Ecophysiological influence of light and mixing on Anabaena circinalis (Nostocales, Cyanobacteria)

Abstract

The physiology (growth rate, biochemical composition and flotation rate) of the toxic cyanobacterium Anabaena circinalis was studied in relation to irradiance and mixing regime in two small-scale (250 ml or 1 l) batch culture experiments and one medium scale (1 m high, 4.4 l), semi-continuous microcosm experiment. In batch cultures, A. circinalis had a relatively high irradiance requirement as indicated by its compensation irradiance, E _c (13 ± 2 μmol m⁻² s⁻¹). The growth efficiency (α_g) of A. circinalis, estimated as the initial slope of the growth-irradiance curve , was in the mid to high range depending on the model applied (Monod: 0.33 ± 0.19 m² mol⁻¹, exponential: 0.43 ± 0.15 m² mol⁻¹). The flotation rate of the A. circinalis population was 0.69 ± 0.11 m d⁻¹ at 100 μmol m⁻² s⁻¹ and decreased at higher irradiance to be negative at irradiances over 135 μmol m⁻² s⁻¹. In a microcosm experiment, different mixing intervals were tested (10 min (MIXED) and 48 h (CALM)) across three irradiance treatments. The irradiance treatments differed in both their degree of vertical light attenuation and mean integrated irradiance. The mixing regime had no effect on growth rate in the high irradiance (HE) and medium irradiance (ME) treatments (mean integrated irradiances 60 and 15 μmol m⁻² s⁻¹, respectively). However, an increase in A. circinalis flotation rate in the lowest irradiance treatment (LE; mean integrated irradiance 7 μmol m⁻² s⁻¹) resulted in a 44% higher growth rate in the CALM microcosm through access to greater irradiance. This suggests that short stable periods (e.g. diurnal stratification) in highly light limited treatments will assist the growth of A. circinalis due to its ability to gain ready access to light in the euphotic zone.

Keywords:

• carbohydrate
• carbon
• compensation irradiance
• cyanobacteria
• growth rate
• flotation rate
• irradiance
• microcosm

Acknowledgements

We thank Ian Jameson, two anonymous reviewers and Prof. Matthew Dring for providing comments that substantially improved the manuscript. MM is grateful to CSIRO Marine Research Laboratories, Hobart, where this work was undertaken and to University of Tasmania, School of Aquaculture, Launceston. This work was funded by Land and Water Resources Research and Development Corporation NEMP grant CSF-1.