Extracellular calcification of Braarudosphaera bigelowii deduced from electron microscopic observations of cell surface structure and elemental composition of pentaliths

• B. bigelowii has a haptonema, and can use it for adhesion to external substrates.
• B. bigelowii has a unique cell surface structure called pentalith-substrate.
• Pentalith-substrate and organic layers probably play some roles in calcification.
• Pentalith calcification probably takes place extracellularly
• Pentaliths contain relatively higher amounts of Mg than typical heterococcoliths.

We have performed morphological and crystallographic studies of Braarudosphaera bigelowii using various light and electron microscopy techniques. A study by light microscopy revealed that B. bigelowii has a haptonema, and can use it for adhesion to external substrates. A study of the pentaliths by transmission electron microscopy indicates that the well-known trapezoidal lamina is formed with foliate crystals having perfectly identical crystallographic orientation. A cytological study shows that the pentaliths of B. bigelowii are surrounded by an organic structure consisting of a pentalith-substrate and thin organic layers. The pentalith-substrate underlies the proximal surface of the pentaliths and extends between the sides of the individual pentaliths, it also extends between the five trapezoidal segments forming a pentalith. Thin organic layers, which apparently originate from ridges of pentalith-substrate, cover the distal surface of the trapezoidal segments. The close association between the pentalith-substrate, organic layers, and pentaliths leads us to the hypothesis that calcification of the pentaliths occurs between the pentalith-substrate and organic layers, extracellularly. The relatively high Mg content observed in pentaliths supports our hypothesis of extracellular calcification.