Studies of the microbial P-cycle during a Lagrangian phosphate-addition experiment in the Eastern Mediterranean

摘要: Microbial uptake of orthophosphate was studied before and during a Lagrangian experiment where added to the surface mixed layer in Cyprus Gyre, Eastern Mediterranean, region previously hypothesized be characterized by P-limited growth both phytoplankton heterotrophic bacteria. The addition ca. 110 nM 16 km2 patch situ led, within 1 day, an increase particulate-P from 8 15 nM, result good agreement with previous microcosm bioassay indicating this system have maximum capacity for consumption between 10 25 phosphate. In samples unperturbed water taken addition, outside, or below experimental patch, turnover time (Tt) <4 h, argued consistent assumption diffusion-limited growth. Upon Tt increased 94 h. Estimates potential rate (Vmax) exceeded more than one order magnitude biological P-requirement (ν) as obtained stoichiometric conversion C-based primary bacterial production values estimated P-requirement. orthophosphate, Vmax decreased level comparable ν. observations are P-starved cells P-replete excess external after addition. Orthophosphate dominated by<1 μm organisms (mean ±SD 0.56±0.03 μm). higher time, shifted towards larger organisms, culminating 5 days near doubling mean size (1.08 distribution standing stock had μm, presence substantial biomass micro-organisms those involved P-uptake. Comparison measured microscope-based estimates indicated microbial food web (70% particulate-P), distributed 25% total bacteria, 40% flagellates, 5% ciliates. Concentration bioavailable phosphate (Sn) relationship Sn=νTi Sn values<1 PO4 increasing afterwards. sum Kt+Sn 0.6–0.2 fraction were range 1–7 outside suggesting half-saturation constant Kt. 69 then dropped over following week back background levels. As reported elsewhere volume, there decline observed chlorophyll concentrations, but positive response copepods. Less clear effects at osmotroph physiology subsequent responses expected web. Two possible mechanisms discussed: (1) chain predators, (2) predators due shift quality rather quantity.