Photosynthesis and calcification of the stromatolitic freshwater cyanobacterium Rivularia

Figures & data

Fig. 1. Features of Rivularia colonies. (a) Diagram of a vertical section through a mature Rivularia colony showing orientation of the trichomes and two bands of calcification. (b) Sampling block for biovolume and CaCO₃ content. (c) Part of the Rivularia colony showing trichomes enclosed by a branched mucilaginous sheath. (d) Vertical view of a Rivularia colony showing trichomes (filled circles) surrounded by firm mucilaginous sheaths (stippled) enclosed in highly hydrated and diffluent mucilage in which calcification occurs (white).

Table 1. Water chemical composition at the two sampling sites. Measurements, excluding pH and temperature, in mmol l⁻¹ unless otherwise stated.

Determinand	Gordale Beck	Kettlewell site
Temperature °C	14	12
pH	8.12	8.29
Ca	1.87	1.68
Mg	0.055	0.24
Na	0.16	0.14
K	0.006	0.012
CT*	3.89	3.93
SO4	0.082	0.11
Cl	0.19	0.17
NO3 (µmol l^−1)	21	14
Ω^**	4.5	6.8
*Total dissolved carbon dioxide, >95% of which is , the remainder being dissolved molecular CO2 and . ^**Calcite saturation index, see text for explanation. Sampled June 2005.

Fig. 2. Trichome density in Rivularia colonies. (a) Number of trichomes per mm² of surface vs depth within colony (mm). (b) Trichome relative biovolume vs depth. Filled circles, R. haematites colonies, open circles, R. biasolettiana.

Fig. 3. Calcium carbonate content (vol. %) of Rivularia with depth. (a) R. haematites. (b) R. biasolettiana. Accurate values are not available for deepest parts of colony owing to breakdown of mucilage. Measurements averaged from duplicate samples taken from single colony.

Fig. 4. Light absorption in Rivularia colonies as a function of depth. Filled circles, R. haematites; open circles, R. biasolettiana. Linear regression lines for R. haematites (full) and R. biasolettiana (broken) are included.

Fig. 5. Dissolved oxygen concentration in a R. haematites colony. Porphyrin planar optode at a surface irradiance of 500 µmol m⁻² s⁻¹. The thin black line indicates the colony surface. Silhouette of colony section shown below.

Fig. 6. Oxygen profile in a R. biasolettiana colony. Mean of three replicate runs showing standard errors at 1 mm intervals. Ordinate gives distance from colony surface in mm. For oxygen, only alternate datapoints are shown on this figure; the original resolution was 50 µm. Filled circles: microelectrode % oxygen. Open circles: pH at a resolution of 100 µm. Irradiance 500 µmol m⁻² s⁻¹ for both measurements.