Microbial activity is a fundamental component of oceanic nutrient cycles. Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. The availability of nutrients in the upper ocean frequently limits the activity and abundance of these organisms. Experimental data have revealed two broad regimes of phytoplankton nutrient limitation in the modern upper ocean. Nitrogen availability tends to limit productivity throughout much of the surface low-latitude ocean, where the supply of nutrients from the subsurface is relatively slow. In contrast, iron often limits productivity where subsurface nutrient supply is enhanced, including within the main oceanic upwelling regions of the Southern Ocean and the eastern equatorial Pacific. Phosphorus, vitamins and micronutrients other than iron may also (co-)limit marine phytoplankton. The spatial patterns and importance of co-limitation, however, remain unclear. Variability in the stoichiometries of nutrient supply and biological demand are key determinants of oceanic nutrient limitation. Deciphering the mechanisms that underpin this variability, and the consequences for marine microbes, will be a challenge. But such knowledge will be crucial for accurately predicting the consequences of ongoing anthropogenic perturbations to oceanic nutrient biogeochemistry

Microbial phosphorus-cycling, with particular emphasis on algal/bacterial competition, was studied in depth profiles through the halocline separating a brackish top layer, rich in nitrate and poor in phosphate, from the underlying coastal water, poor in both nitrate and phosphate, in the Sandsfjord area, western Norway. At 2 stations along the axis of natural freshwater outflow, physiological P-deficiency of algae and bacteria in the brackish layer was inferred from rapid luxury consumption of added PO: -by organisms in size fractions > l pm and 1-0.2 pm, respectively. In a branch of the fjord without natural freshwater outlets, luxury consumption in the brackish layer was less, and without a clear difference between the 2 water layers. High luxury consumption coincided with short (< 30 min) turnover times for ~0 2 -a n d strong bacterial dominance of 32P0,"--uptake, suggesting bacterial superiority as competitors during P-limiting conditions. Estimation of P-specific maximum uptake rate and affinity for PO:--uptake from isotope dilution experiments indicated bacterial superiority at low and algal superiority at high concentrations. Although most of the 32P hydrolyzed from added y -~T 3 2 P was initially liberated as free 32P04, partitioning of incorporated 32P between size fractions > l pm and 1-0.2 pm was found to depend on whether label was added as y -~T " P or as 3 2~0 , , -, indicating that coupling of uptake to hydrolysis by cell-bound enzymes could modify the outcome of algalbacterial phosphorus competition. Disappearance rate of "P from the 1-0.2 pm size fraction following initial labeling and a subsequent cold chase with PO:-, was used to estimate the flow-rate of phosphorus through the microbial food web. Combined with measured kinetic constants for PO: -uptake. alkaline phosphatases and 5'nucleotidases, a coherent flow-scheme could only be obtained assuming very low (< 1 nmol I-') concentrations of PO;'-and nucleotides. Chemically measured concentrations of dissolved organic phosphorus (DOP) more than 2 orders of magnitude above the estimated nucleotide level and with an estimated turnover time of ca 500 h, are consistent with the view that this large Preservoir consists mainly of slowly hydrolyzable polymers.

Abstract. We investigated the identity of the limiting nutrient of the pelagic microbial food web in the Mediterranean Sea using nutrient manipulated microcosms during summer 2008. Experiments were carried out with surface waters at the center of anticyclonic eddies in the Western Basin, the Ionian Basin, and the Levantine Basin. In situ, the ratio of N to P was always higher in both dissolved and particulate organic fractions compared to the Redfield ratio, suggesting a relative P-starvation. In each experiment, four different treatments in triplicates (addition of ammonium, phosphate, a combination of both, and the unamended control) were employed and chemical and biological parameters monitored throughout a 3-4 day incubation. Temporal changes of turnover time of phosphate and ATP, and alkaline phosphatase activity during the incubation suggested that the phytoplankton and heterotrophic prokaryotes (Hprok) communities were not P-limited at the sites. Furthermore, Correspondence to: T. Tanaka (tsuneo.tanaka@obs-vlfr.fr) statistical comparison among treatments at the end of the incubation did not support a hypothesis of P-limitation at the three study sites. In contrast, primary production was consistently limited by N, and Hprok growth was not limited by N nor P in the Western Basin, but N-limited in the Ionian Basin, and N and P co-limited in the Levantine Basin. Our results demonstrated the gap between biogeochemical features (an apparent P-starved status) and biological responses (no apparent P-limitation). We question the general notion that Mediterranean surface waters are limited by P alone during the stratified period.

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