Morphological record of oxygenic photosynthesis in conical stromatolites

Abstract

Conical stromatolites are thought to be robust indicators of the presence of photosynthetic and phototactic microbes in aquatic environments as early as 3.5 billion years ago. However, phototaxis alone cannot explain the ubiquity of disrupted, curled, and contorted laminae in the crests of many Mesoproterozoic, Paleoproterozoic, and some Archean conical stromatolites. Here, we demonstrate that cyanobacterial production of oxygen in the tips of modern conical aggregates creates contorted laminae and submillimeter-to-millimeter-scale enmeshed bubbles. Similarly sized fossil bubbles and contorted laminae may be present only in the crestal zones of some conical stromatolites 2.7 billion years old or younger. This implies not only that cyanobacteria built Proterozoic conical stromatolites but also that fossil bubbles may constrain the timing of the evolution of oxygenic photosynthesis.

Fig. 1.

Center of a conical stromatolite [Atar Formation, Atar Group, Mauritania, Late Mesoproterozoic (53, 54), sample provided by A. Maloof]. (A) Mosaic photomicrograph of the central zone (thin section). The white rectangle surrounds the area shown in B, and the arrows point to fossil bubbles. (Scale bar, 1 cm.) (B) A typical nearly-circular submillimeter feature surrounded by dark laminae in the middle of the stromatolite (white arrow). This fossil bubble is found on top of a larger, irregularly shaped feature surrounded by contorted laminae (outlined by a dashed line) that may be a former mat-trapped bubble. (Scale bar, 1 mm.) (C) Fossil bubbles are absent from laminae on the sides of the same conical stromatolite. (Scale bar, 1 mm.) (D) Histogram showing the diameters (in millimeters) of fossil bubbles in 18 well-preserved Proterozoic conical stromatolites. Bubbles were identified as submillimeter- and millimeter-diameter features with nearly-circular cross-sections enclosed by laminae.

Fig. 2.

Oxygen and bubble production in modern conical aggregates dominated by cyanobacteria. (A) Bubbles often form at the tips of cones and are partly or completely covered by biofilm. Note that these bubbles did not lift the mat up into a conical structure. (Scale bar, 5 mm.) (B) Profiles of oxygen (filled circles) and gross photosynthetic rate (white bars) and oxygen uptake rate (gray bars) within a cone. Depth 0 mm denotes the tip of the cone and −6 mm is the bottom of the cone. (C) Transmitted-light micrograph of a 30-μm-thick thin section of a cone showing disrupted fabrics, large voids (former blisters), and bubbles in the center. Multiple pores surrounded by cyanobacterial filaments were present in the series of successive thick sections, confirming that the porosity was not an artifact. The sample had an elliptical cross-section, and it was sectioned along the minor axis. (Scale bar, 1 mm.)

Fig. 3.

Some of the oldest conical stromatolites with possible bubble-like features in the central zone. (A) Central zone of a conical stromatolite from Lime Acres Formation at Lime Acres, Griqualand, South Africa (2.52–2.55 Ga) (21) (Scale bar, 2 mm.) (B) Small diapirs surrounded by contorted laminae at the crests of a conical stromatolite from the Meentheena Member of the Tumbiana Formation, Australia (≈2.7 Ga) (25).