Preprints
https://doi.org/10.5194/bg-2021-140
https://doi.org/10.5194/bg-2021-140

  03 Aug 2021

03 Aug 2021

Review status: this preprint is currently under review for the journal BG.

A stable ultrastructural pattern despite variable cell size in Lithothamnion corallioides

Valentina Alice Bracchi, Giulia Piazza, and Daniela Basso Valentina Alice Bracchi et al.
  • Department of Earth and Environmental Sciences, University of Milano-Bicocca, Milan, 20126, Italy

Abstract. Recent advances on the mechanism and pattern of calcification in coralline algae lead to contradictory conclusions. Coralline calcification appears biologically induced, as suggested by the dependency of its elemental composition on environmental variables. However, evidence of a biologically controlled calcification process, resulting in distinctive patterns at the scale of family, was also observed. In order to clarify the matter, five collections of Lithothamnion corallioides from the Atlantic Ocean and the Mediterranean Sea, across a wide depth range (12–66 m) have been analyzed for morphology, anatomy and cell wall crystal patterns of both perithallial and epithallial cells, in order to detect possible ultrastructural changes. L. corallioides shows the alternation of tiers of short-squared and long-ovoid/rectangular cells along the perithallus, forming a typical banding. The perithallial cell length decreases according to water depth and growth-rate, whereas diameter remains constant. Our observations confirm that both epithallial and perithallial cells show primary (PW) and secondary (SW) calcite walls. Rectangular tiles, with the long axis parallel to the cell membrane forming a multi-layered structure, characterize the PW. Flattened squared bricks characterize the SW with roundish outlines enveloping the cell and showing a zigzag pattern. Long and short cells have different thickness of PW and SW, with a thicker SW and PW in short cells. Epithallial cells are one up to three flared cells, with the same shape of the PW and SW crystals. Despite the diverse seafloor environments and the variable L. corallioides growth-rate, the cell walls maintain a consistent ultrastructural pattern, with unaffected crystal shape and arrangement. A comparison with two congeneric species, L. minervae and L. valens, showed similar ultrastructural patterns in SW, but evident differences in the PW crystal shape. Our observations point to a biological control rather than an induction of the calcification process in coralline algae, and suggest a possible new morphological diagnostic tool for species identification, with relevant importance for paleontological application. Finally, secondary calcite, in form of dogtooth crystals that fill the cell lumen, has been observed. It represents a form of early diagenesis in living collections which can have implications in the reliability of climate and paleoclimate studies based on the geochemistry techniques.

Valentina Alice Bracchi et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2021-140', Anonymous Referee #2, 20 Aug 2021
  • RC2: 'Comment on bg-2021-140', Anonymous Referee #1, 23 Aug 2021

Valentina Alice Bracchi et al.

Valentina Alice Bracchi et al.

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Short summary
Crustose coralline algae, important bio-engineers in marine environment, secretes high-Mg calcite crystals forming cell walls with different shape and orientation. The precise description of such structures in Lithothamnion corallioides from different seas (Atlantic and Mediterranean) and depths, reveals that crystals are unaffected by different environmental conditions of the living sites, suggesting that the biomineralization process is biologically-controlled rather than induced.
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