Dissolved esterase activity as a tracer of phytoplankton lysis: Evidence of high phytoplankton lysis rates in the northwestern Mediterranean

Phytoplankton cell lysis is perceived to be an important loss process in th e sea, although a quantification of this process has proved elusive. A rece ntly developed method, based on the measurement of dissolved esterase activ ity (EA), was used to estimate the release of esterases following phytoplan kton cell lysis in an effort to evaluate the importance of this process as a loss factor in the summer phytoplankton of the northwestern Mediterranean Sea. Implicit in this method was the assumption that only the lysis of phy toplankton cells caused these enzymes to be released to the medium. This as sumption was tested by analyzing the presence and release of esterases by m arine bacteria, heterotrophic flagellates, and heterotrophic ciliates, all isolated from the Blanes Bay (northwestern Mediterranean, Spain), and by ph ytoplankton grown in culture (Synechococcus elongatus, Dunaliella sp., Chlo rella sp., Phaeodactyllum tricornutum, and Chaetoceros decipiens). The diss olved EA found during the growth, stationary, and decay phases of microhete rotrophs (bacteria, flagellate, and ciliate) was negligible when compared t o that found for phytoplanktonic cultures. Differences in cell volume expla ined the differences in cell EA among the organisms, but heterotrophs showe d lower cell EA (10-50-fold) than phytoplanktonic cells of similar cell siz e. These results support the assumption that microheterotrophs do not contr ibute significant amounts of EA to the dissolved pear, allowing the use of the method to estimate phytoplankton lysis. Independent estimates of cell, loss in phytoplankton cultures, derived from cell cycle analysis, confirmed the estimates of cell lysis obtained from the measurement of dissolved EA. During the study conducted in the Mediterranean Sea, the water column was s trongly stratified, showing a deep (40-55 m) chlorophyll a (Chl a) maximum (DCM; 1.25 +/- 0.09 mu g liter(-1)) and low surface Chl a concentrations (0 .09 +/- 0.008 mu g liter(-1)). Phytoplankton lysis rates ranged between 0.0 26 d(-1) and 1.9 d(-1), and they declined significantly with depth; the fas test rates were found in surface waters and the slowest ones at the DCM. De spite the fast gross growth rates of surface phytoplankton las calculated f rom phytoplankton biovolume and oxygen production), the calculated lysis ra tes represented a considerable proportion of gross phytoplankton growth rat e (50%) at the surface, whereas they were comparatively less important at t he DCM (7%). These results provide strong evidence that phytoplankton lysis can be an important loss factor in the surface waters of this stratified, oligotrophic sea. Phytoplankton lysis could provide the loss factor needed to explain the low phytoplankton biomass despite fast growth and low grazin g rates in the northwestern Mediterranean surface waters. The high lysis ra te of phytoplankton in surface waters represents an important path by which primary production may fuel the growth of microheterophic organisms, consi stent with the high respiration rate of the surface community examined. The conclusion that phytoplankton lysis rates can occur at rates high enough t o influence food web dynamics and biogeochemical cycles in the oligotrophic ocean should stimulate research on this largely neglected loss factor in p hytoplankton ecology.