Abstract
Este artículo contiene 14 páginas, 10 figuras, 3 tablas.
Phytoplankton cell lysis is perceived to be an important loss process in the sea, although a quantification of this
process has proved elusive. A recently developed method, based on the measurement of dissolved esterase activity
(EA), was used to estimate the release of esterases following phytoplankton 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 phytoplankton cells caused these enzymes to be
released to the medium. This assumption was tested by analyzing the presence and release of esterases by marine
bacteria, heterotrophic flagellates, and heterotrophic ciliates, all isolated from the Blanes Bay (northwestern Mediterranean,
Spain), and by phytoplankton grown in culture (Synechococcus elongatus, DunaZieZZa sp., ChZoreZZu sp.,
Phaeoductyllum tricornutum, and Chaetoceros decipiens). The dissolved EA found during the growth, stationary,
and decay phases of microheterotrophs (bacteria, flagellate, and ciliate) was negligible when compared to that found
for phytoplanktonic cultures. Differences in cell volume explained the differences in cell EA among the organisms,
but heterotrophs showed lower cell EA (lo-50-fold) than phytoplanktonic cells of similar cell size. These results
support the assumption that microheterotrophs do not contribute significant amounts of EA to the dissolved pool,
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 strongly stratified, showing a deep
(40-55 m) chlorophyll a (Chl a) maximum (DCM; 1.25 ? 0.09 pg liter-‘) and low surface Chl a concentrations
(0.09 + 0.008 pug liter-‘). Phytoplankton lysis rates ranged between 0.026 d-’ and 1.9 d-l, and they declined
significantly with depth; the fastest rates were found in surface waters and the slowest ones at the DCM. Despite
the fast gross growth rates of surface phytoplankton (as calculated from phytoplankton biovolume and oxygen
production), the calculated lysis rates represented a considerable proportion of gross phytoplankton growth rate
(50%) at the surface, whereas they were comparatively less important at the DCM (7%). These results provide
strong evidence that phytoplankton lysis can bc 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 grazing rates in the northwestern Mediterranean surface waters. The high lysis rate of
phytoplankton in surface waters represents an important path by which primary production may fuel the growth of
microheterophic organisms, consistent with the high respiration rate of the surface community examined. The
conclusion that phytoplankton lysis rates can occur at rates high enough to influence food web dynamics and
biogeochemical cycles in the oligotrophic ocean should stimulate research on this largely neglected loss factor in
phytoplankton ecology.
This work was supported by grants AMB93-0729, MAR91-0359,
and AMB94-1019 from CICYT (Spanish Commission of Science
and Technology), by the European Commission under the MAST
program (contract MAS3-CT96-0045), and by a grant from the
GRUPO IFA. M. P S. was supported by a grant from the Regione
Autonoma della Sardegna, and M. F! M. was supported by a grant
from the CE.
Peer reviewed