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.