Due to the exponentially rising atmospheric carbon dioxide partial pressure (pCO₂) and its partial absorption by the ocean (−30%, Sabine et al., 2004), the saturation state of surface seawater (Ω) with respect to calcium carbonate minerals (CaCO₃) will decrease together with seawater pH by the end of the century (−0.2 to −0.4 pH unit depending on the IPCC scenario and seasonality; Orr et al., 2005; Bindoff et al., 2019). Such decrease will greatly impact negatively major calcifying organisms and some coastal carbonate ecosystems (Guinotte and Fabry, 2008; Fabricius et al., 2011; Agostini et al., 2018) while enhancing carbonate dissolution (Andersson et al., 2007; Andersson et al., 2008; Krumins et al., 2013; Stubler and Peterson, 2016; Schönberg et al., 2017).

Cyronak et al. (2014) showed that the average pCO₂ may have increased faster in coral reefs than in the atmosphere and the open ocean over the past 2 decades (−3.5-fold, i.e., + 6.6 ± 1.4 µatm.y⁻¹) due to additional local disturbances resulting from human activities (e.g., eutrophication), thus putting these ecosystems even more at risk under ocean acidification. Notwithstanding the fact that reef ecosystems will greatly be degraded by the end of the century due to this factor but also ocean warming and local disturbances (e.g., storm impacts and rising runoffs), the latest SROCC report (Bindoff et al., 2019) highlighted the lack of information regarding the sensitivity and adaptive capacity of coral reef organisms and ecosystems to climate change impacts inducing bias in projections.

While several studies highlighted the negative effect of ocean acidification (combined or not with other factors) on growth, abundance and calcification rates of the main reef framebuilders, i.e., corals and calcifying algae under more or less controlled conditions and over short term (e.g., Langdon and Atkinson, 2005; Kuffner et al., 2008; Pandolfi et al., 2011; Comeau et al., 2014; Johnson et al., 2014), at the global reef scale the relationship between net reef community calcification and aragonite saturation state varies greatly from one area to the next (e.g., Shamberger et al., 2011; Falter et al., 2012; Shaw et al., 2012; Page et al., 2016). Such variations may be explained by local biological adaptations of calcifiers to the natural variability of their environmental conditions (Vargas et al., 2022) and/or variability of carbonate dissolution processes in both reef sediments and hard substrates (Pandolfi et al., 2011; see summary Table 2 in Tribollet et al., 2019); reef budget depending on the equilibrium between reef calcification and dissolution. Eyre et al. (2014) showed that despite possible adaptations of calcifiers, changes in reef dissolution are more rapid than changes in reef calcification under ocean acidification, enhancing net reef dissolution (see also Andersson and Gledhill 2013).

Reef dissolution comprises the thermodynamically driven carbonate dissolution (seawater chemistry with Ω < 1), the bacteria driven dissolution (release of CO₂ through organic matter remineralization) and the biogenic carbonate dissolution driven mainly by boring microflora and sponges (Tribollet and Golubic, 2011; Schönberg et al., 2017). Among those processes, the spatial and temporal variability of biogenic dissolution rates due to microboring flora remains poorly known (Carreiro-Silva et al., 2005; Tribollet, 2008b; Grange et al., 2015; Schönberg et al., 2017), most probably because this process which occurs at the microscale is considered as negligible, or is ignored and/or too difficult to study. However, this process may explain an important part of the reef dissolution variability (Andersson and Gledhill 2013). Tribollet et al. (2019) showed indeed that biogenic dissolution due to boring microflora dissolve up to 20% of carbonates deposited by reef calcifiers under ambient conditions (dissolution estimated on average at 11 mmol CaCO₃.m⁻² of reef.d⁻¹ in both hard and sediment substrates via microscopy techniques). They also highlighted that mature microboring communities dominated by the chlorophyte Ostreobium sp. colonizing experimental blocks of dead coral skeleton can produce consequently, significant amount of seawater alkalinity both under low controlled light intensity (during photosynthesis) and at night, at ambient saturation states (at Ω_Arag 3–3.5). They finally showed that this production is enhanced by 50% under ocean acidification conditions (at Ω_Arag = 2), confirming trends obtained for biogenic dissolution rates in dead corals measured by microscopy or buoyant weight by Tribollet et al. (2009), Reyes-Nivia et al. (2013) and Enochs et al. (2016). Meanwhile, Tribollet et al. (2019) suggested a potential negative feedback of this biogenic dissolution on ocean acidification; a feedback that could benefit to reef calcifiers and ecosystems, at least at the microscale. Thus, to better understand dynamics of the biogenic dissolution process resulting from microborers’ metabolic activity and possible implications on reef resilience, it is necessary to further investigate the relationship between the production of seawater alkalinity by microborers and the saturation state of aragonite under natural conditions and at various spatial (from substrate to ecosystem scale) and temporal scales (day, season, year).

Here we studied the capacity of natural mature microboring communities dominated by Ostreobium sp. at dissolving natural coral rubble and consequently at producing seawater alkalinity under different light regimes and natural variations of aragonite saturation states. Two experiments were carried out, one in an indoor flume under controlled light and the other one outdoor under natural light, in Hawaii (Kaneohe Bay). Seawater DIC was not artificially modified as we did not test the effects of ocean acidification on the metabolism of natural microboring communities; this being reported elsewhere (see Tribollet et al., 2009; Reyes-Nivia et al., 2013; Enochs et al., 2016; Tribollet et al., 2019).

Observations of rubbles showed that epiliths colonizing substrate surfaces were dominated by green short turfs. Rare crustose coralline algae and boring polychaetes (1 or 2) were observed on some rubbles, as well as rare traces of grazing. Observations under light microscopy revealed that rubbles were colonized by mature communities of microborers dominated by the chlorophyte of the genus Ostreobium (Figure 1B; Chazottes et al., 1995; Grange et al., 2015). Once epiliths were gently brushed off, rubbles showed an intense yellowish-greenish colour indicative of the presence of a majority of phototrophic endoliths inside dead coral skeletons (Figure 1A).

Although chasmo- and crypto-endoliths including phototrophs (see definitions in Golubic et al., 1981) are present in coral pores (Yang et al., 2019; Pernice et al., 2020), most of the biomass in dead corals or coral rubble is generally attributed to microboring flora (Wanders, 1977; Vooren 1981; Fine & Loya 2002). Our light microscopy observations of a few pieces of decalcified coral skeleton confirmed the abundance of the photrotrophic eukaryote Ostreobium sp. within the natural microboring communities studied in our rubble set exposed to colonization for over 12 months (Figure 1B). The alga Ostreobium sp. is known to dominate mature microboring communities in dead coral skeletons after more than 6 months of exposure (Chazottes et al., 1995; Le Campion-Alsumard et al., 1995; le Bris et al., 1998; Tribollet et al., 2006; Tribollet, 2008a; Grange et al., 2015). The green turfs growing on rubble surfaces were also similar to those observed on experimental coral blocks exposed to colonization during at least 6 months at 3 m depth near Coconut Island (Tribollet et al., 2006; see also Hatcher and Larkum, 1983). The presence of only a few polychaetes in rubbles was not surprising as after 6 months of exposure, the experimental blocks studied by Tribollet et al. (2006) presented only a few traces of boring polychaetes at a lagoonal site nearby HIMB laboratory. Polychaetes are generally more abundant in dead coral after several years of exposure to colonization as shown by Kiene and Hutchings (1994) and Tribollet and Golubic (2005). Moreover, Hutchings et al. (1992) pointed out that polychaetes are generally less abundant in shallow lagoon reefs than in other environments.

In our experiments, we did not observe the formation of dense epilithic biofilms at the surface of our coral debris after brushing, unlike Leggat et al. (2019). These authors suggested that corals freshly killed by marine heat waves (i.e., denuded coral skeletons) are immediately and intensely colonized by epilithic biofilms dominated by Ostreobium sp. that originate from the interior of the skeleton (thus growing outward), generating strong carbonate dissolution of coral skeletons. This is unlikely as the green alga Ostreobium sp. is a cryptic sciaphile siphonale avoiding intense light intensities to prevent photoinhibition (Fine et al., 2005; Ralph et al., 2007). This alga has developed a large repertoire of specific pigments to live in extremely low light environments such as in coral skeletons (Shashar and Stambler, 1992; Koehne et al., 1999; Iha et al., 2021). It also actively dissolves carbonates by creating galleries that perfectly fit the shape of its filaments (e.g., Chazottes et al., 1995; Carreiro-Silva et al., 2005; Tribollet et al., 2009; Wisshak et al., 2011). Although Massé et al. (2018) and Massé et al. (2020) showed that Ostreobium filaments can live freely in seawater and can be epilithic under certain conditions (see also Kobluk and Risk, 1977), they also never observed the formation of dense biofilms of Ostreobium sp. on dead corals within a few days. This is especially true since more exposure to light, if not too rapid (Fine et al., 2005), stimulates phototrophic microborers’ growth inside carbonates (Schneider and Le Campion-Alsumard 1999; Fine and Loya, 2002; Tribollet and Golubic 2005; Iha et al., 2021). Here we thus present results obtained from typical mature microboring communities that developed in naturally dead corals in reef lagoonal shallow waters.

Brushed coral rubbles under a constant light in the indoor flume (350 µmol photons.m⁻².s⁻¹) had a Pg/R ratio of about 1, suggesting much less limitation of microborers’ photosynthesis than in Tribollet et al. (2019)’s study (Pg/R ratio <1 in experimental coral blocks colonized by mature microboring communities dominated by Ostreobium sp. under 200 µmol photons.m⁻².s⁻¹). Interestingly, net carbonate dissolution measured in the indoor flume was close to biogenic dissolution measured either by buoyant weight on recently killed corals colonized mostly by Ostreobium and regularly brushed (27–42 mmol.m⁻².d⁻¹; Reyes-Nivia et al., 2013), or by microscopy techniques on 1 year or three years-old experimental coral blocks grazed by fishes (8–30 mmol.m⁻² d⁻¹; Tribollet 2008a; see Table 2 in Tribollet et al., 2019). Removal of epiliths by brushing coral rubble may have indeed simulated a moderate scrapping pressure. Grazers including scrapers and excavators, remove substrate surfaces (Clements et al., 2017) to feed on endolithic phototrophs (especially Ostreobium sp.). That way they allow more light reaching microborers and consequently, stimulate their growth inside substrates until they reach their new depth of compensation (Tribollet and Golubic, 2005).

The three incubations carried out under natural daylight showed that natural coral rubbles can release alkalinity in seawater at rates as high as 2.8–3.6 mequiv.m⁻².h⁻¹ (i.e., a CaCO₃ dissolution rate of 1.4–1.8 mmol.m⁻².h⁻¹). This is the first time that CaCO₃ biogenic dissolution is reported under natural daylight conditions, simultaneously with significant rates of net photosynthesis (13–25 mmol C.m⁻².h⁻¹). Assuming a two-fold increase in CaCO₃ dissolution due to the use of HCl (consistent with results found during acidification experiments in New Caledonia as well as in literature; Tribollet et al., 2009; Tribollet et al., 2019; Reyes-Nivia et al., 2013), the alkalinity production by coral rubbles under ambient pCO₂ would still be high (1.4–1.8 mequiv.m⁻².h⁻¹). Interestingly, net CaCO₃ biogenic dissolution rate and net photosynthesis increased with time in the three outdoor incubations (up to Ω_Arag < 6.4), suggesting that constant brushing, combined with some environmental factors such as nutrient availability and temperature, certainly stimulated microborer growth (in depth and/or by branching) over the duration of the experiment. Rates of biogenic dissolution in coral skeletons are known to be related to the biomass of microborers under natural light cycles (Reyes-Nivia et al., 2013; Grange et al., 2015).

By taking into account all experiments, the higher was net photosynthetic activity the higher was net biogenic dissolution (Figure 4), which is a priori counter intuitive as rising pH due to photosynthesis is not thermodynamically in favour of carbonate dissolution. This further supports the hypothesis that alkalinity release is related to the metabolic activity of phototrophic microborers (Tribollet et al., 2019). Light availability and DIC speciation did probably however, influenced pathways of carbon acquisition and thus, CaCO₃ dissolution by microborers. Massé et al. (2020) showed that seawater DIC (most probably CO₂ according to Tribollet et al., 2009; Tribollet et al., 2019) is the main source of C involved in Ostreobium photosynthesis. However, these authors also reported several other possible sources of C that could be used depending on the DIC concentration in seawater (i.e., HCO₃ ⁻ or CO₂ released during carbonate dissolution and organic C remineralized by Ostreobium associated bacteria). The recycling of inorganic C from dissolved CaCO₃ was also shown in the boring cyanobacteria, Mastigocoleus testarum, when DIC was limiting (Guida and Garcia-Pichel, 2016).

In the present study, the logarithmic relationship presented Figure 4 (R₂ = 0.996; p < 0.0001) shows that biogenic dissolution increased rapidly as a function of net photosynthesis at low light intensities (≤350 µmol photons.m⁻².s⁻¹), but was less dependent on net photosynthetic rates under high natural daylight intensities (740–1300 µmol photons.m⁻².s⁻¹). In the case of the outdoor incubations, it is unlikely that CO₂ escape into the atmosphere resulted in over-estimation of net photosynthesis because seawater was on average under-saturated with respect to CO₂ (<90%). Although we cannot exclude that non-boring phototrophic chasmo- and cryptoendoliths may have benefited from increased amounts of light (Marcelino et al., 2018; Yang et al., 2019), we suggest that most probably the decreasing pCO₂ over the course of the outdoor incubations (Figure 3D) in combination with high light availability promoted the use of carbon concentrating mechanisms (CCMs) by Ostreobium filaments which dominated microboring communities. The use of CCM was suggested for the first time by Tribollet et al. (2009) in an acidification experiment, and more recently by Massé et al. (2020) depending on environmental conditions.

For four out of the five drift incubations (indoor and outdoor incubations combined), CaCO₃ dissolution rates did not change over time (Figure 2A and3A) although Ω_Arag was clearly increasing (Figure 2C and 3C). Variations in Ω_Arag were driven by variations in A_T and DIC, which were in turn driven by organic carbon metabolism (net photosynthesis under natural daylight conditions) and/or CaCO₃ dissolution. It thus may not be appropriate to attempt to relate CaCO₃ fluxes to Ω_Arag variations (see Andersson and Gledhill, 2013; Chauvin, 2013). Ostreobium response to variations in the parameters of the carbonate system could also be non-linear, as found for some marine calcifiers (Maier et al., 2013) and bioeroding communities dominated by boring sponges (Wisshak et al., 2012; Stubler and Peterson, 2016). Total bioerosion rates (particulate + dissolved CaCO₃) for the boring clionaid sponge, Cliona orientalis, were similar under present (pCO₂ = 393 ± 56 µatm) and slightly lowered pCO₂ conditions (pCO₂ = 339 ± 37 µatm), suggesting that the sponge was able to compensate for the less favourable conditions (Wisshak et al., 2012). In a similar manner, the somewhat higher range in pCO₂ (pCO₂ = 339–512 μatm; Ω_Arag = 2.85-3.78) during the indoor flume experiment did not affect net biogenic dissolution rates, which were also similar at Ω_Arag 3 and 3.5 (pCO₂ = 570 and 437 µatm respectively) during the acidification experiment in New Caledonia conducted by Tribollet et al. (2019). It is therefore likely that a threshold somewhere between Ω_Arag 2 and 3 must be reached before promoting increased CaCO₃ dissolution by microboring communities dominated by Ostreobium. Above Ω_Arag 3, microboring communities dominated by Ostreobium seem to be able to maintain a constant carbonate dissolution rate.

Surprisingly, elevated CO₂ at the beginning of the incubations performed under natural daylight in the outdoor experiment (757 ± 31 μatm, due to the initial addition of HCl) did not cause an increase of CaCO₃ dissolution early in the incubations like it was shown in controlled acidification experiments under low light intensities (Tribollet et al., 2009; Reyes-Nivia et al., 2013; Tribollet et al., 2019). It is hypothesized that high light availability promoted CO₂ fixation by RUBISCO, preventing acidification of the intracellular medium of boring filaments. In contrast, during the third incubation performed under natural daylight, CaCO₃ dissolution was followed by some CaCO₃ precipitation when the tank seawater Ω_Arag reached values higher than 6.4 and pCO₂ decreased down to 30 µatm (Figure 3A and 3C). Conditions where pH is above 9, Ω is very high while pCO₂ is close to 13 μatm, are known to stimulate nucleation of CaCO₃ in seawater (Morse and He, 1993; Wurgaft et al., 2016). It is most probable here that chemical conditions at the interface (boundary layer) between carbonate rubbles (behaving as carbonate nucleus) and seawater were even more extreme than those measured in the middle of the tank at the end of the third incubation, creating conditions for CaCO₃ precipitation. We also hypothesize that under such extreme conditions, either 1) the available energy was allocated by phototrophic microborers (mainly Ostreobium sp.) to CCM to support the Calvin cycle instead of supporting other processes such as the Ca²⁺ ATPase dependent pumps, and/or 2) the use of HCO₃ ⁻ as a source of CO₂ for photosynthesis via the implication of carbonic anhydrase enzymes in Ostreobium filaments (Shashar and Stambler 1992; Iha et al., 2021) may have produced significant amounts of OH⁻ (Giordano et al., 2005) which, combined to H⁺ protons, could have stopped the biogenic dissolution process (this implies that Ostreobium filaments used Ca²⁺/H⁺ ATPase dependent pumps similar to those involved by the bioeroding cyanobacterium Mastigocoleus testerum; (Garcia-Pichel et al., 2010), a hypothesis supported by Krause et al. (2019) and Iha et al. (2021)), and/or 3) the number of Ca²⁺ pumps was limited so despite an important photosynthetic activity (providing energy), biogenic dissolution could be limited or stopped. In the case of cultured euendolithic cyanobacteria, Garcia-Pichel et al. (2010) showed that calcium release at the substrate surface resulted in a local super-saturation with respect to CaCO₃ creating conditions for a micrite layer formation. More recently, Krause et al. (2019) highlighted the capacity of Ostreobium filaments to uptake Ca²⁺ at their apical thallus tip and to excrete it at the other end of its thallus, most probably in the coral pore spaces causing reprecipitation of CaCO₃ in the form of secondary aragonite. In contrast, in the indoor flume, micritization most probably did not contribute to the difference recorded between alkalinity production in the light and in the dark during the indoor flume experiment because Ω_Arag was at most 3.7 and no curvature of the A_T vs. time relationships was observed before Ω_Arag reached about 5 (Figure 3A and 3C). Nevertheless, this process could probably lower alkalinity production by microborers on some reef flats where extreme diurnal variability in carbonate chemistry is recorded (e.g. Ω_Arag up to 6.5 during the day at Lady Elliot reef, Great Barrier Reef; Shaw et al., 2012).

We show here that microboring communities dominated by the phototrophic green alga Ostreobium sp. are most probably the main drivers of reef dead coral skeleton dissolution observed under a large range of saturation states in seawater, at least between 2 and 6.4. In previous studies on microboring flora, no traces of random dissolution by endolithic bacteria inside coral skeletons were observed under scanning electron microscopy (Tribollet and Golubic, 2011; Tribollet et al., 2019) although bacteria are known to be associated to Ostreobium filaments (unpublished data) and the boring cyanobacterium Solentia sp. (see Figure 10 in Le Campion-Alsumard et al., 1996). Those bacteria are preferentially located inside filaments (personal observations) or in polysaccharide envelope layers of cells (Le Campion-Alsumard et al., 1996). We suggest that phototrophic microborers are thus able to keep dissolving reef carbonate substrates under higher saturation states of aragonite than those measured in our study as long as they have enough light and available C sources, as nitrogen source is not limiting (see Massé et al., 2020; Pernice et al., 2020). We thus strongly stress the importance for more investigations to better understand the roles of the phototrophic microboring flora in reef carbonate budget and resilience as they have paradoxical activities: they are able to rise pH, Ω_Arag, and seawater alkalinity under daylight while they dissolve actively dead carbonate substrates enhancing reef fragility and degradation. Those roles should be studied in more or less mature endolithic communities in rubbles and dead coral substrates as for instance, the epilithic cover (Chazottes et al., 2002) and the presence of macroborers such as sponges and bivalves can greatly influence the composition of microboring flora communities and consequently, the biogenic dissolution rates (Grange et al., 2015; Schönberg et al., 2017; Fordyce et al., 2020). There is also a specific need to better understand the functional roles played by the various genotypes and phenotypes of Ostreobium as Massé et al. (2020) highlighted a large diversity in response to environmental constraints such as the type of habitat. Finally, we also suggest that microborer metabolic activity could counteract the effects of acidification at least at the local scale of very shallow reefs with long water residence time (Page et al., 2016) and in some living calcifiers such as massive corals which are heavily colonized by Ostreobium sp. compare to branching corals (Le Campion-Alsumard et al., 1995; Godinot et al., 2012; Massé et al., 2018) and crustose coralline algae (Tribollet and Payri, 2001; Reyes-Nivia et al., 2014). They could indeed benefit to their host through a potential transfer of photoassimilates (e.g. in Mediterranean subtropical corals: Fine and Loya, 2002; Sangsawang et al., 2017), photoprotection (Galindo-Martinez et al., 2022), and/or through the rise of pH and alkalinity at the microscale inside their host under various environmental conditions (Nothdurft and Webb, 2009; Garcia-Pichel et al., 2010; Reyes-Nivia et al., 2014; Krause et al., 2019; Tribollet et al., 2019 and present study).