Prevalence of Ca2�-ATPase-Mediated Carbonate Dissolution amongCyanobacterial Euendoliths

E. L. Ramírez-Reinat and F. Garcia-Pichel

School of Life Sciences, Arizona State University, Tempe, Arizona, USA

Recent physiological work has shown that the filamentous euendolithic cyanobacterium Mastigocoleus testarum (strain BC008)is able to bore into solid carbonates using Ca2�-ATPases to take up Ca2� from the medium at the excavation front, promotingdissolution of CaCO3 there. It is not known, however, if this is a widespread mechanism or, rather, a unique capability of thismodel strain. To test this, we undertook a survey of multispecies euendolithic microbial assemblages infesting natural carbonatesubstrates in marine coastal waters of the Caribbean, Mediterranean, South Pacific, and Sea of Cortez. Microscopic examinationrevealed the presence of complex assemblages of euendoliths, encompassing 3 out of the 5 major cyanobacterial orders. 16SrRNA gene clone libraries detected even greater diversity, particularly among the thin-filamentous forms, and allowed us to cate-gorize the endoliths in our samples into 8 distinct phylogenetic clades. Using real-time Ca2� imaging under a confocal laserscanning microscope, we could show that all communities displayed light-dependent formation of Ca2�-supersaturated zones inand around boreholes, a staple of actively boring phototrophs. In 3 out of 4 samples, boring activity was sensitive to at least oneof two inhibitors of Ca2�-ATPase transporters (thapsigargin or tert-butylhydroquinone), indicating that the Ca2�-ATPasemechanism is widespread among cyanobacterial euendoliths but perhaps not universal. Function-community structure correla-tions point to one particular clade of baeocyte-forming euendoliths as the potential exception.

Euendolithic cyanobacteria, also known as boring, microbor-ing, excavating, perforating, or tunneling cyanobacteria, can

penetrate a variety of calcareous substrates, such as shells, deadcoral, and limestone, by means of chemical dissolution. Asideform cyanobacteria, only fungi and some eukaryotic algae are ca-pable of such boring activity. A wide variety of euendolithic cya-nobacteria has been described during almost 2 centuries of natu-ralistic studies (4, 9, 14, 23). While only a small proportion of thesespecies are capable of boring, this select group has representativesin several of the major taxonomic groups of cyanobacteria. Theycan be found in diverse geographical locations worldwide (1, 2, 5,24), where they are involved in the diagenesis of the substrates theycolonize. In most instances, they weaken the structure of the solidsubstrate with their tunneling (26, 27), but in other cases, thereprecipitation of carbonates as micrite, a by-product of dissolu-tion, forms the cement that binds carbonate sand grains into stro-matolites (21).

The mechanism that allows cyanobacterial endoliths to bore, asubject of long-standing controversy (6, 11), has been recentlyelucidated in the model filamentous strain Mastigocoleus testarumBC008 (20). It is based on the uptake and transcellular transport ofCa2� (12). This is driven, at least partly, by P-type ATPases. As it ispresently understood, the trichomes import free Ca2� from theinterstitial space between the boring cell and mineral (“boringfront”), lowering the concentration of the ion in the interstitialspace below that of saturation and thus promoting the localizeddissolution of the calcium carbonate at the boring front. Intracel-lular Ca2� is transported from cell to cell along the filament, likelywith the aid of Ca2�-specific pumps or channels, and excreted atthe end farthest from the boring front. This ATP-powered extru-sion results in very strong Ca2� supersaturation around the bore-holes. The mechanism, shown in M. testarum strain BC008, differsfrom the previously tacitly accepted hypothesis contending thatboring is powered by mere secretion of acid. Because of the diver-sity of boring cyanobacteria in nature, it is only natural to question

if the strategy of BC008 is universal. While a mechanism commonto all cyanobacterial microborers seems plausible, it is difficult toevaluate a priori on the basis of phylogenetic relatedness, giventhat, in fact, explicit modern phylogenetic work on euendoliths isextremely restricted; references 5 and 8 provide the two sequencesavailable.

The overarching goal of this work was to address the potentialuniversality of the boring mechanism among cyanobacterial eu-endoliths, particularly given that the model strain cannot bore ondolomitic substrates (20). The boring mechanism was investi-gated empirically in complex microbial assemblages collectedfrom distinct geographical locations worldwide, so as to encom-pass as diverse as possible a range of subjects, and compared to theone recently described for M. testarum BC008.

MATERIALS AND METHODSField samples. Euendolith-infested biogenic carbonates (shells and deadcoral skeleton pieces) were collected from four coastal regions, the Carib-bean (San Juan, Puerto Rico, 18°45=N, 65°96=W), North Pacific Sea ofCortez (Baja California, 30° 92=N, 114°70=W), Mediterranean (L’Alguer,Sardinia, 40°33=N, 8°19=E), and South Pacific (Whakatane, New Zealand,37°31=S, 177°11=E). Infested samples were selected owing to their typicalgreen-to-gray coloration. They were air dried before transport to the lab-oratory and then rehydrated in a mixture of Provasoli’s enriched seawater(PES) medium (19, 20) at pH 8.3 and filtered seawater in a 1:1 (vol/vol)ratio. Using intertidal samples, which naturally suffer desiccation cycles,ensured that microbial activity would resume upon rewetting. Thereafter,samples were kept at 25°C under constant light, provided by white fluo-

Received 23 August 2011 Accepted 19 October 2011

Published ahead of print 28 October 2011

Address correspondence to F. Garcia-Pichel, E-mail: ferran@asu.edu.

Copyright © 2012, American Society for Microbiology. All Rights Reserved.

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rescent lamps at a light intensity of 30 �mol (photons) m�2 s�1, beforeanalyses. Fragments were analyzed within a week of collection to mini-mize potential changes in community composition or a significant loss ofbiological activity.

Confocal laser scanning microscopy. Small fragments, ranging from2 to 6 mm2 in size, were broken off the samples using sterile pliers and/ora sterilized hammer. Fragments were scrubbed briskly with a small sterilepaintbrush to remove any superficial biomass or biofilms, so as to ensurethat no epiliths were included in the analyses, rinsed twice in sterile PESmedium, and prepared for confocal microscopy analysis as described pre-viously (12). Briefly, fragments were incubated for 1 h in PES mediumwith 1 �M Calcium Green-5N ([CG5N] Molecular Probes, Eugene, OR)at room temperature under incandescent light at 30 �mol of photons.After incubation, the fragments were fixed to custom-made, slide-mounted chambers, which were then filled with PES containing CG5N ata final concentration of 1 �M. The slides were then placed on the stage of aLeica TCS-SP2 confocal laser scanning microscope and observed under a40� oil immersion objective. Epifluorescence was used to evaluate thefragments for infestation and viability (presence of photosynthetic pig-ments in cells) prior to laser scanning. Visible laser lines of Ar/Kr (488 nm)and Kr (546 nm) were used to excite the samples in confocal mode. Con-focal laser microscopy allowed us to optically section the areas around thecalcite chip surface without physically disturbing any gradients that hadformed. Single-plane horizontal fluorescent images were stored in twoseparate channels: green fluorescence for Ca2� and red autofluorescencefor chlorophyll a. Green fluorescence intensity was converted into abso-lute Ca2� concentrations using a two-point calibration where the signalinside the solid chip corresponded to zero Ca2� and the signal far awayfrom the chip corresponded to the medium in equilibrium with calcite (10mM Ca2�). Curves of concentration versus fluorescence for CalciumGreen in PES medium were run separately in a spectrofluorometer(Turner Designs). In PES medium at pH 8.3, the dissociation constant ofthe dye was around 5 mM, and a linear approximation described therelationship as well as a hyperbolic one in the measured range. The linearrelationship was used for extrapolation into the supersaturated region, asit will bias by defect (i.e., it is a conservative concentration estimator [seereference 12]). Dynamic changes in fluorescence with time, as used here,were monitored on a single optical section focused on the surface of thecarbonate, in the region around the borehole entrance. Images were ana-lyzed with the Leica Confocal Software (Leica Microsystems Inc., Ban-nockburn, IL).

Testing for boring activity and its light dependence. To test for activeboring, we imaged [Ca2�] in situ at the surface of infested fragments on asingle optical section. This area at the borehole entrance has the strongest[Ca2�] supersaturation (��10 mM) in pure-culture experiments usingBC008. Fragments were illuminated with the stage illumination (whitelight, 50 �mol photons m�2 s�1) for 1 h before initial imaging. To dem-onstrate light dependence, a characteristic of boring by phototrophs, illu-mination was turned off for at least 1 h and the Ca2� concentration wasimaged again. To assess recovery, the light was turned on and fragmentswere incubated for another hour before the measurements were repeated.

Effect of calcium transport inhibitors on boring. To probe the in-volvement of P-type Ca2�-ATPase enzymes in the boring mechanismof field euendoliths, the inhibitors thapsigargin (TG; Alexis Biochem-icals, Lausen, Switzerland) and tert-butylhydroquinone (TBHQ; Spec-trum Chemicals, Gardena, CA) were used. The initial concentrationwas 100 �M, which was increased in sequential experiments if negativeresults were obtained. The experiments using the minimal concentra-tion with a measurable effect are reported. Carbonate fragments wereincubated under constant light for 1 h, and single-optical-plane mea-surements were made at the sample surface before addition of theinhibitors. After initial addition of the inhibitors [Ca2�] dynamicswere monitored for at least 1 h.

Extraction of euendoliths and microscopic observations. Euendo-liths were extracted from the carbonate samples used in the confocal mi-

croscopy experiments described above by an EDTA-based dissolutionmethod based on that of Wade and Garcia-Pichel (25). Briefly, carbonatefragments were washed in sterile PES and placed on a mesh basket on topof the column of a filtration apparatus. There, an iced-cooled, sterile so-lution of 100 mM EDTA, pH 5, was allowed to drip over the sample at arate of 0.5 ml min�1, slowly dissolving the carbonate matrix. Cells thusexposed were removed by gently brushing the surface, allowed to flowwith the dripping EDTA solution, and collected on a (2-�m-pore-size)polycarbonate filter at the bottom of the column. Filters were washed withsterile distilled water (�5 ml) while still in the column, collected, andplaced in 5 ml of sterile PES medium. After gentle centrifugation (4,000 �g,10 min), filters were removed and cells were resuspended in sterile PESmedium. Wet mounts were prepared in glass slides for observation underbright-field optics in a compound microscope. Cell diameter measure-ments were performed with a calibrated ocular micrometer.

DNA extraction and amplification. Endolith biomass was extractedby EDTA dissolution as described above (25). Nucleic acids were thenextracted from the biomass of the same samples used in physiologicalexperiments and microscopy with the UltraClean Soil DNA kit (MoBioLaboratories, Inc., Solana Beach, CA) according to the manufacturer’srecommendations. After extraction, genomic DNA quantity and size weredetermined by electrophoresis on 1% agarose gels stained with ethidiumbromide. Approximately 10 ng of DNA extract was used as the template togenerate small-subunit rRNA gene (16S rRNA) amplicons by PCR ampli-fication. 16S rRNA fragments (ca. 700 bp long) were amplified using theprimer set CYA106F/CYA781R, which is specific for cyanobacteria (25).The thermal cycle consisted of an initial denaturation at 94°C for 5 min; 35cycles of 94°C for 1 min, 60°C for 1 min, and 72°C for 1 min; and a finalextension at 72°C for 5 min. Quantification of PCR products was per-formed as explained previously for genomic DNA.

Clone libraries. To gauge the diversity of euendoliths, clone librariesof the 16S rRNA gene PCR products were constructed separately for eachfragment used in the inhibitor experiments. These were generated by li-gating 16S rRNA amplicons into a TOPO 2.1 cloning vector and trans-forming chemically competent E. coli cells by following the manufactur-er’s instructions (Invitrogen). Ten individual colonies were selected fromthe library, and each was grown in 3 ml of Luria broth with kanamycin (50�g/ml) overnight at 37°C in a shaking water bath. A Qiagen Plasmid Prepkit was used to isolate the plasmids according to the manufacturer’s in-structions (Qiagen Inc.). EcoRI was used to check for the correct insertsize. Double-stranded plasmid DNA was sequenced in an Applied Biosys-tems 3730 sequencer.

Phylogenetic reconstructions. A total of 11 clones were obtainedfrom Italian samples, 16 from those from Mexico, 10 from those fromNew Zealand, and 8 from those from Puerto Rico. Sequences were alignedwith MEGA 4.0 (22) and checked for noncoding bases, which accountedfor 1% or less of the total sequence length. All noncoding ends werediscarded. These new sequences were aligned alongside other cyanobac-terial 16S rRNA partial sequences retrieved from the National Center forBiotechnology Information database using the basic local alignmentsearch tool. The cyanobacterial sequences most similar to our clones wereused to establish the initial alignment, and sequences representative of alltaxonomic groups of cyanobacteria (and plant plastids) were added topopulate the alignment; a betaproteobacterium sequence was used to rootthe phylogenetic tree. In all, 93 sequences were used to construct the mainphylogenetic tree, which contains all clones from all sites. All positionscontaining gaps and missing data were eliminated. Two algorithms wereused, neighbor joining (NJ) and the maximum parsimony (MP), bothwith 1,000 bootstrap replicates. Bootstrap values, or the percentage ofreplicate trees where a group of sequences clustered together, are shownnext to the respective nodes.

Nucleotide sequence accession numbers. Sequences obtained duringthis work have been submitted to GenBank and assigned accession num-bers JN810702 to JN810746.

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RESULTSMorphology of euendoliths. Morphogenera were identified bycomparison with traditional morphotaxa (4, 14). Figure 1 nonex-haustively illustrates their diversity in our samples. In the Italiansamples (IT) (Fig. 1, upper left panel), we found pseudofilamen-tous, nonheterocystous forms, resembling Hyella or Solentia, thatranged from 10 to 15 �m in diameter and displayed a range ofbluish-to-greenish colorations. We also found abundant thin, fil-amentous, nonheterocystous, Plectonema-like forms withtrichome diameters of about 2 to 3 �m. In the New Zealand frag-ments (NZ) (Fig. 1, upper right panel), we detected an unidenti-fied, thin, true-branching, apparently nonheterocystous form 2 to3 �m in diameter and of uncertain assignment. In this fragment,we also commonly found nonheterocystous, baeocyte-formingPleurocapsa- or Mysoxarcina-like morphotypes with various veg-etative cell diameters (5 to 15 �m). In the Mexican (MX) samples(Fig. 1, lower left panel), nonheterocystous, grayish, Hyella-likepseudofilaments 10 to 15 �m in cell diameter were common, ac-companied by 2- to 3-�m-thick, filamentous, nonheterocystous,Plectonema-like types of elongated cells. Other thin, filamentous,nonheterocystous forms with shorter cells 2 to 3 �m in diameterand blue-green in color were seen there as well. Lastly, in thePuerto Rican fragments (PR) (Fig. 1, lower right panel), filamen-tous, true-branching, heterocystous forms were observed thatranged from 5 to 10 �m in diameter, with the typical morphologyof Mastigocoleus, as well as filamentous, nonheterocystous formswith apparent baeocyte formation, ranging from 4 to 10 �m indiameter, whose assignment is more difficult.

Boring activity and its light dependence. Incubations in thelight were performed on all fragments to assess if Ca2� supersat-uration, a proxy for boring activity by phototrophic euendoliths(12), was reached at the surface of the substrate. All fragmentsexamined displayed this clearly (an example is shown in Fig. 2 ).Separate killed controls did not. Figure 3 depicts extracellularCa2� concentrations and their dynamic response to sequentialdarkening and illumination in the areas around boreholes, asquantified from a series of sequential images such as that shown inFig. 2. All samples exhibited significant supersaturation, with anaverage [Ca2�] of 20 to 60 mM in the light, and in all cases, dark-ening resulted in [Ca2�] relaxing back to the concentration of themedium in dissolution equilibrium with calcite; for PES mediumthis is around 10 mM (12). Upon sequential onset of illumination,again in all cases, there was clear recovery of supersaturation lev-els. Thus, all samples tested contained significant boring activity,most of which can be attributed to phototrophic organisms thatrequire light to power calcite dissolution. This, however, does notnecessarily require a biological calcium transport system to be atwork, in that similar results would have been obtained, for exam-ple, by localized and intense acidification of the medium, whichwould force the dissolution of calcite and the release of excessmetal.

Effect of Ca2�-ATPase inhibitors on boring. Figure 4 illus-trates the effect of the Ca2�-ATPase inhibitor TG on the surface[Ca2�]. Average Ca2� supersaturation levels on the fragmentsused for these experiments and before inhibitor addition rangedfrom 40 to 70 mM. TG was effective in inhibiting boring activity in

FIG 1 Morphological diversity of extracted euendoliths. Upper left (IT),forms similar to Hyella and Plectonema. Upper right (NZ), Plectonema-like(PL) and Pleurocapsa/Mysoxarcina (PM). Lower left (MX), Hyella (H) andPlectonema (PL) morphotypes. Lower right (PR), unclassified morphotypes,baeocyte-forming pseudofilaments (U), and Mastigocoleus-like filaments (M).IT, Italy; MX, Mexico; NZ, New Zealand; PR, Puerto Rico. Bars, 10 �m.

FIG 2 Surface of a shell fragment (sample from New Zealand, top view)containing a natural assemblage of live phototrophic euendoliths, as imagedby confocal laser scanning microscopy. Green fluorescence intensity denotesextracellular Ca2� concentration as reported by the fluorophore CG5N. Redautofluorescence is from chlorophyll and/or phycobilins. Yellow/orange fluo-rescence indicates colocalization of both signals. High Ca2� concentrationscolocalize with boreholes or near terminal cells of filamentous microborers.

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the NZ and PR fragments, with free Ca2� reaching saturationlevels (�10 mM) soon after the treatment. The IT and MX frag-ments were apparently not significantly affected, even at high con-centrations. Figure 5 illustrates the effects of the specific Ca2�-ATPase inhibitor TBHQ. The average Ca2� supersaturation levelsin all of the fragments used here ranged from 32 to 75 mM initially.Treatment with TBHQ resulted in effective inhibition of Ca2�

supersaturation in the IT, NZ, and PR samples. This effect wasdose dependent, with IT responding to 5 mM TBHQ but not to aprevious 1 mM treatment (data not shown). The MX samples didnot respond to TBHQ (up to 10 mM). Thus, all but the MX sam-ples were sensitive to at least one of the two P-type ATPase inhib-itors tested.

Clone libraries and phylogenetic reconstruction. The phylo-genetic placement of clone sequences was inferred using the NJand MP algorithms with 1,000 bootstrap replicates. Both algo-rithms generated trees with similar topologies; only the NJ tree isshown for the sake of simplicity. Figure 6 illustrates the phyloge-netic analysis of 93 16S rRNA partial sequences, including 45clones, 42 cyanobacteria, 3 plasmid sequences, and 1 member ofthe betaproteobacteria (Chromobacterium violaceum JCM 1249)as an outgroup. In order to ensure proper correlation, we used thespent samples from the inhibition experiments to obtain 16SrRNA gene clone libraries. A total of 11 sequences were obtained

FIG 3 Dynamics of Ca2� concentrations around the surface boreholes ofinfested carbonate fragments measured by laser scanning fluorescence micro-scopic imaging of live samples during shifts in illumination. Dotted lines rep-resent 1 standard deviation of the population mean (solid symbol) of at least 10measurements. IT, Italy; MX, Mexico; NZ, New Zealand; PR, Puerto Rico.

FIG 4 Dynamics of Ca2� concentrations around the surface boreholes ofinfested carbonate fragments measured by laser scanning fluorescence micro-scopic imaging of live samples and effect of the addition of the P-type Ca2�-ATPase inhibitor TG (1 mM). Dotted lines represent 1 standard deviation ofthe population mean (solid symbol) of at least 10 measurements. IT, Italy; MX,Mexico; NZ, New Zealand; PR, Puerto Rico.

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from Italy (Italy sequences), 16 were from Mexico (Baja se-quences), 10 were from New Zealand (NZ sequences), and 8 werefrom Puerto Rico (PR sequences). Eight clades (A, B, B1, C, D, E, F,and G) were assigned to clone sequences that cluster together.Clade A includes sequences allied to the heterocystous cyanobac-teria in taxonomic groups IV (order Nostocales) and V (orderStigonematales) and includes the sequence from model strainMastigocoleus sp. strain BC008. Clade B includes baeocyte-forming genera (Mysoxarcina/Pleurocapsa/Staniera) in group II

(order Pleurocapsales). Clade B1 contains sequences that fallwithin clade B but are not well resolved and do not have any closeneighboring sequence. Clade C contains group II sequences andincludes Chroocococcidiopsis and Solentia sp. strain HBC10, a de-

FIG 6 NJ phylogenetic tree based on 93 16S rRNA gene partial sequences (559bp). The sequence of C. violaceum JCM 1249 was used as an outgroup. Lettersdenote clades of euendolithic phylotypes. Colors indicate the geographic regionsof origin (red, Italy; green, Mexico; magenta, New Zealand; blue, Puerto Rico).Denominations of entries correspond to those given in the database and are notnecessarily taxonomically correct. Bootstrap values for 1,000 trees are indicated atthe nodes. The scale bar represents 1% estimated sequence divergence.

FIG 5 Dynamics of Ca2� concentrations around the surface boreholes ofinfested carbonate fragments measured by laser scanning fluorescence micro-scopic imaging of live samples and effect of the addition of the P-type Ca2�-ATPase inhibitor TBHQ (1 to 10 mM). Dotted lines represent 1 standarddeviation of the population mean (solid symbol) of at least 10 measurements.IT, Italy; MX, Mexico; NZ, New Zealand; PR, Puerto Rico.

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monstrably boring cyanobacterium (8). Clade D includes a clusterthat falls within group II but does not have a closely allied se-quence in databases. Clades E and F contain sequences exclusive tothe New Zealand sample which fall within group III (order Oscil-latoriales; nonheterocystous and filamentous). Clade E containsLeptolyngbya types (also nonheterocystous and filamentous), withits closest neighbor being Leptolyngbya sp. strain ITAC101 isolatedfrom a marine sponge; clade F does not have a closely matchingsequence in the databases. Clade G includes Leptolyngbya-typesequences (group III), with the closest neighbor being Leptolyng-bya sp. strain HBC1, an isolate from marine carbonate microbial-ites (8). Apart from the assigned clades, there are three clone se-quences, all likely allied to Leptolyngbya that stand alone: PR 10e,Baja 5h, and Baja 6a. From these, only Baja 5h has a close neigh-boring sequence pair (Leptolyngbya sp. strain PCC 7373 and Phor-midium sp. strain SAG 80.79).

DISCUSSIONMicroborer diversity. Microscopic observations of extracted eu-endoliths revealed a broad diversity of morphogenera in our sam-ple collection. While we acknowledge that present taxonomic di-visions of the cyanobacteria offer just a convenient, rather than aphylogenetically correct, view of diversity, it is still noteworthythat the euendoliths detected here encompassed 3 out of the 5major cyanobacterial taxonomic groups (group II, order Pleuro-capsales; group III, order Oscillatoriales; group V, order Stigone-matales), excluding at this time groups I and IV, Chroococcales andNostocales, respectively. Most common were pseudofilamentous,nonheterocystous, baeocyte-forming cyanobacteria (group II),which could be seen in all four of the geographical regions sur-veyed. This result is not surprising, as it is from this group II of thecyanobacteria that many morphogenera with microboring spe-cies, such as Cyanosaccus (17), Hyella (3, 16), Hormathonema (7,14), and Solentia (17, 18) have been described. The importance ofthis group is particularly evident in the sard samples, where all ofthe forms we found can be assigned to it, at least morphologically.In the fragments from New Zealand and Mexico, forms assignableto groups II and III were found. In Mexico, we found Hyella/Solentia-like cyanobacteria among others that closely resembledmembers of the genus Plectonema. In the New Zealand sam-ples, we found Pleurocapsa/Mysoxarcina-like forms amongPlectonema-like types. Plectonema is another genus that containsboring species (e.g., Plectonema terebrans; 4, 15). In the fragmentsfrom Puerto Rico, extracted filaments displayed a variety of mor-phologies, with many (pseudo)filamentous, nonheterocystoustypes capable of apparent baeocyte formation (likely group II),but we were unable to assign them accurately to an existing mor-phogenus. We also detected filamentous, true-branching formsthat displayed lateral heterocysts (group V, order Stigonematales)closely resembling the morphological characteristics of Mastigo-coleus, another genus with a single, microboring species, M. testa-rum.

This broad morphological diversity is partly consistent withthe variety of clades detected according to phylogenetic recon-structions of 16S rRNA clone libraries. Four out of eight cladesfound (B, B1, C, and D) were assigned to clusters of sequences thatfall within baeocyte-forming (group II) types. The relative abun-dance of sequences within this group, compared to all other phy-lotypes, indicates that the majority of euendoliths found are pseu-dofilamentous, nonheterocystous, and baeocyte forming, which

correlates with the common microscopic observations of Hyella-and Solentia-like forms and other baeocyte formers. Clade A in-cludes one sequence found only in Puerto Rico, and the sequenceof the model boring organism M. testarum BC008, which wasoriginally isolated from carbonates in this geographic region (5).This finding is not surprising, as it correlates fully with micro-scopic observations. The only conspicuous disagreement is in therelative diversity and abundance of Leptolyngbya-like, thin-filamentous cyanobacteria. Traditional studies have reported onlya single species of microborer among this group: Plectonema (Lep-tolyngbya) terebrans (4, 14, 15). Yet we found at least three distinctclades in our survey (E, F, and G) and a few singletons, which likelyrepresent several different generic entities. Obviously traditionalmorphological surveys have severely underestimated the diversityof thin-filamentous microborers. This is perhaps not surprising,since other studies have shown that much of the “hidden” geneticdiversity in cyanobacteria resides in the morphologically simplestforms, either filamentous or unicellular (10). Our phylogeneticdata suggest that many species, perhaps even genera within groupIII (Oscillatoriales), are capable of boring and that this diversityremains largely unexplored. No cultivated isolates of this type ex-ist currently in public culture collections. Some of the stand-aloneclone sequences from Mexico and Puerto Rico might correlatewith their respective yet-to-be-identified morphotypes. Thus, arenewed and sustained effort to find cultivated isolates for some ofthese clades and unidentified forms is necessary to fully describethe diversity of euendoliths in a comprehensive and useful man-ner.

Universality of the boring mechanism. Having establishedthat all samples were active with respect to boring activity and thatphototrophs accounted for most of the boring activity in thesesubstrates, the molecular surveys discussed above ensured that thediversity of microborers was large in all samples and covered manyof the groups of cyanobacterial microborers known from the lit-erature. Thus, the fact that in most instances at least one of theP-type Ca2�-ATPase inhibitors could abolish boring activity in-dicated that these enzymes are central to the process of boring inmost cyanobacterial euendoliths, as they are in strain BC008. Inthis sense, the mechanism seems to be quite widespread. Not un-expectedly, however, the degree of sensitivity differed among sam-ples. We knew from laboratory experiments that inhibitors suchas TG can work in some organisms and not in others (13), and thisis why our experimental design called for the use of two indepen-dent inhibitors. In our interpretation, sensitivity to at least one ofthe two implies the involvement of the target enzymes. Only inone case, the samples from the Sea of Cortez (MX), there was noresponse to either TG or TBHQ, even at high concentrations. Thisprevents us from calling the response universal. The insensitivitymay speak for the presence of a different boring mechanism in theeuendoliths of this sample. If so, then we would need to postulatethat a group of cyanobacteria that is highly represented in theMexican clone libraries, but not in others, is probably responsiblefor the alternative behavior. Clade B1 seems to be the only groupfitting this criterion. Alternatively, clade B1 may possess a form ofcalcium transporters that are particularly insensitive to the inhib-itors used. Unfortunately, no cultivated representatives of thisclade are yet available to test the hypothesis of functional differ-ences.

The empirical analysis of the boring mechanism carried outhere with geographically distinct, complex euendolithic microbial

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assemblages reveals similar physiological response to core exper-iments performed with M. testarum BC008, suggesting that theboring mechanism, as it is understood in the latter, is probablywidespread, if perhaps not universal. Our work also revealed thatthe genetic diversity of cyanobacterial euendoliths, particularlythe nonheterocystous filamentous form, has been underestimatedin the past and provides a phylogenetic basis for the study of cya-nobacterial euendolith biodiversity.

ACKNOWLEDGMENTS

We thank Dörte Hoffmann for assistance in the construction of clonelibraries, as well as Ankita Kothari and Carmen M. Reinat for assistancewith sample collection.

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