Cultivation and inorganic carbon uptake of the rare desmid Oocardium stratum (Conjugatophyceae)

Abstract:

The desmid Oocardium stratum is sporadically and exclusively found in active meteogene travertine habitats. The restriction to these peculiar biotopes is probably linked to specific carbon requirements. We studied inorganic carbon (C_i) uptake under controlled laboratory conditions. First, we developed a method for isolating and cultivating O. stratum. Colonies visible to the naked eye (green pinhead-shaped formations) were collected from a tufa spring in Lower Austria. We then grew unialgal cultures on solidified agar prepared with Wood's Hole culture medium, buffered with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) to pH 7 and enriched with CaCO₃. Scanning and transmission electron microscopy were applied to study the condition of cells shortly after they started in vitro growth. For C_i uptake, pH-drift experiments were performed including the external carbonic anhydrase inhibitor acetazolamide and the inhibitor of membrane-bound adenosine triphosphatase (ATPase) sodium orthovanadate. We compared the C_i affinity of Oocardium to Acutodesmus acuminatus and Chlorella sp. grown under identical conditions. Whereas A. acuminatus was common in slightly alkaline, eutrophic surface waters, Chlorella sp. was isolated from a soil sample. Owing to its peculiar habitat with abundant free CO₂, we expected a clear preference for CO₂, no enzymatic catalysis of C_i uptake and absence of active HCO₃⁻ uptake for Oocardium. Our experiments confirmed these hypotheses. In contrast to A. acuminatus, we found no HCO₃⁻ uptake in Chlorella sp. and O. stratum. Moreover, enzyme inhibition assays did not suggest activity of those enzymes that are commonly responsible for active C_i uptake.

Key Words::

• Acetazolamide (AZ)
• Algae cultivation
• Carbon uptake
• Oocardium
• pH drift
• Sodium orthovanadate (VAN)

ACKNOWLEDGEMENTS

We thank the staff of the Wassercluster Lunz GmbH for providing basic equipment and laboratory space. Caroline Linhardt kindly assisted in the field work. Thanks to Bernhard Braun for providing isolates of Chlorella sp. EM was supported by the considerate help of Marie Luise Weidinger. Thanks to Elly Spijkerman for her patience during discussions and her valuable hints regarding the experimental setup.