Alkaline phosphatase activities among populations of the colony-forming diazotrophic cyanobacterium Trichodesmium spp. (cyanobacteria) in the Red Sea

Alkaline phosphatase activities of the diazotrophic marine cyanobacterium Trichodesmium were studied among natural populations in the northern Red Sea and in laboratory cultures of Trichodesmium sp, strain WH9601, Open-water tuft-shaped colonies of Trichodesmium showed high alkaline phosphatase activities with 2.4-11.7 mu mol p-nitrophenylphosphate (PNPP) hydrolyzed.mug chl a(-1).h(-1), irrespective of date or origin of the sample. Coastal populations of the Trichodesmium tuft colonies had low alkaline phosphatase activities with 0.2-0.5 mu mol PNPP.mug chl a(-1).h(-1). An exception was the Trichodesmium fall maximum, when both tuft colonies and the plankton community (< 100 mum) had alkaline phosphatase activities of 0.6-7.4 mu mol PNPP.mug chl a(-1).h(-1). Likewise, the more rare puff and bowtie colonies of Trichodesmium spp. in coastal waters had elevated alkaline phosphatase activities (0.8-1.6 mu mol PNPP.mug chi a(-1).h(-1)) as compared with tuft colonies coinhabiting the same waters. Intact filaments of tuft-forming Trichodesmium sp, strain WH9601 from phosphate-replete cultures had a base alkaline phosphatase activity of 0.5 mu mol PNPP.mug chl a(-1)h(-1). This activity underwent a 10-fold increase in phosphate-deplete cultures and in cultures supplied with glycerophosphate as the sole P source. The elevated level of alkaline phosphatase activity was sustained in P-deplete cultures, but it declined in cultures with glycerophosphate, The decline is suggested to result from feedback repression of alkaline phosphatase synthesis by the phosphate generated in the glycerophosphate hydrolysis, The enhanced alkaline phosphatase activities of Trichodesmium spp, populations provide evidence that P stress is an important factor in the ecology of Trichodesmium in the northern Red Sea.