Phytoplankton and bacterial alkaline phosphatase activities in relation to phosphate and DOP availability within the Gironde plume waters (Bay of Biscay)
Introduction
The ability of microorganisms to acquire phosphorus (P) from dissolved organic phosphorus (DOP) compounds, requires the action of phosphatases, catalyzing the hydrolysis of phosphate monoesters and liberating inorganic phosphate and organic matter. Part of this released phosphate could be taken up directly by microorganisms since phosphatases are membrane-bound or located in the periplasmatic space (coupled uptake) (Cembella et al., 1984). Considerable parts of APA are dissolved and contribute to the phosphate pool which is not directly taken up. Alkaline phosphatases, the most active in marine waters, hydrolyse a wide range of organic P compounds due to their low specificity for organic moiety compared to more specific phosphatases such as 5′-nucleotidases (Ammerman and Azam, 1985). In addition, alkaline phosphatase activity (APA) was shown to be sensitive to phosphate availability and particularly to the intracellular phosphate pool of microorganisms (Gage and Gorham, 1985, Myklestad and Sakshaug, 1983). As a result, it has often been used as an indicator of the phosphorus nutritional status of phytoplankton communities. As such an indicator, it has largely been used in lake waters where phosphorus was generally the limiting factor of microbial communities (Berman, 1970, Pettersson and Jansson, 1978); on the contrary few studies have been conducted in marine waters (Hoppe, 2003) even though a recent surge in interest has been pointed out (Sebastian et al., 2004, Hoppe and Ullrich, 1999, Li et al., 1998, Nausch, 1998). However, few surveys gather APA, DOP, phosphate monoesters (Hydrolysable DOP) and phosphate data in marine systems on a seasonal scale compared to previous studies in lake waters (Chrost and Overbeck, 1987).
APA studies have mainly focused on algae (Fitzgerald and Nelson, 1966) and have scarcely dealt with bacterial activities even though bacteria are also known to have significant APA (Martinez and Azam, 1993). Most papers, including recent ones, have evaluated total APA on unfiltered samples (Hoppe and Ullrich, 1999, Nausch, 1998) or separate particulate and dissolved APA, taking into account that algae or bacteria comprise the bulk of particulate APA (Rose and Axler, 1998). Some studies have evaluated APA from different plankton fractions (Gambin et al., 1999, Gonzalez-Gil et al., 1998) but surveys dealing with simultaneously algal and bacterial fractions are relatively scarce (Chrost and Overbeck, 1987). Even if size fractionation by filtration is never completely satisfying (overlapping size) it gives useful indications as regards the major microorganisms contributing to APA.
The Spanish coast of the Bay of Biscay with its weak and localized freshwater inputs (OSPAR, 2000) is characterized by spring phytoplankton blooms dominated by diatoms (Varela, 1996) which is a classic setting like the phytoplankton dynamics in the temperate waters in the North Atlantic (Longhurst, 1998). Until now, this scheme was applied to the whole Bay of Biscay. The French continental shelf is characterized by significant freshwater inputs from large rivers (Gironde and Loire, annual mean water outflows of 900 m3 s−1); however, the biological production of these waters is largely unknown. Previous studies conducted in the Gironde plume waters pointed out that spring phytoplankton blooms actually did occur (May 1995). However, they were atypical with a size distribution of chlorophyll a (chl a) and primary production dominated by small cells (<3 μm), already severely phosphorus (P) limited (Herbland et al., 1998), and grazed daily by microzooplankton (Sautour et al., 2000). In a first paper (Labry et al., 2001), we highlighted the occurrence of winter blooms in the Gironde plume, with the same characteristics as typical spring blooms in temperate waters. In a second paper (Labry et al., 2002), we confirmed that algal growth was phosphorus limited at the end of the winter blooms and in spring. This early P limitation favoured the development of small cells at the beginning of spring and the subsequent presence of spring blooms composed of pico and nanophytoplankton (as in May 1995), which ultimately acts upon the nature of the whole plankton food web.
The objectives of the present paper are to evaluate the seasonal importance of DOP in terms of stocks (evaluation of hydrolysable DOP) and fluxes (APA) for phytoplankton and bacterial communities in the Gironde plume by in situ measurements and in vitro bioassays simultaneously conducted, simulating phosphate depleted and phosphate repleted conditions.
Section snippets
Sampling strategy
Six cruises were carried out in 1998 and 1999 from early winter to late spring (see Labry et al., 2002 for their names and dates). They were composed of a grid of stations covering the spread of the Gironde plume or a part of the plume where sampling using conventional parameters (nutrients, chl a, bacteria numbers, particulate organic phosphorus POP and DOP) was carried out within the water column. In addition to the usual parameters, determination of APA and their kinetic parameters as well
Results
The general trends from the stations presented in this paper were in accordance with those from the grid of stations, concerning chl a, bacteria numbers, phosphate, DOP, POP (see Labry et al., 2002). Phytoplankton dynamics was characterized by late winter blooms of diatoms and late spring blooms of pico-nanophytoplankton although bacterial biomass did not change throughout the seasons (Table 1).
Seasonal evolution of phosphorus and total APA
The same seasonal evolution in phosphate found in 1998 and 1999 and matching previous investigations (Artigas, 1998, Herbland et al., 1998) indicates that the same seasonal pattern occurs year by year. The highest phosphate values observed in early winter corresponded to wind-driven vertical mixing, high inputs from the Gironde inflow and low biological activities. The decrease to an undetectable level thereafter (<0.02 μM) corresponded to the consumption by late winter phytoplankton blooms (
Acknowledgements
This research was supported by the French “Programme National Environnement Côtier”-atlantic working site and “Programme National sur le Déterminisme du Recrutement”-GLOBEC. We wish to thank Martine Breret, Françoise Mornet and Christophe Arnaud, respectively, for their assistance regarding bacterial counts, the analysis of DOP and phosphate monoesters. [SS]
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