the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Apr 2026
Biosignatures of microbial mats in Pleistocene coral reef cores from IODP Expedition 389 (Hawaiian Drowned Reefs)
Abstract. We systematically document surfaces and biosignatures of Pleistocene reefal microbialites (Marine Isotope Stages 7–6) recovered during IODP Expedition 389 (Hawai’ian Drowned Reefs). Microbialites are abundant within Pleistocene coral reef successions and offer valuable archives of environmental information under Quaternary climate variability. However, relatively little is known about biofilm-forming microbial consortia, because biosignature preservation is usually very poor. The microbial crusts studied here form encrustations as much as 20 cm thick, ranging from laminated to thrombolitic, within the coral reef framework. Scanning electron microscopy (SEM) of samples from the windward (humid) Hilo and Kohala and the leeward (arid) Kawaihae sides of the Island of Hawai‘i reveals exceptionally well-preserved microbial fabrics that developed during Marine Isotope Stages (MIS) 7–6. Humid side samples exhibit abundant preserved putative exopolymeric substance (EPS) matrices, mineralized filaments, and near spherical, multilobate aggregates that resemble protodolomite spherules formed by modern cyanobacteria or extant coccoid cyanobacteria (e.g., Gloeocapsa-type). In either case, the surfaces appear to have been formed with significant aid of cyanobacteria suggesting formation in a euphotic setting. The microbialites from the arid side display peloidal microfabrics with fewer preserved physical biosignatures, typical of cryptic reefal microbialites, but the surfaces suggest confinement by an organic biofilm. The occurrence of pyrite framboids and huntite-like crystals in the wet-side samples suggests local redox gradients consistent with both sulphate reducing bacteria and cyanobacteria mediating carbonate precipitation. These findings provide the first direct evidence for euphotic microbial mat communities contributing to microbialite formation in Pleistocene drowned Hawai’ian reefs, and, to our knowledge, in Indopacific reefs and beyond. The outstanding preservation of mineralized EPS and microbial morphotypes highlights the potential of these Pleistocene reefal microbialites as sensitive archives of palaeoenvironmental conditions and microbial diversity under glacio-eustatic forcing and associated environmental changes.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1564', Nora Noffke, 30 Apr 2026
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RC2: 'Comment on egusphere-2026-1564', Joachim Reitner, 27 May 2026
The authors describe intriguing various microbial mats mineralized in high-Magnesium calcite and aragonite with stromatolitic structures from Pleistocene reef systems (MIS 7–6) in Hawaii (Big Island), which were collected during the IODP 389 expedition. They describe a variety of mineralized structures, including possible bacterial remains such as filaments and cell-like structures, which they attribute to cyanobacteria. Samples were collected on the rainy side of the island near Hilo and the arid side of the island near Kawaihae, which exhibit significant microfacies differences. In carbonate systems, the preservation of morphologically intact microbial cells is rather rare and, when it occurs, is due to specific taphonomic processes based on high concentrations of divalent cations (e.g., Ca²⁺, Mg²⁺) and significantly increased carbonate alkalinity. These factors are presumably responsible for a very rapid collapse of the inhibitory function of EPS. Typical examples are the calcimicrobes, including Girvanella, most known from the Paleozoic. The authors describe a variety of structures, but it is difficult to make sense of all the details. It is also unclear what they mean by “preserved EPS”? An important point. Biochemically, EPS molecules are predominantly acidic polysaccharides and proteins (e.g., lectins), which typically serve a matrix function during mineralization and prevent metabolic ion-exchange processes between microbial cells and the surrounding environment from rapidly blocking the cell surfaces. Stromatolitic and so-called thrombolitic structures are a result of this process. The structures observed here are the result of this metabolic process. Possible organic EPS residues were not analyzed; an attempt would be worthwhile if they are discussing the preservation of EPS (e.g., analysis of sugar residues and protein residues, such as amino acids). A major shortcoming of this very interesting study is that no biogeochemical analyses were performed, nor were any detailed geochemical or isotopic analyses. EDX and XRD analyses are helpful but do not adequately substitute for these. Microbial mats cannot be validly reconstructed using morphological analyses alone. These precise data are also needed to reconstruct the original environment. The layered microbialites would also allow for a high-resolution environmental reconstruction.
It is interesting to note that so-called cryptic aphotic microbialites are observed in the arid zone. They consist predominantly of peloidal structures, as are typical for protein-rich microbial mats, often associated with flat-growing sponges. In this system, framboidal pyrite occurs, which likely originates from microbial sulfate reduction. Furthermore, the authors describe huntite-like crystals, a Ca-Mg carbonate, detected traces with XRD analyses.
The authors describe in great detail two paleoecologically interesting microbial communities from Pleistocene reefs around Big Island (Hawaii). These microbial post-reef systems are frequently observed and represent an important process in the reef carbonate factory. These structures can also be identified in many fossil reef structures. The authors could cite examples here. As already mentioned, a detailed geochemical and biogeochemical analysis of the structures found is missing to support the microbial nature of the presumed microbes.
In any case the presented paper is well organized, the quality of the figures is excellent and the reference list is state of the art. However, I recommend the use of a Raman spectrometer for determining the spatial mineralogy (e.g., Mg-calcite, huntite-dolomite, etc.) as well as for Corg+Carb analyses. I also consider the analysis of stable isotopes (δ13C and δ18O), and very important, lipid chemical analyses (e.g. Pyrolysis-GC-MS, good for small sample amounts) of the various microbialite structures. This should make it possible to distinguish microbial biomarkers and perhaps also the differentiation of phototrophic, sulfate-reducing, and general heterotrophic bacteria and other microbes.
Citation: https://doi.org/10.5194/egusphere-2026-1564-RC2
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The manuscript describes microbial mats dominated by photoautotrophic microbes in modern Hawaiian reef systems. The microbial mats develop in a carbonate system. Such systems are commonly not known to preserve microbial textures such as filaments, EPS, and others. The reason is a quick recrystallization of the carbonate. However, this material presented by Westphal et al. shows a multitude of microscopic textures that allows reconstructing of the ancient microbial community and its interaction with the past environmental conditions. The manuscript offers a highly detailed description of these features. The discussion is based on many references relevant. Overall, the study is convincing and sheds light on microbial mat composition in a carbonate system. However, the text is a bit difficult to read, and some minor revisions should be considered. The Study Are section includes already some results that should be moved into the results section. There are also so many illustrations that it is difficult to navigate the text. I suggest trying to reduce the ilustations and descriptions to some major features that are typical for the individual layers (if possible). Pu your best foot forward and chose the best examples. Also, please note that figures and panels should be discussed in the text in their order, e.g. Fig. x A shows...; Fig. X B is a ... etc. This allows the reader to follow along. I made a few suggestions in the attached PDF.