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Analyses carried out per Ostreobium strain in endolithic versus free-living growth form. Controls are either bleached coral carbonate skeleton or residual skeletal organic matrix (after carbonate decalcification) of Pocillopora acuta host. (Strain 022 was lost since the analyses); subcult.: replicate subculture, techn. replicates: technical replicates.
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The filamentous chlorophyte Ostreobium sp. dominates shallow marine carbonate microboring communities, and is one of the major agents of reef bioerosion. While its large genetic diversity has emerged, its physiology remains little known, with unexplored relationship between genotypes and phenotypes (endolithic versus free-living growth forms). Here...
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Context 1
... this study, we developed an in vitro approach to compare the physiological characteristics of endolithic versus free-living Ostreobium filaments isolated from the fast growing, small polyp coral model species Pocillopora acuta Lamarck (1816). We genotyped nine strains of Ostreobium based on amplicon sequencing of the rbcL plastid gene marker and characterized successive subcultures of these strains in either endolithic (coral carbonate eroding) or free-living growth habit to provide (see Table 1): (i) photosynthetic and accessory pigment composition, (ii) fatty acid composition, (iii) bulk tissue δ 13 C Tables S5 Figure 5 and Supporting Information Table S6 and δ 15 N stable isotope values and (iv) inorganic C and N assimilation patterns, measured via uptake of 13 Cbicarbonate and 15 N-nitrate stable isotope tracers. ...Context 2
... removal of coral tissues, branched filaments with typical Ostreobium morphology emerged after $3 weeks from the skeleton of two out of three colonies (Ostreobium filaments did not emerge from the skeleton of one of the three colonies), forming yellowish-green tufts of filaments which were pulled out or cut with a scalpel to initiate cultures in freeliving form. Siphoneous filaments had a diameter varying between 5 and 12 μm ( Table 1). ...Context 3
... algal isolates were assigned to three specieslevel Ostreobium genotypes (clades) defined by 99% sequence similarity thresholds of the chloroplast-encoded RuBisCo large subunit rbcL gene (see Genbank Accession numbers in Table 1). The rbcL phylogeny showed that the majority (7/9) of Ostreobium strains (obtained from two out of three host coral colonies) clustered into one P1 clade (99% similarity over $729 nt for 6 strains, and over 375 nt for strain 022). ...Context 4
... the pigment composition of Ostreobium strains belonging to lineage P1 (strains of less frequent lineage P12/P14 were not studied) was similar for endolithic and free-living forms (three and five individual strains, respectively, see Table 1). Indeed, similar photosynthetic (chlorophyll) and accessory (carotenoid) pigments were detected for both growth forms in high-performance liquid chromatography (HPLC) fingerprints of organic extracts, although their relative abundance varied (Supporting Information Fig. S2b; with mean retention times (±standard deviation) for the 58 peaks detected provided in the Supporting Information Table S1). ...Context 5
... the pigment composition of Ostreobium strains belonging to lineage P1 (strains of less frequent lineage P12/P14 were not studied) was similar for endolithic and free-living forms (three and five individual strains, respectively, see Table 1). Indeed, similar photosynthetic (chlorophyll) and accessory (carotenoid) pigments were detected for both growth forms in high-performance liquid chromatography (HPLC) fingerprints of organic extracts, although their relative abundance varied (Supporting Information Fig. S2b; with mean retention times (±standard deviation) for the 58 peaks detected provided in the Supporting Information Table S1). In contrast, profiles of control bleached skeletons (uncolonized by endolithic Ostreobium) contained only three peaks, rarely observed and at very low intensity in endolithic Ostreobium (Supporting Information Table S1). ...Context 6
... similar photosynthetic (chlorophyll) and accessory (carotenoid) pigments were detected for both growth forms in high-performance liquid chromatography (HPLC) fingerprints of organic extracts, although their relative abundance varied (Supporting Information Fig. S2b; with mean retention times (±standard deviation) for the 58 peaks detected provided in the Supporting Information Table S1). In contrast, profiles of control bleached skeletons (uncolonized by endolithic Ostreobium) contained only three peaks, rarely observed and at very low intensity in endolithic Ostreobium (Supporting Information Table S1). ...Context 7
... cultures were obtained from the free-living cultures, by colonization during several months of pre-bleached native coral carbonate chip substrates (see below). Strain vouchers have been deposited in the Collection of microalgae and cyanobacteria of the Muséum national d'Histoire naturelle in Paris, France (Table 1). ...Context 8
... assignation of Ostreobium strains was determined by BLASTn against GenBank database. All sequences generated during this study have been deposited in Genbank under accession numbers MK095212 to MK095220 (Table 1). ...Context 9
... Ostreobium isolates (five strains) and their corresponding endolithic growth forms (three strains, due to limited available biomass) belonging to lineage P1 (see Table 1, less frequent strains P12/P14 were not studied) were washed three times with large volumes (1:25) of autoclaved deionized water to mechanically remove most surface bacteria and dinoflagellate contaminants and decrease the salt concentration before organic extractions. To evaluate variability in pigment profile across subcultures within same strain, two successive passages (i.e. ...Context 10
... evaluate variability in pigment profile across subcultures within same strain, two successive passages (i.e. subcultures) of three free-living Ostreobium strains were analyzed (see Table 1). Furthermore, bleached Pocillopora acuta coral skeletons (uncolonized substrate, n = 3) were analyzed as controls for potential residual pigments. ...Context 11
... and free-living Ostreobium strains (total eight strains, from P1 and P12/P14, see Table 1), as well as control bleached skeletons (n = 4), were lyophilized and ground rapidly (a few seconds) to powder. Two to three subcultures were analysed for each of 1 endolithic and 3 free-living Ostreobium strains, to evaluate variability in fatty acid composition across subcultures within same strain. ...Context 12
... dual labeling experiments were conducted for endolithic and their free-living Ostreobium counterparts (n = 4 pairs, two per each genetic lineage P1 and P12/P14; see Table 1). Skeletons colonized by endolithic forms were brushed to remove epilithic filaments as described above, and all culture samples (free-living and endolithic growth forms) were transferred for 3 days before the labeling experiment in Artificial SeaWater (ASWadapted from Harrison et al., 1980) in order to rinse the nutrient rich PES medium. ...Context 13
... increase replication of measurements of C and N stable isotope values across subcultures, strains, and genetic lineages, additional samples were analysed, of endolithic Ostreobium strains (not decalcified, or decalcified with formic acid 5%) and their corresponding free-living forms, and control bleached skeleton substrate (see Table 1). ...Context 14
... this study, we developed an in vitro approach to compare the physiological characteristics of endolithic versus free-living Ostreobium filaments isolated from the fast growing, small polyp coral model species Pocillopora acuta Lamarck (1816). We genotyped nine strains of Ostreobium based on amplicon sequencing of the rbcL plastid gene marker and characterized successive subcultures of these strains in either endolithic (coral carbonate eroding) or free-living growth habit to provide (see Table 1): (i) photosynthetic and accessory pigment composition, (ii) fatty acid composition, (iii) bulk tissue δ 13 C Tables S5 Figure 5 and Supporting Information Table S6 and δ 15 N stable isotope values and (iv) inorganic C and N assimilation patterns, measured via uptake of 13 Cbicarbonate and 15 N-nitrate stable isotope tracers. ...Context 15
... removal of coral tissues, branched filaments with typical Ostreobium morphology emerged after $3 weeks from the skeleton of two out of three colonies (Ostreobium filaments did not emerge from the skeleton of one of the three colonies), forming yellowish-green tufts of filaments which were pulled out or cut with a scalpel to initiate cultures in freeliving form. Siphoneous filaments had a diameter varying between 5 and 12 μm ( Table 1). ...Context 16
... algal isolates were assigned to three specieslevel Ostreobium genotypes (clades) defined by 99% sequence similarity thresholds of the chloroplast-encoded RuBisCo large subunit rbcL gene (see Genbank Accession numbers in Table 1). The rbcL phylogeny showed that the majority (7/9) of Ostreobium strains (obtained from two out of three host coral colonies) clustered into one P1 clade (99% similarity over $729 nt for 6 strains, and over 375 nt for strain 022). ...Context 17
... the pigment composition of Ostreobium strains belonging to lineage P1 (strains of less frequent lineage P12/P14 were not studied) was similar for endolithic and free-living forms (three and five individual strains, respectively, see Table 1). Indeed, similar photosynthetic (chlorophyll) and accessory (carotenoid) pigments were detected for both growth forms in high-performance liquid chromatography (HPLC) fingerprints of organic extracts, although their relative abundance varied (Supporting Information Fig. S2b; with mean retention times (±standard deviation) for the 58 peaks detected provided in the Supporting Information Table S1). ...Context 18
... the pigment composition of Ostreobium strains belonging to lineage P1 (strains of less frequent lineage P12/P14 were not studied) was similar for endolithic and free-living forms (three and five individual strains, respectively, see Table 1). Indeed, similar photosynthetic (chlorophyll) and accessory (carotenoid) pigments were detected for both growth forms in high-performance liquid chromatography (HPLC) fingerprints of organic extracts, although their relative abundance varied (Supporting Information Fig. S2b; with mean retention times (±standard deviation) for the 58 peaks detected provided in the Supporting Information Table S1). In contrast, profiles of control bleached skeletons (uncolonized by endolithic Ostreobium) contained only three peaks, rarely observed and at very low intensity in endolithic Ostreobium (Supporting Information Table S1). ...Context 19
... similar photosynthetic (chlorophyll) and accessory (carotenoid) pigments were detected for both growth forms in high-performance liquid chromatography (HPLC) fingerprints of organic extracts, although their relative abundance varied (Supporting Information Fig. S2b; with mean retention times (±standard deviation) for the 58 peaks detected provided in the Supporting Information Table S1). In contrast, profiles of control bleached skeletons (uncolonized by endolithic Ostreobium) contained only three peaks, rarely observed and at very low intensity in endolithic Ostreobium (Supporting Information Table S1). ...Context 20
... cultures were obtained from the free-living cultures, by colonization during several months of pre-bleached native coral carbonate chip substrates (see below). Strain vouchers have been deposited in the Collection of microalgae and cyanobacteria of the Muséum national d'Histoire naturelle in Paris, France (Table 1). ...Context 21
... assignation of Ostreobium strains was determined by BLASTn against GenBank database. All sequences generated during this study have been deposited in Genbank under accession numbers MK095212 to MK095220 (Table 1). ...Context 22
... Ostreobium isolates (five strains) and their corresponding endolithic growth forms (three strains, due to limited available biomass) belonging to lineage P1 (see Table 1, less frequent strains P12/P14 were not studied) were washed three times with large volumes (1:25) of autoclaved deionized water to mechanically remove most surface bacteria and dinoflagellate contaminants and decrease the salt concentration before organic extractions. To evaluate variability in pigment profile across subcultures within same strain, two successive passages (i.e. ...Context 23
... evaluate variability in pigment profile across subcultures within same strain, two successive passages (i.e. subcultures) of three free-living Ostreobium strains were analyzed (see Table 1). Furthermore, bleached Pocillopora acuta coral skeletons (uncolonized substrate, n = 3) were analyzed as controls for potential residual pigments. ...Context 24
... and free-living Ostreobium strains (total eight strains, from P1 and P12/P14, see Table 1), as well as control bleached skeletons (n = 4), were lyophilized and ground rapidly (a few seconds) to powder. Two to three subcultures were analysed for each of 1 endolithic and 3 free-living Ostreobium strains, to evaluate variability in fatty acid composition across subcultures within same strain. ...Context 25
... dual labeling experiments were conducted for endolithic and their free-living Ostreobium counterparts (n = 4 pairs, two per each genetic lineage P1 and P12/P14; see Table 1). Skeletons colonized by endolithic forms were brushed to remove epilithic filaments as described above, and all culture samples (free-living and endolithic growth forms) were transferred for 3 days before the labeling experiment in Artificial SeaWater (ASWadapted from Harrison et al., 1980) in order to rinse the nutrient rich PES medium. ...Context 26
... increase replication of measurements of C and N stable isotope values across subcultures, strains, and genetic lineages, additional samples were analysed, of endolithic Ostreobium strains (not decalcified, or decalcified with formic acid 5%) and their corresponding free-living forms, and control bleached skeleton substrate (see Table 1). ...Similar publications
A coral reef is an underwater ecosystem characterized by stony corals and reef rubble. Reef rubble is formed by the aragonite and calcite skeletons of dead scleractinian corals and other calcareous organisms. In the ocean, reef rubble contributes to numerous benefits to the marine environment, including the formation of coral reefs, the provision o...
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... The resulting product was quantified and pass to a centration of 30 ng/ul so that it reacted adequately in the amplification process. Next, the rbcL gene was amplified with primers rbcLF250 [5'GATATTGARCCTGTTGTTG GTGAAGA 3 '], and rbcLR670 [5' CCAGTTTCAGCTTGWGCTTTATAAA 3 '], based on the sequence of amino acids (Masse et al. 2020). The mixture to carry out the PCR had a total of 15 ul distributed as follows, 6.35 ul of water, 2 ul of MgCl 2 , 3 ul of buffer, 0.75 ul BSA (Bovine Serum Albumin, 20ng /ul), 0.6/ul of each primer, 1 ul Taq DNA polymerase, and 1 ul of genomic DNA. ...
Ostreobium comprise endolithic algae commonly seen in conjunction with scleractinian corals. In the past, it was solely recognized as a coral skeleton bioeroder. Yet, their relationship with corals is critical because they give photosynthetic byproducts and help the coral when it loses its primary symbionts due to stress. The variety of these algae among coral species of the genus Porites in the tropical easter Pacific and western Atlantic was investigated here. We extracted Ostreobium samples from seven Porites species including two from the Tropical Easter Pacific-TEP ( P. panamensis, P. lobata ) and five from the Caribbean (P . furcata, P. porites, P. colonensis, P. branneri, and P. astreoides ). We also compared the new rbcL sequences from algae found within various coral species from other parts of the world. A biogeographic analysis and two methodologies, PTP (Poisson tree process) and GMYC (General Mixed Yule-Coalescent), were used to delineate the different species. The findings revealed a significant degree of genetic diversity within Ostreobium , with more than 15 groups of no more than three individuals and 40 individual lineages. Its origins date back to the Ordovician, 500 Ma, and it does not appear to preserve host-specificity. The sampled locations include a wide variety of Ostreobium still, biogeographically, varied patterns were confirmed, making it impossible to pinpoint the precise origins of most clades. The ancestry analyses revealed convergent events for the emergence of Ostreobium in a few genera of local corals, but the phenomenon also occurred in genera from far-off places.
... The resulting product was quantified and pass to a centration of 30 ng/ul so that it reacted adequately in the amplification process. Next, the rbcL gene was amplified with primers rbcLF250 [5'GATATTGARCCTGTTGTTG GTGAAGA 3 '], and rbcLR670 [5' CCAGTTTCAGCTTGWGCTTTATAAA 3 '], based on the sequence of amino acids (Masse et al. 2020). The mixture to carry out the PCR had a total of 15 ul distributed as follows, 6.35 ul of water, 2 ul of MgCl 2 , 3 ul of buffer, 0.75 ul BSA (Bovine Serum Albumin, 20ng /ul), 0.6/ul of each primer, 1 ul Taq DNA polymerase, and 1 ul of genomic DNA. ...
... Their thallus is composed of a network of interwoven, branching, giant coenocytic filaments (siphons), which are 10 to 20 µm in diameter and up to several hundred µm in length, with multiple nuclei and chloroplasts that move by cytoplasmic streaming. When endolithic (bioeroding form), these filaments erode galleries within the carbonate, with epilithic three-dimensional "tufts of filaments" emerging at the substratum surface, which can be isolated as a free-living growth form [5,7,8]. In contrast, the coenocytic thalli of other siphonous Bryopsidale green algae, such as Bryopsis and Codium (Bryopsidineae) or Caulerpa and Halimeda (Halimedineae), form macroscopic three-dimensional 2 of 26 algal bodies, with morphologically differentiated basal anchoring rhizoids and frond-like axes (sometimes bearing lateral branches and cortical utricles) [9]. ...
... There is growing consensus that these photoautotroph algal endoliths and their associated prokaryotes have an overlooked functional role in the health of their coral host (reviewed by Pernice et al. [17], Ricci et al. [18], and van Oppen and Blackall [19]): they bloom during coral-bleaching episodes, and support bleached coral recovery via the suggested provision of alternative photosynthetic carbon nutrients [20][21][22]. Other suggested roles involve contribution to nitrogen cycling [7,23] and increased host photoprotection via modifications of skeletal optical properties [24]. ...
... In culture experiments, a pioneer study [7] detected fatty acid bacterial markers in Ostreobium strains isolated from aquarium-propagated Pocillopora coral colonies. A recent survey of the taxonomical composition of the bacterial communities associated with Ostreobium strains noted differences between phylotypes [27]; however, this was noted without robust comparison of multiple cultures for each strain and experimental manipulation of abiotic factors. ...
Microscopic filaments of the siphonous green algae Ostreobium (Ulvophyceae, Bryopsidales) colonize and dissolve the calcium carbonate skeletons of coral colonies in reefs of contrasted salinities. Here, we analyzed their bacterial community’s composition and plasticity in response to salinity. Multiple cultures of Pocillopora coral-isolated Ostreobium strains from two distinct rbcL lineages representative of IndoPacific environmental phylotypes were pre-acclimatized (>9 months) to three ecologically relevant reef salinities: 32.9, 35.1, and 40.2 psu. Bacterial phylotypes were visualized for the first time at filament scale by CARD-FISH in algal tissue sections, within siphons, at their surface or in their mucilage. Ostreobium-associated microbiota, characterized by bacterial 16S rDNA metabarcoding of cultured thalli and their corresponding supernatants, were structured by host genotype (Ostreobium strain lineage), with dominant Kiloniellaceae or Rhodospirillaceae (Alphaproteobacteria, Rhodospirillales) depending on Ostreobium lineage, and shifted Rhizobiales’ abundances in response to the salinity increase. A small core microbiota composed of seven ASVs (~1.5% of thalli ASVs, 19–36% cumulated proportions) was persistent across three salinities in both genotypes, with putative intracellular Amoebophilaceae and Rickettsiales_AB1, as well as Hyphomonadaceae and Rhodospirillaceae also detected within environmental (Ostreobium-colonized) Pocillopora coral skeletons. This novel knowledge on the taxonomic diversity of Ostreobium bacteria paves the way to functional interaction studies within the coral holobiont.
... High salinity stress on coral reefs is also caused by brine 121 discharge from desalination plants (Petersen et al., 2018), a local pollution by-product from 122 geo-engineering operations to supply increasingly scarce freshwater resources. 123 High salinity is known to impact the physiology of tropical corals (aquarium- (Massé et al., 2020;Pasella et al., 2022). 138 Here, the diversity of Ostreobium-associated bacteria was investigated in two distinct 139 lineages, each exposed to three distinct salinity levels. ...
... 138 Here, the diversity of Ostreobium-associated bacteria was investigated in two distinct 139 lineages, each exposed to three distinct salinity levels. Experiments were conducted on two 154 Ostreobium strains and acclimatization to 3 salinities 155 The bacterial communities associated to two Ostreobium strains, 010 (MNHN-ALCP- Provasoli, 1968;Massé et al., 2020). Originally isolated from their coral host 169 in the presence of low doses of penicillin (100 U/ml) and streptomycin (100 µg/mL) 170 antibiotics, they were propagated for 4.5 years, with penicillin and streptomycin (Massé et al.,171 2020) to prevent overgrowth by opportunistic bacteria. ...
... Although routinely used in animal cell 172 culture, these generic antibiotics are known to be not efficient against most native marine 173 bacteria (Domart-Coulon and Blanchoud, 2022). And, indeed, bacterial fatty acid markers 174 were consistently detected within chloroform extracts of these strains (Massé et al., 2020). 175 However, to remove antibiotics selection pressure on the algal microbiota, penicillin was 176 omitted for the last 9 months and streptomycin for the last month before sampling for DNA 177 extraction. ...
Microscopic filaments of the siphonous green algae Ostreobium (Ulvophyceae, Bryopsidales) colonize and dissolve the calcium carbonate skeletons of coral colonies, in shallow-water reef environments of contrasted salinities. Their bacterial composition and plasticity in response to salinity remain unknown. Here, we analyzed the bacteria associated with coral-isolated Ostreobium strains from two distinct rbcL lineages, representative of IndoPacific environmental phylotypes, that had been pre-acclimatized (>9months) to three ecologically-relevant reef salinities: 32.9, 35.1 and 40.2 psu. Bacterial phylotypes were visualized at filament scale by CARD-FISH in algal tissue sections, localized to the surface, within filaments or in the algal mucilage. Ostreobium-associated communities, characterized by bacterial 16S rRNA metabarcoding of cultured thalli and corresponding supernatants, were structured by host genotype more than salinity and partly overlapped with those of environmental (Ostreobium-colonized) coral skeletons. Alphaproteobacteria dominated the thalli communities, enriched in Kiloniellaceae or Rhodospirillaceae depending on algal genotype. A small core microbiota composed of 7 ASVs (~1.5% of thalli ASVs, 19%-36% cumulated proportions), shared by multiple cultures of both Ostreobium genotypes and persistent across 3 salinities, included putative intracellular Amoebophilus and Rickettsiales bacteria. This novel knowledge on the taxonomic diversity of Ostreobium bacterial associates paves the way to functional interaction studies within the coral holobiont.
... Within the past few years, several studies have thus investigated the genetic diversity of the endolithic microbiome, and especially that of the dominant euendolith, the chlorophyte Ostreobium sp. (Marcelino and Verbruggen, 2016;Sauvage et al., 2016;Yang et al., 2016;Del Campo et al., 2017;Masséet al., 2020) and its possible implications in coral growth, physiology and photoprotection (Sangsawang et al., 2017;Masséet al., 2018;Galindo-Martıńez et al., 2022). Conversely, the species composition, distribution and abundance of microboring communities in living corals remain poorly known and most studies focused only on communities located within the first few centimeters below coral tissues of adult colonies (Odum and Odum, 1955;Lukas, 1973;Kühl and Polerecky, 2008;Fordyce et al., 2021;Galindo-Martıńez et al., 2022). ...
... Here such an inverse relationship was not seen probably because the bulk density was studied instead of the microdensity. Fordyce et al. (2021) reported that corals with a low skeletal microdensity are more colonized by phototrophic microboring communities as they would benefit from greater and easier access to nutrients (organic matrix: see Masséet al., 2020;Iha et al., 2021) than corals with a high density. Interestingly and similarly to microborers abundance, the coral bulk density, calcification and linear extension rates in our study were strongly correlated to SSTA, rainfall and cumulative insolation ( Table 2); factors reported previously by Lough and Barnes (1997) and Lough and Cantin (2014). ...
... Godinot et al. (2012) showed that a rapid vertical coral growth can 'dilute' microborers as they cannot keep up with their host's fast growth. The possible consequences of such a drastic decrease in microborers' abundance in massive corals such as Diploastrea sp. may be important as several authors suggested that Ostreobium sp. may play a key role in coral health, especially during coral bleaching recovery, by both providing photoprotection (Galindo-Martinez et al., 2022) and photoassimilates (Schlichter et al., 1995;Schlichter et al., 1997;Sangsawang et al., 2017;Masséet al., 2020;Iha et al., 2021). More coral cores should thus be studied to confirm the observed trends in order to better understand the possible implications of an important decrease of microborers abundance in living corals especially the stress-tolerant ones such as Porites sp. ...
Coral reefs are increasingly in jeopardy due to global changes affecting both reef accretion and bioerosion processes. Bioerosion processes dynamics in dead reef carbonates under various environmental conditions are relatively well understood but only over a short-term limiting projections of coral reef evolution by 2100. It is thus essential to monitor and understand bioerosion processes over the long term. Here we studied the assemblage of traces of microborers in a coral core of a massive Diploastrea sp. from Mayotte, allowing us to explore the variability of its specific composition, distribution, and abundance between 1964 and 2018. Observations of microborer traces were realized under a scanning electron microscope (SEM). The area of coral skeleton sections colonized by microborers (a proxy of their abundance) was estimated based on an innovative machine learning approach. This new method with 93% accuracy allowed analyzing rapidly more than a thousand SEM images. Our results showed an important shift in the trace assemblage composition that occurred in 1985, and a loss of 90% of microborer traces over the last five decades. Our data also showed a strong positive correlation between microborer trace abundance and the coral bulk density, this latter being particularly affected by the interannual variation of temperature and cumulative insolation. Although various combined environmental factors certainly had direct and/or indirect effects on microboring species before and after the breakpoint in 1985, we suggest that rising sea surface temperature, rainfall, and the loss of light over time were the main factors driving the observed trace assemblage change and decline in microborer abundance. In addition, the interannual variability of sea surface temperature and instantaneous maximum wind speed appeared to influence greatly the occurrence of green bands. We thus stress the importance to study more coral cores to confirm the decadal trends observed in the Diploastrea sp. from Mayotte and to better identify the main factors influencing microboring communities, as the decrease of their abundance in living massive stress tolerant corals may have important consequences on their resilience.
... 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). ...
... Light availability and DIC speciation did probably however, influenced pathways of carbon acquisition and thus, CaCO 3 dissolution by microborers. Massé et al. (2020) showed that seawater DIC (most probably CO 2 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., HCO 3 − or CO 2 released during carbonate dissolution and organic C remineralized by Ostreobium associated bacteria). ...
... 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 pCO 2 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. ...
Bioerosion, resulting from microbioerosion or biogenic dissolution, macrobioerosion and grazing, is one the main processes involved in reef carbonate budget and functioning. On healthy reefs, most of the produced carbonates are preserved and accumulate. But in the context of global change, reefs are increasingly degraded as environmental factors such as ocean warming and acidification affect negatively reef accretion and positively bioerosion processes. The recent 2019 SROCC report suggests that if CO2 emissions in the atmosphere are not drastically reduced rapidly, 70%–99% of coral reefs will disappear by 2,100. However, to improve projections of coral reef evolution, it is important to better understand dynamics of bioerosion processes. Among those processes, it was shown recently that bioeroding microflora which actively colonize and dissolve experimental coral blocks, release significant amount of alkalinity in seawater both by day and at night under controlled conditions. It was also shown that this alkalinity production is enhanced under ocean acidification conditions (saturation state of aragonite comprised between 2 and 3.5) suggesting that reef carbonate accumulation will be even more limited in the future. To better understand the conditions of production of alkalinity in seawater by boring microflora and its possible consequences on reef resilience, we conducted a series of experiments with natural rubble maintained under natural or artificial light, and various saturation states of aragonite. We show here that biogenic dissolution of natural reef rubble colonized by microboring communities dominated by the chlorophyte Ostreobium sp., and thus the production of alkalinity in seawater, can occur under a large range of saturation states of aragonite, from 2 to 6.4 under daylight and that this production is directly correlated to the photosynthetic activity of microboring communities. We then discuss the possible implications of such paradoxical activities on reef resilience.
... Improvements to culturing cell lines of nonmodel organisms will allow us to study the metal economy of different holobiont partners in the absence of one another. Several coral holobiont partners can currently be reared in culture including cnidarian cell lines, hundreds of strains of Symbiodiniaceae, a subset of Ostreobium strains, and thousands of bacterial strains Massé et al., 2020;Roger et al., 2021). Until recently, coral cell lines were only viable for short time periods (12 days), but a subset of coral cell lines are now viable for 6 months (Kawamura et al., 2021). ...
The juxtaposition of highly productive coral reef ecosystems in oligotrophic waters has spurred substantial interest and progress in our understanding of macronutrient uptake, exchange, and recycling among coral holobiont partners (host coral, dinoflagellate endosymbiont, endolithic algae, fungi, viruses, bacterial communities). By contrast, the contribution of trace metals to the physiological performance of the coral holobiont and, in turn, the functional ecology of reef‐building corals remains unclear. The coral holobiont's trace metal economy is a network of supply, demand, and exchanges upheld by cross‐kingdom symbiotic partnerships. Each partner has unique trace metal requirements that are central to their biochemical functions and the metabolic stability of the holobiont. Organismal homeostasis and the exchanges among partners determine the ability of the coral holobiont to adjust to fluctuating trace metal supplies in heterogeneous reef environments. This review details the requirements for trace metals in core biological processes and describes how metal exchanges among holobiont partners are key to sustaining complex nutritional symbioses in oligotrophic environments. Specifically, we discuss how trace metals contribute to partner compatibility, ability to cope with stress, and thereby to organismal fitness and distribution. Beyond holobiont trace metal cycling, we outline how the dynamic nature of the availability of environmental trace metal supplies can be influenced by a variability of abiotic factors (e.g. temperature, light, pH, etc.). Climate change will have profound consequences on the availability of trace metals and further intensify the myriad stressors that influence coral survival. Lastly, we suggest future research directions necessary for understanding the impacts of trace metals on the coral holobiont symbioses spanning subcellular to organismal levels, which will inform nutrient cycling in coral ecosystems more broadly. Collectively, this cross‐scale elucidation of the role of trace metals for the coral holobiont will allow us to improve forecasts of future coral reef function.
... For example, euendolithic green algae almost exclusively belong to the class Ulvophyceae, in which the ability to bore evolved independently over 20 times [64], while the cyanobacterium Acaryochloris marina Miyashita and Chihara (2003) has been recorded for the first time in the skeleton of live corals using eDNA [64] and produces chlorophyll-d, allowing it to use far-red light for photosynthesis and thus to occupy niches depleted of visible light [72]. Finally, the correct identification of the euendolithic species is fundamental in the study of euendolithic communities and their impacts, as different strains within the same species complex (e.g., Ostreobium quekettii Bornet and Flahault, 1889) can differ in their physiology [73,74]. ...
Photoautotrophic euendolithic microorganisms are ubiquitous where there are calcium
carbonate substrates to bore into and sufficient light to sustain photosynthesis. The most diverse and abundant modern euendolithic communities can be found in the marine environment. Euendoliths, as microorganisms infesting inanimate substrates, were first thought to be ecologically irrelevant. Over the past three decades, numerous studies have subsequently shown that euendoliths can colonize living marine calcifying organisms, such as coral skeletons and bivalve shells, causing both sub-lethal and lethal damage. Moreover, under suitable environmental conditions, their presence can have surprising benefits for the host. Thus, infestation by photoautotrophic euendoliths has significant
consequences for calcifying organisms that are of particular importance in the case of ecosystems underpinned by calcifying ecosystem engineers. In this review, we address the nature and diversity of marine euendoliths, as revealed recently through genetic techniques, their bioerosive mechanisms, how environmental conditions influence their incidence in marine ecosystems and their potential as bioindicators, how they affect live calcifiers, and the potential future of euendolithic infestation in the context of global climate change and ocean acidification.
ABSTRACT
An undescribed variety of the green alga Chlorella cf. vulgaris was found living in the octocoral (soft coral) Sarcophyton sp., which also has dinoflagellate zooxanthellae. The green alga is present in very small numbers and is capable of photoheterotrophy based on growth in the presence of an inhibitor of photosynthesis, DCMU, with glycerol provided as a carbon source. The presence of zoochlorellae (Chl a + b), in soft corals appears to have been unsuspected. The easily cultured green alga was unicellular,
unflagellated and very small, c. 2 to 6 μm in diameter and had a relatively high Chl b content (Chl b/a = 0.379 ± 0.063). Rapid light curves using PAM fluorometry on the cultured cells showed that Yield vs. irradiance had a low maximum yield (Ymax, 0.388 ± 0.0132; Irradiance ½ point, E½Ymax = 139 ± 11.1 μmol
photon m–2 s–1). The overall optimum irradiance (Eopt) for the alga was 314 ± 18.8 μmol photon m–2 s–1. The alga can photosynthesize at 50% or more of the optimum rate from 70 to 900 μmol photon m–2 s–1 but would be severely inhibited under full sunlight. Most naturally occurring low irradiance habitats may
have a low median irradiance but are exposed intermittently to very high irradiances and so a relatively high Eopt is advantageous in minimizing photochemical damage. Like the green algal symbiont Elliptochloris marina of sea anemones, Chlorella cf. vulgaris is not exclusively marine: it grows better in
freshwater medium where it has different photosynthetic characteristics (higher Ymax, Eopt and ETRmax). Chlorella cf. vulgaris appears to be capable of photoheterotrophy and it is not clear if it is a symbiont, a commensal or a parasite.
Parrotfish are key agents of bioerosion and sediment production in coral reef ecosystems; however, their dietary targets and therefore potential sources of variation in carbonate cycling lack resolution. Here we address this knowledge shortfall in our current understanding of parrotfish diets by testing the concept that protein-rich micro-photoautotrophs are the target prey for many Scarinine parrotfishes. We focus at fine spatial scales on the feeding substrata of 12 syntopic Indo-Pacific parrotfish species at mid-shelf sites around Lizard Island, Great Barrier Reef, Australia. We followed individual parrotfish on snorkel until biting was observed, and then extracted a reef core around each bite. The surface of each bite core was scraped to ~1 mm for quantitative microscopic analysis (up to 630 × magnification) and for 16S and 18S rRNA metabarcoding. The most dominant photoautotrophic group in terms of surface cover was filamentous cyanobacteria, followed by crustose coralline algae. Epiphytic, epilithic, endophytic and endolithic filamentous cyanobacteria were consistent bite core biota. Although the density of filamentous cyanobacteria on bite cores was largely consistent among the 12 parrotfish species, the quantitative microscopic data and rRNA metabarcoding revealed distinct differences between parrotfish species in the taxonomic composition of core biota. Our data provide further evidence that these syntopic parrotfish species partition feeding resources.
