carbonate substrates in warm temperate (Färber et al, 2015) and shallow tropical settings (Carreiro-Silva et al, 2009Enochs et al, 2016;Kiene et al, 1995;Mao Che et al, 1996). Phaeophila sp.…”

“…While several studies showed that an elevated pCO 2 with a pH of 7.9 increases rates of biogenic dissolution by at least 50% (Enochs et al, 2016;Reyes-Nivia et al, 2013;Tribollet et al, 2009), our data show the opposite trend. Rates of biogenic dissolution were indeed reduced by a factor 4 to 7 and 3 to 6 depending on sites and substrate types at the acidified sites at Ischia and Faial, respectively, compared to ambient pH sites.…”

“…The growth stimulation of microboring communities dominated by Ostreobium increased rates of biogenic dissolution by 50%. This enhanced biogenic dissolution by elevated pCO 2 was recently confirmed by two studies, one conducted under different controlled conditions (Reyes-Nivia et al, 2013) and another one under naturally acidified tropical waters (Enochs et al, 2016). It was thus suggested that biogenic dissolution of carbonate substrates will increase significantly by the end of the century, potentially offsetting carbonate precipitation, together with bioeroding sponges and bacteria activity in reef ecosystems (Andersson et al, 2015;Reyes-Nivia et al, 2013;Schoenberg et al, 2017;Tribollet et al, 2009Tribollet et al, , 2014Wisshak et al, 2013).…”

“…Chazottes et al (1995) reported that about 20% of coral skeleton surfaces are colonized by pioneer microborers after 2 months and that no more than 30% of surface areas are bioeroded by mature communities after 6-12 months of exposure in coral reef ecosystems under ambient pH (see also Tribollet, 2008a). Similarly, Enochs et al (2016) found a percent of colonization of 20% in carbonates exposed at ambient pH in Maug reef (Mariana Islands) during 3 months. In naturally acidified seawater at Maug (pH ≥ 7.9), the percent of colonization was still mainly due to Phaeophila sp.…”

“…Similar communities were also observed in dead shells of Callista sp. in shallow waters of the Faial channel (Azores) after 3 months of exposure (Wisshak et al, 2011) and in coral skeletons and shells in Global Biogeochemical Cycles 10.1002/2016GB005575 different coral reefs after 2-3 months (Carreiro-Silva et al, 2005;Chazottes et al, 1995;Enochs et al, 2016;Grange et al, 2015;Kiene et al, 1995). In contrast at Ischia, communities of microborers were expected to be mature as substrates were exposed during 6 months to colonization, but instead, they were immature (dominance of the chlorophyte Phaeophila sp.…”