2012JSB

Visualization of sub-daily skeletal growth patterns in massive Porites corals grown in Sr-enriched seawater

ストロンチウム高濃度海水で飼育した塊状ハマサンゴによる日輪形成の可視化

We performed high resolution marking experiments using seawater with elevated Sr concentration to investigate the timing and ultrastructure of skeletal deposition by massive Porites australiensis corals. Corals were cultured in seawater enriched with Sr during day-time only, night-time only or for one full-day. Cross sections of skeletal material were prepared and the Sr incorporated into the newly deposited skeleton analyzed by electron probe microanalysis. These regions of Sr incorporation were then correlated with skeletal ultrastructure. Massive Porites coral skeletons are composed of two types of microstructural elements – the “centers of calcification” and the surrounding fibrous structural region. Within the fibrous structural region, alternative patterns of etch-sensitive growth lines and an etch-resistant fibrous layer were observed. In the full-day samples, high-Sr bands extended across both growth lines and fibrous layers. In day-time samples, high-Sr regions corresponded to the fibrous layer, while in the night-time samples high-Sr regions were associated with an outermost growth line. These distinct growth patterns suggest a daily growth pattern associated with the fibrous region of massive P. australiensis corals, where a pair of narrow growth lines and a larger fibrous layer is seen as a daily growth region.

塊状ハマサンゴの骨格の微細骨格構造形成とそのタイミングを調べるために，ストロンチウム濃度を高くした海水を用いて高解像度マーキング実験を行った．サンゴを昼間，夜間，丸一日，ストロンチウム濃度を高くした海水中で飼育した．その骨格断面を作成し，ストロンチウムを取り込んだ新たな成長部位を電子線プローブマイクロアナライザーで分析し，特定された成長部位と微細骨格構造とを比較した．ハマサンゴ骨格は”center of calcification（石灰化中心）”と呼ばれる部位と，その周りを囲む繊維状骨格部位の2つの微細骨格構造で形成されていた．繊維状骨格部位はさらに，酸に溶けやすい細い成長線と酸に溶けにくい繊維状結晶層が交互に重なり合っている構造であった．丸一日の飼育実験では，ストロンチウムの濃集部位は繊維状結晶層と成長線にまたがって見られた．昼間の飼育実験では，ストロンチウムの濃集部位は繊維状結晶層に見られ，夜間の飼育実験では，ストロンチウムの濃集部位は最も外側の成長線に見られた．このような明瞭な成長パターンは塊状ハマサンゴの繊維状骨格部位において，日周輪が繊維状結晶層と成長線を一対として形成されている事を示している．

丸一日の飼育実験結果 Full-day experiment

夜間の飼育実験結果 Night-time experiment

日中の飼育実験結果 Day-time experiment