Ecology, morphology and physiology of Chroothece richteriana (Rhodophyta, Stylonematophyceae) in the highly calcareous Río Chícamo, south-east Spain

Abstract

The ecology of Chroothece was studied in the highly calcareous Río Chícamo, south-east Spain, in order to explain its success there, but rarity elsewhere. The river, which originates mainly from an underground aquifer, has water with high conductivity, sulphate and nitrate but low phosphate concentrations, the latter mainly organic. Chroothece occurs in mats and in lobed colonies reaching 4 cm in the broadest dimension. The colony surface consists of one layer of cells, each of which is attached to a stalk, which dichotomizes when the cell divides; stalks often extend to the colony base. The central region of many mat cells and almost all colony cells has a yellow to orange-brown colour, associated with the numerous lipid droplets densely covering the surface of the pyrenoid and arms of the star-shaped chloroplast. Field material and laboratory isolates indicate that stalk formation occurs under moderate P limitation and both stalks and cell sheath show high phosphatase activities. This also occurred in a culture collection strain maintained for 30 years in a very P-rich medium, but then transferred to a moderately P-limiting medium (c. 0.9 mg l⁻¹). We suggest that colony formation is initiated by aggregation of motile cells following P pulses in the water. Comparisons are made with Rivularia, a competitor in this nitrate-rich river, in spite of being a N₂-fixer. One difference is that Chroothece cells lie at the periphery of the colonies and are therefore exposed to maximum sunlight, whereas Rivularia trichomes grow inside colonies with photoprotection by scytonemin. However, the ability to withstand heavy grazing pressure may be an especially important factor favouring Rivularia here.

Key words:

• calcareous Chroothece
• colony
• lipid
• organic phosphatase activities
• phosphate
• river
• Rivularia
• semi-arid

Acknowledgements

We thank the Seneca Foundation of Murcia Region for their support (grant 05762/PI/07) and Consejería de Universidades, Empresa e Investigación of Murcia Region (PEPLAN-S5). Sulphate and ethylene were measured at the Servicio de Instrumentación Científica de la Universidad de Murcia using ion and gas chromatography (DIONEX ICS-2000, AGILENT 7683B), and some microscopy done at the Servicio Microscopía de la Universidad de Murcia and Serveis Cientificotècnics de la Universidad de Barcelona. Joaquín Hernández Bastida helped with elemental and sulphate analysis of colonies. We are also most grateful to those who helped in various ways, including sampling and laboratory measurements by José Pedro Marín Murcia, Maribel Sánchez and also Dr Allan Pentecost of the Freshwater Biological Association, Cumbria, UK and Dr Neil T.W. Ellwood of Universita Roma Tre, Italy, for useful discussions.