AME 38:203-213 (2005)  -  doi:10.3354/ame038203

ABSTRACT: 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 µg l^–1 d^–1 (corresponding to 50, 100 and 150 µg 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 µg 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.

KEY WORDS: Algae–bacteria interaction · Chemostat · Continuous cultivation system · Cryptomonas phaseolus · Eutrophication · Phosphorus · Phytoplankton–bacteria paradox