Origin and characteristics of the zooplankton phosphatase activity in a coastal ecosystem of the Mediterranean sea (Toulon Bay).

In Toulon Bay (France), very high phosphatase activities have been found in the zooplankton fraction>90 microm. This work was intended to specify their origin. For that purpose, larvae, juvenile and adult Crustacea (Copepods: Calanoids, Cyclopoids, Branchiopods: Cladocera, and Cirripeds) were isolated. Their activities were measured using paranitrophenyl phosphate dissolved in sea water in order to calculate Km (the enzyme half saturation concentration) and Vmax (the reaction rate when the enzyme is saturated with substrate). Vmax were referred to protein contents of the isolated organisms to calculate specific activities. For all zooplankton groups high and low affinity phosphatase activities were found. The low affinity enzyme was responsible for at least 70% of the total phosphatase activity. Its specific activity was higher for larvae than for copepodites and adults. In Cirriped nauplii this activity was particularly high with values which were several hundred times higher than that in other Crustacea. These enzymes had optimum pH close to 8.4, magnesium requirement and were competitively inhibited by orthophosphate. Experiments with intact and lysed Cirriped nauplii confirmed that living organisms had only a weak external activity and showed that most of the activity of these larvae was primarily intracellular.

Role of DOP on the alkaline phosphatase activity of size fractionated plankton in coastal waters in the NW Mediterranean Sea (Toulon Bay, France).

The particulate material was fractionated into 5 size classes (>90µ, 50-90µ, 6-50µ, 1-6µ, and <1µ). DOP was analysed as easily (DOPh, DOPpa) and less easily hydrolysable compounds (DOPox). Based on Vmax, 94% of the high affinity AP activity was due to <50µ cells and 77% to <1µ cells. 83% of the low affinity activity was due to >90µ cells. The high affinity activities were negatively correlated with DOP for the <50µ classes. These correlations came mostly from DOPox. They were more significant when NO3+NO2 concentrations were high, when DIP concentrations were low and when N/P ratio was >10. At lower N/P ratios, AP was more significantly correlated with DIP. The low affinity activities showed significant negative correlation with DIP and with DOP and DOPox for the >90µ class. The inhibition of AP activities by DOPox may originate from stable compounds interfering with DIP for the control of AP synthesis.

Analysis of the role of DOP on the particulate phosphatase activity in Toulon Bay (N.W. Mediterranean Sea, France).

Alkaline phosphatase (AP) activity was studied on homogenates of particulate material in relation to the concentrations of AP-hydrolysable (DOPpa) and AP-non hydrolysable (DOPr) phosphorus. AP activity had high and low Km components. The high affinity activity came from ectoenzymes. It was negatively and significantly correlated with DOPr as with the sum of DIP and DOPr, but not with DOPpa. However negative correlations with DOPpa existed when DIP concentrations decreased. Significant correlations with the sum of DIP and DOPpa attest of additive effects of DIP and DOPpa. The low affinity activity came from "endoenzymes". This activity was also negatively correlated with DOP (DOPpa and DOPr). DOPpa and to a lesser extent DOPr influenced also positively the protein and/or Chlorophyll biomasses of the particulate material. We hypothesize that the correlations of the AP activity with DOP come from regenerated phosphate sequestered in cells and not released in the environment with DIP.

The relationships between particulate and soluble alkaline phosphatase activities and the concentration of phosphorus dissolved in the seawater of Toulon Bay (NW Mediterranean).

The activities of particulate and soluble phosphatase were analyzed monthly for 1 year in the coastal ecosystems of the NW Mediterranean Sea. The mean contribution of the particulate activity increased from 56% at an MUF-P concentration of 30 µM to 77% at 0.04 µM. This particulate activity was negatively correlated with the DIP, DOP and TDP concentrations when the activities were related to the seawater volume, chlorophyll a or the protein concentration. The TDP correlations were highly significant (p: 0.001). The DOP correlations were significant (p: 0.04) and became highly significant (p: 0.009) at low DIP concentrations (<0.13 µM). The DIP correlations were significant (p: 0.04) only at low DOP concentrations (<0.18 µM). Thus, the effects of seawater DIP and DOP were found to be linked. The soluble activity exhibited distinct phosphatase fractions with high (0.5-29.5 µM) and low (0.02-2 µM) Km values, but none exhibited significant correlations with phosphorus compounds.

Role of sea water DIP and DOP in controlling bulk alkaline phosphatase activity in N.W. Mediterranean Sea (Toulon, France).

The regulation of alkaline phosphatase activity by dissolved inorganic (DIP) and organic phosphorus (DOP) and the contribution of DOP as phosphorus source were studied monthly in Toulon Bay (NW Mediterranean, France) in 2005-2006. The concentrations of DIP and DOP varied respectively from 0 to 0.185µM and from 0 to 0.329µM. The bulk activities (Vm, Km, Vm/Km) were measured using MUFP as substrate. Its high affinity component (Km: 0.05-1.00µM) was negatively correlated with the sum of the concentrations of DIP and DOP but not with these compounds taken independently. A negative correlation with DIP was found when the concentrations of DOP were lower than 0.08µM. A negative correlation with DOP was shown when the concentrations of DIP were lower than 0.05µM. This high affinity component can be considered as a valuable indicator for the potential utilization of the compounds which contribute to the intracellular phosphorus pool.