Galaxea, Journal of Coral Reef Studies
Online ISSN : 1883-3969
Print ISSN : 1883-0838
ISSN-L : 1883-0838
22 巻 , 1 号
選択された号の論文の10件中1~10を表示しています
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Original paper
  • Kazuhiko FUJITA, Toru SASAKI, Sho KOYANO, Masaaki CHINEN, Chuki HONGO, ...
    2020 年 22 巻 1 号 p. 9-25
    発行日: 2020年
    公開日: 2020/10/02
    ジャーナル フリー
    電子付録

    Reefal microbial crusts (RMCs) are fine-grained, non-skeletal carbonate crusts coating coralgal reef frameworks, and are locally common in late Quaternary reef deposits. They are interpreted as microbial carbonates produced by the growth and metabolism of benthic bacterial communities. Key questions remain concerning their uneven spatio-temporal distributions, formation process and controlling factors; the crusts have not yet been reported in the Ryukyu Archipelago. Here we report the first occurrence of brownish, few-centimeter-thick, fine-grained, non-skeletal crusts in a Middle Holocene reef core recovered at Naha New Port Pier, off the western coast of Okinawa Island, the Ryukyu Archipelago. The outer morphology of the crusts is either knobby or flat. The meso-scale fabric of the crusts is generally clotted and structureless, while a few crusts are weakly laminated or digitate. The slab core shows a biological succession from a bioeroded coral oriented upward, overlain by thin crusts of coralline algae and encrusting foraminifers, ending in a brownish, fine-grained, non-skeletal crust. Surface elemental mapping shows that the crusts are composed mainly of Ca and Mg (i.e., Mg-containing carbonate). X-ray diffraction analysis indicates that the crusts are composed predominantly of high-magnesium calcite, subordinate with aragonite and quartz. Petrographic observations show that the crusts are made mainly of peloidal micrite with irregular voids (cavities), associated with silt-sized bioclastic and siliciclastic grains. The crusts develop within a particular core depth and age range (4.6-6.1m depth; ca. 7ka), from which the crusts change downward and upward into encrusting bryozoan and foraminiferal crusts filled with micrite. Based on our observations, compared with previous studies, we conclude that brownish, non-skeletal carbonate crusts found in this study are RMCs, similar to those found in the last glacial, last deglacial and Holocene reef deposits from other coral reef regions (e.g., Tahiti and Great Barrier Reef). The RMCs likely developed in a low light/darker, semi-enclosed environment within primary cavities of high-energy, shallow-water coralgal frameworks. Since the study area is located adjacent to a river mouth and directly exposed to terrigenous sediment input from river runoff, multiple, local and global environmental factors associated with Holocene transgression and reef formation likely influenced the development of RMCs in the study area.

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Original paper
  • Sushma MATTAN-MOORGAWA, Pramod Kumar CHUMUN, Nawsheen TALEB-HOSSENKHAN ...
    2020 年 22 巻 1 号 p. 37-50
    発行日: 2020年
    公開日: 2020/12/30
    ジャーナル フリー

    This study examined the effects of thermal stress (28 and 31°) and biochemical stress (sponge Haliclona madrepora and ascidian Didemnum molle crude methanolic extracts) on density of coral tissue balls (TBs) and on their symbiont photosystem II functioning. Coral TBs were obtained from scleractinian corals harbouring symbionts Cladocopium [Lithophyllon repanda, Pocillopora damicornis and Acropora muricata] and symbionts Durusdinium [Acropora muricata]. Thermal stress experiments at 28 and 31°C tended to have negative effect on TB density from A. muricata (harbouring Cladocopium or Durusdinium) and L. repanda (harbouring Cladocopium) but not from P. damicornis (harbouring Cladocopium). Effective quantum yield, ΦPSII, decreased at temperature 31°C in A. muricata (Cladocopium) but remained stable in A. muricata (Durusdinium). Combined thermal (31°C) and biochemical (50µg ml-1 D. molle extract) stressors had a relatively more pronounced effect on TB density, but did not affect ΦPSII in the three species, irrespective of symbiont genus. Thermal stress and 200µg ml-1 D. molle affected ΦPSII in A. muricata (Cladocopium) and L. repanda (Cladocopium) as compared to P. damicornis (Cladocopium). Thermal stress and 50µg ml-1 H. madrepora affected TB density in three coral species but caused a drop in ΦPSII only in A. muricata (Cladocopium). Thermal stress and 200µg ml-1 H. madrepora affected both TB density and ΦPSII in the three tested coral species. These results suggest a variable susceptibility among corals, with A. muricata (Cladocopium) being the most susceptible and P. damicornis (Cladocopium) the least susceptible to studied thermal and biochemical stressors.

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