The effect of different phosphorus loads (L P ) 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 'phytoplankton-bacteria paradox' (sensu Bratbak & Thingstad 1985). This hypothesis states that the ratio of bacterial to algal abundance changes to the benefit of bacteria with decreasing L P . However, the phenomenon was originally investigated by simultaneously altering L P 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 L P of 21, 41 or 62 g l -1 d -1 (corresponding to 50, 100 and 150 g P l -1 ). Higher L P 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 L P , while the bacteria-algae 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 L P (from 44 to 73). An enrichment experiment with the 50 g P l -1 treatment demonstrated that bacteria at the lowest L P were co-limited by P and C, and that increased P stimulated mainly the algal fraction. The phytoplankton-bacteria 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 L P ; (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 L P ; (4) large oscillations in the bacteria-algae ratios at the lowest L P 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.
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Mindl et al. (2005) studied this question.