Rising carbonate dissolution due to bioeroding microflora under climate change - an overlooked buffer process?
Résumé
Since the industrial era, the atmospheric partial pressure of CO2 (pCO2) has been rising. Consequently, the world’s ocean is getting warmer and acidified. By the end of the century, IPCC models in the worst case scenario predict an increase of sea surface temperature of 4°C and a decrease of seawater pH estimated at 0.3-0.4 pH-units. As a consequence, the saturation state of surface seawater (Ω) with respect to calcium carbonate minerals (CaCO3) will also decrease. All these climatic factors are expected to affect calcification and dissolution processes, putting for instance in jeopardy coral reef ecosystems which are entirely made of carbonates. Among those processes, biogenic dissolution of carbonates due to bioeroding microflora (or euendoliths), which comprise cyanobacteria, algae and fungi, has been the most overlooked process and is currently not taken into account in biogeochemical models. So far, rates of biogenic dissolution were estimated by quantifying the volume of calcium carbonate removed by bioeroding filaments using microscopy observations. Although those rates are significant (up to 1.1 kg CaCO3 dissolved per m2 per year in coral reefs), the question is how much alkalinity bioeroding microflora are able to release in the ocean, and how they are influenced by climate change (pH and temperature). In addition, all experiments recently carried out which highlighted the positive effects of ocean warming and acidification on biogenic dissolution, were realized under controlled conditions (mesocosms) in tropical regions over short periods of time (2-3 months). The long term dynamics of the process of biogenic dissolution under natural conditions remains poorly known. Here we present results of five experiments carried out in tropical (Hawaii, New Caledonia reefs) and temperate regions (Ischia in Italy) at different time scales (a few hours up to 4 years), to show that (1) the amount of alkalinity produced by bioeroding microflora is significant (as high as 71 mequiv m-2 d-1 which converts to a CaCO3 dissolution rate of 1.3 kg m-2 y-1 under constant light conditions in tropical regions), (2) biogenic dissolution can occur under various saturation states (0.8 < Ω ≤ 5 both in temperate and tropical regions) and increases under rising pCO2 (by 50% to 250% depending on conditions) as long as the saturation state is above 1 (otherwise carbonate dissolution due to chemical conditions limits euendolith development and thus, biogenic dissolution), and (3) biogenic dissolution is more efficient (x2.7) when new carbonate substrates become available for colonization by microboring communities in the summer season (higher temperature, light intensities, etc…) than in the winter season in tropical regions as colonization by the main agents of biogenic dissolution is faster in summer than in winter. These results suggest that at least in coral reef systems, global warming and ocean acidification will most probably stimulate the process of carbonate bioegenic dissolution due to microboring flora, accelerating the transition from a net coral reef accretion towards net coral reef dissolution. We estimate that today, at ambient temperature and pH in coral reef ecosystems, at most 20% of produced carbonates are dissolved by bioeroding microflora. By 2100, these organisms may be responsible for the dissolution of up to 70% of reef carbonates that are expected to be produced. If all dissolution processes are taken into account, i.e. chemical dissolution driven by water chemistry and bacteria metabolic activity, biogenic dissolution by bioeroding flora and biogenic dissolution by macroborers (such as boring sponges), reef carbonate budget may become negative much earlier than 2100. The capacity of carbonate biogenic dissolution due to bioeroding flora in buffering seawater remains however, unknown and needs to be investigated in order to better understand carbon biogeochemical cycles and to improve predictions of the fate of carbonate coastal systems.