GLOETRICHIA-ECHINULATA, A COLONIAL CYANOBACTERIUM WITH A UNIQUE PHOSPHORUS UPTAKE AND LIFE STRATEGY

Epilimnetic colonies of Gloeotrichia echinulata were harvested from 200-300 l of water in Lake Erken with filtration through appropriate plankton nets (200 or 70 mum). Phosphate uptake characteristics, phosphorus (P) status and photosynthesis of the colonies were determined twice a week during July and August 1991. Phosphate uptake was analysed according to the simple force-flow relationship of Falkner et al. (Arch. Microbiol., 152, 353-361, 1989). The threshold concentration of P uptake below which uptake ceases for energetic reasons, was constantly much higher than the epilimnetic soluble reactive phosphorus (SRP) concentration, so that the planktonic colonies were unable to acquire any P in the epilimnion. Neither did organic P seem to be a source of P for planktonic colonies. Gloeotrichia echinulata has a unique life strategy in comparison to other common genera of bloom-forming cynanobacteria. Its P assimilation and growth are completely separated both in time and space; growth is preceded by benthic P assimilation. Epilimnetic growth was based solely on internally stored P and growth rates fitted the Droop model well. Depletion of stored P restricts the length of the planktonic phase to 15-20 days under 'optimal' growth conditions. Wind-induced surface drift seemed to be the most important loss factor from the epilimnion. Massive recruitment of P-rich benthic colonies accounted for two-thirds of total net internal P loading observed between mid-July and mid-August (3.8 mg P m-2 day-1).