Mineral nutrient limitation seems to stimulate phytoplankton excretion of extracellular organic carbon (EOC). Bacterial growth on EOC requires additional uptake of mineral nutrients. A paradoxical situation is thereby created: algae stressed by lack of mineral nutrients respond in a manner whereby they stimulate their competitors for the lacking nutrients. To investigate this paradoxical ecological relation, a simple mathematical model was analysed and compared to a biological model system. The biological model system consisted of the diatom Skeletonema costaturn and a strain of a marine bacterium cultured together in chemostats with phosphate as the limiting mineral nutrient. Increasing mineral nutrient limitation by decreasing dilution rate resulted in chemostat equilibria with more bacteria and less algae, confirming the proposed paradox. In a certain sense, algae outcompeted themselves at low dilution rates. Predictions of the simple mathematical model were found to be in agreement both with observed distribution of phosphorus between the 2 populations and w~t h amount of algal carbon biomass in the mixed culture. Implications of results for natural aquatic ecosystems are briefly discussed.

In light of evidence suggesting that both phytoplankton and bacteria in the Mediterranean Sea are limited by the availability of phosphorus rather than of nitrogen, and that most of the P in the photic zone during summer stratification exists as dissolved organic compounds (DOP), we address the question of how these observations may interact with the 'biological pump' transporting carbon to deep waters. From theoretical considerations, the C storage via sinking particles should function better in a P-than in an N -h t e d system. It is argued, however, that the microbial food web during summer stratification has a net accumulation of dissolved organic carbon (DOC) and DOP. The hmited data available suggest a high DOC : DOP value which would make downwards transport of DOC with winter deep water formation a potentially effective mechanism in C sequestration from the atmosphere. Part of the DOC accumulating in the photic zone appears to be readily biodegradable. This is in conflict with a simple model of phytoplankton-bacterial competition for phosphate since phytoplankton, as an inferior competitor, would be expected to be reduced in biomass until autochthonous production of organic C falls to a level where bacteria become C-lmuted. The conflict is resolved by including microzooplankton grazing as a controlling factor of bacterial biomass.

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Viral lysis of bacteria has been suggested to be a quantitatively important process in the removal of bacteria, and potentially also in the production of DOC, in the ocean. In order to investigate the quantitative role of viruses in the pelagic microbial ecosystem, a die1 study was undertaken, comprising measurements of particulate and dissolved primary production, bactenal production, bacterial grazing by flagellates, bacterial lysis by viruses, and production of viruses. Estimates of algal excretion (4.7 pm01 C 1-' d-I) and bacterial carbon consumption (3.0 pm01 C I-' d-l) were in reasonable balance. Predation (2.9 pm01 C 1-' d-l) exceeded bacterial production (1.5 pm01 C 1-' d-l) estimates by a factor of 2. Major difficulties in balancing the budget emerged however, from the estimates of vual lysis of bacteria (9.2 pm01 C I-' d-l), which exceeded bacterial production by a factor of 6. Despite these problems, results support the idea that viral lysis may b e a quantitatively significant process that needs to b e incorporated into budgets of microbial C-transfer.