Gloeotrichia echinulata, a colonial cyanobacterium with a unique phosphorus uptake and life strategy

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Abstract
Epilimnetic colonies of Gloeotrichia echinulata were harvested from 200–300 I of water in Lake Erken with filtration through appropriate plankton nets (200 or 70 μm). 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 Pin 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 ).

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