Recent experimental evidence indicates the importance in some pelagic systems of mixotrophic protists that combine photosynthetic ability with the ability to ingest bacteria. If both bacteria and phytoplankton are mineral nutrient limited, this should provide the mixotrophs with the double benefit of combining removal of their competitor with ingestion of the limiting nutrient in pelleted form. It is the objective of this study to expand the classical theories of competition and predation to explore the effect on the microbial food web of one trophic group possessing both strategies. In a chemostat scenario, we analyzed the two—species situation of a mixotroph preying on mineral—nutrient—limited bacteria, and also the situations when the mixotroph in addition has to compete with specialized photoautotrophic and phagotrophic protists, each superior to the mixotroph in their specialized nutritional modes. In the mixotroph—bacteria relationship, somewhat paradoxically, high predatory abilities will reduce the quantitative importance of predation in the mixotroph's nutrition. The reason is a strong reduction in prey abundance, allowing the mixotroph to survive as a photoautotroph despite its low competitive ability. In the three—species case with mixotrophs, bacteria, and specialized phagotrophs, it is shown that the mixotroph can compensate for a "price" paid in reduced affinity for bacterial prey by a sufficiently high affinity for mineral nutrients. In the other three—species case where the mixotroph has to compete with a specialized photoautotroph, the situation is more complex; there is an optimum value for the mixotroph's predatory ability at which mixotroph biomass is maximized. In the general situation with all four species (bacteria, mixotrophs, and specialized auto— and phagotrophs) potentially present, different mixotrophic strategies will alter the equilibrium composition of the consortium, with the mixotroph being most successful with a high affinity for nutrients and an intermediate affinity for bacteria. In the simple form used here, the model predicts no equilibrium with all four species simultaneously present. The theory is in principle directly applicable to laboratory experimentation.

The factors controlling prokaryote abundance and activity along salinity gradients were investigated in the Bras del Port solar saltern system (Alacant, Spain) in May 1999. Specific growth rates were high and prokaryote abundance relatively low at the lowest (seawater) salinities; the opposite was found at higher salinities. Experiments were carried out in representative salterns at salinities of 4 to 37%, to test whether prokaryote abundance and growth rate were (1) limited by inorganic or organic nutrients (nutrient addition experiments), (2) limited by cell abundance (dilution experiments), or (3) affected by zooplankton cascading down to affect the prokaryote predators. Lowsalinity ponds were limited by organic nutrients, while high-salinity ponds responded slightly only to dilution. Zooplankton affected prokaryote growth rates particularly in the medium-salinity ponds. In the low salinity ponds, zooplankton effects were weak and probably indirect (through increased supply of organic matter). Neither organic matter limitation nor zooplankton predation pressure affected prokaryote development in the higher salinity ponds. We suggest that 3 types of functional communities occur in the same saltern system: (1) an active, substrate-limited community in the low salinity ponds; (2) an active, grazer-controlled community in the medium salinity ponds; and (3) a possibly dormant, probably substrate-limited, community in the high salinity ponds. However, the results at the highest salinities were equivocal, because the dilution manipulation had detrimental effects, artificially decreasing the contribution of the haloarchaea, which were essential contributors to the total activity in the saltern. Bacterial taxonomic community composition was also determined in these experiments by denaturing gradient gel electrophoresis (DGGE) analyses on 16S rRNA genes, and showed very small changes in community composition in the experimental manipulations. Together with the known microbial community structure and composition at differing salinities along the gradient, our results show that functional aspects of the microbial food web also vary between salterns.

As part of an investigation of the relationship between diversity and productivity, measurements were made in a solar saltern of carbon fixation, nitrate and ammonium uptake and microzooplankton grazing at salt concentrations ranging from 4 to 37%. Elevated photosynthetic pigment concentrations were present in ponds of intermediate (5-11%) and high (>32%) salinity but rates of primary production and nutrient uptake were generally reduced at the highest salinity. Maximum primary production was measured at 8% salinity and chlorophyll-specific carbon fixation also maximised at this salinity. Ammonium was the dominant nitrogen source throughout the salinity gradient; turnover times of ammonium were from 2 to 14 days. Nitrate turnover times were very long ( approximately 100 days) at salinities <22% but at 37% salinity, nitrate was taken up rapidly by the microbial assemblage in the light and turnover times for the ambient nitrate concentrations in the 37%-salinity pond were between 6 and 12 days. There were large changes in C:N uptake ratio. At salinities <11%, the C:N uptake ratio was higher than the Redfield ratio. However, at >22% salinity, the C:N uptake ratio was approximately 1. That is, much more nitrate and ammonium were taken up than would be expected from the observed carbon-fixation rates. Although primary production declined with decreasing phytoplankton diversity along the salinity gradient, there was no clear relationship between heterotrophic activity and microbial biodiversity.

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