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AME 38:203-213 (2005)
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Abstract
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Effects of phosphorus loading on interactions of algae and bacteria: reinvestigation of the phytoplankton bacteria paradox in a continuous cultivation system
Birgit Mindl1, Bettina Sonntag1, Jakob Pernthaler2, Jaroslav Vrba3, Roland Psenner1,Thomas Posch1,*
1Institute of Zoology and Limnology, University of Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria 2Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany 3Hydrobiolog
ical Institute AS CR, Na sádkách 7, 370 05 Ceské Budejovice, Czech Republic
*Corresponding author. Email: thomas.posch@uibk.ac.at
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ABSTRACT: The effect of different phosphorus loads (LP) on the phosphorus (P) and carbon (C) content (biomass) of algae and bacteria was assessed in continuous culture. We tested if a mixed freshwater microbial assemblage co-cultured with a
phytoflagellate (Cryptomonas phaseolus) would comply with the phytoplanktonbacteria paradox (sensu Bratbak & Thingstad 1985). This hypothesis states that the ratio of bacterial to algal abundance changes to the benefit of
bacteria with decreasing LP. However, the phenomenon was originally investigated by simultaneously altering LP and microbial growth rates, and it is unclear to which extent it can be assigned to either parameter. Therefore, we set up
3 chemostat systems in triplicate at equal dilution rates, but with daily LP of 21, 41 or 62 µg l1 d1 (corresponding to 50, 100 and 150 µg P l1). Higher LP led to a
5-fold increase in total algal abundance and biomass but to less than a doubling of these parameters in the bacterial assemblage. Total biomass ratios of bacteria to algae changed from 0.18 to 0.06 with increasing LP, while the
bacteriaalgae total phosphorus ratios decreased from 0.80 to 0.17. The cellular C:P ratio of algae remained similar at all P concentrations, whereas the molar C:P ratios of bacterial cells significantly increased at higher LP (from 44 to
73). An enrichment experiment with the 50 µg P l1 treatment demonstrated that bacteria at the lowest LP were co-limited by P and C, and that increased P stimulated mainly the algal fraction. The
phytoplanktonbacteria paradox at the level of a mixed microbial assemblage is thus characterised by the following aspects: (1) bacteria profit from their high affinity to P and are better competitors at lower LP; (2) although algae
compete with bacteria for P, P-limited algae release extracellular C that stimulates growth of their bacterial competitors; (3) when bacteria depend on algae as their sole source of organic C, this provides a feedback mechanism by which algae limit the
abundance of their competitors at higher LP; (4) large oscillations in the bacteriaalgae ratios at the lowest LP point to a greater instability of this interaction with stronger P competition. However, bacteria were not able to
outcompete C. phaseolus, as algae were their only C source.
KEY WORDS: Algaebacteria interaction · Chemostat · Continuous cultivation system · Cryptomonas phaseolus · Eutrophication · Phosphorus · Phytoplanktonbacteria paradox
Full text in pdf format
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Published in AME Vol.
38, No. 3
(2005) on March 18
Print ISSN: 0948-3055; Online ISSN: 1616-1564.
Copyright © Inter-Research, Oldendorf/Luhe, 2005
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