Rhodolith beds are major CaCO3 bio-factories in the tropical South West Atlantic

Rhodoliths are nodules of non-geniculate coralline algae that occur in shallow waters (<150 m depth) subjected to episodic disturbance. Rhodolith beds stand with kelp beds, seagrass meadows, and coralline algal reefs as one of the world’s four largest macrophyte-dominated benthic communities. Geographic distribution of rhodolith beds is discontinuous, with large concentrations off Japan, Australia and the Gulf of California, as well as in the Mediterranean, North Atlantic, eastern Caribbean and Brazil. Although there are major gaps in terms of seabed habitat mapping, the largest rhodolith beds are purported to occur off Brazil, where these communities are recorded across a wide latitudinal range (2°N – 27°S). To quantify their extent, we carried out an inter-reefal seabed habitat survey on the Abrolhos Shelf (16°50′ – 19°45′S) off eastern Brazil, and confirmed the most expansive and contiguous rhodolith bed in the world, covering about 20,900 km². Distribution, extent, composition and structure of this bed were assessed with side scan sonar, remotely operated vehicles, and SCUBA. The mean rate of CaCO₃ production was estimated from in situ growth assays at 1.07 kg m⁻² yr⁻¹, with a total production rate of 0.025 Gt yr⁻¹, comparable to those of the world’s largest biogenic CaCO₃ deposits. These gigantic rhodolith beds, of areal extent equivalent to the Great Barrier Reef, Australia, are a critical, yet poorly understood component of the tropical South Atlantic Ocean. Based on the relatively high vulnerability of coralline algae to ocean acidification, these beds are likely to experience a profound restructuring in the coming decades.

Effects of diurnally oscillating pCO2 on the calcification and survival of coral recruits

Manipulative studies have demonstrated that ocean acidification (OA) is a threat to coral reefs, yet no experiments have employed diurnal variations in pCO₂ that are ecologically relevant to many shallow reefs. Two experiments were conducted to test the response of coral recruits (less than 6 days old) to diurnally oscillating pCO₂; one exposing recruits for 3 days to ambient (440 µatm), high (663 µatm) and diurnally oscillating pCO₂ on a natural phase (420–596 µatm), and another exposing recruits for 6 days to ambient (456 µatm), high (837 µatm) and diurnally oscillating pCO₂ on either a natural or a reverse phase (448–845 µatm). In experiment I, recruits exposed to natural-phased diurnally oscillating pCO₂ grew 6–19% larger than those in ambient or high pCO₂. In experiment II, recruits in both high and natural-phased diurnally oscillating pCO₂ grew 16 per cent larger than those at ambient pCO₂, and this was accompanied by 13–18% higher survivorship; the stimulatory effect on growth of oscillatory pCO₂ was diminished by administering high pCO₂ during the day (i.e. reverse-phased). These results demonstrate that coral recruits can benefit from ecologically relevant fluctuations in pCO₂ and we hypothesize that the mechanism underlying this response is highly pCO₂-mediated, night-time storage of dissolved inorganic carbon that fuels daytime calcification.