Nature of phosphorus limitation in the ultraoligotrophic eastern Mediterranean.

Phosphate addition to surface waters of the ultraoligotrophic, phosphorus-starved eastern Mediterranean in a Lagrangian experiment caused unexpected ecosystem responses. The system exhibited a decline in chlorophyll and an increase in bacterial production and copepod egg abundance. Although nitrogen and phosphorus colimitation hindered phytoplankton growth, phosphorous may have been transferred through the microbial food web to copepods via two, not mutually exclusive, pathways: (i) bypass of the phytoplankton compartment by phosphorus uptake in heterotrophic bacteria and (ii) tunnelling, whereby phosphate luxury consumption rapidly shifts the stoichiometric composition of copepod prey. Copepods may thus be coupled to lower trophic levels through interactions not usually considered.

Particulate and Dissolved Organic Carbon Production by the Heterotrophic Nanoflagellate Pteridomonas danica Patterson and Fenchel.

> Abstract We established a budget of organic carbon utilization of a starved heterotrophic nanoflagellate, Pteridomonas danica, incubated in batch cultures with Escherichia coli as model prey. The cultures were sampled periodically for biomass determinations and total organic carbon dynamics: total organic carbon, total organic carbon <1 µm, and dissolved organic carbon (DOC, <0.2 µm). During the 22 h incubation period, P. danica underwent biovolume variations of 3.2-fold. Gross growth efficiency was 22% and net growth efficiency 40%. P. danica respired 33% and egested 44% of the ingested E. coli carbon during lag and exponential growth phases. The form of the organic carbon egested varied. Of the total ingested carbon, 9% was egested in the form of DOC and occurred mainly during the exponential growth phase; 35% was egested in the form of particulate organic carbon (POC), ranging in size from 0.2 to 1 µm, and took place during the lag phase. P. danica could have reingested as much of 58% of this previously produced POC during the exponential growth phase as food scarcity increased. We concluded that POC egestion by flagellates could represent a significant source of submicrometric particles and colloidal organic matter. In addition, flagellate reingestion of egested POC could play a nonnegligible role in the microbial food web. Finally, the methodology reported in this study has proved to be a useful tool in the study of carbon metabolism in aquatic microorganisms.http://link.springer-ny.com/link/service/journals/00248/bibs/37n4p276.html

Extracellular enzyme activity: Indications for high short-term variability in a coastal marine ecosystem.

Extracellular α-glucosidase, ß-glucosidase, and aminopeptidase activity variations (measured by use of fluorogenic substrate analogs) at a coastal station in the Mediterranean Sea were investigated over a 1-year period. A 27-h cycle and daily measurements were made in a summer situation. We observed strong relative diurnal variations, compared to seasonal variations, in α- and ß-glucosidase. Within 24 h, 0-100% of both α- and ß-glucosidase were found in the dissolved phase. The aminopeptidase activities did not show a strong diurnal variation, but day to day variations were similar in magnitude to seasonal changes. Consistently, high proportions of all three enzymes were found in the dissolved phase on a seasonal scale. Seasonal measurements at 50- and 100-m depths showed a weak negative dependency on depth for extracellular enzyme activity. The potential importance of both hourly and daily changes in extracellular enzyme activity and of free enzymes is considered.

Rates of digestion of bacteria by marine phagotrophic protozoa: temperature dependence.

The effect of temperature on length of time for digestion of bacteria was evaluated, by using fluorescently labeled bacteria (FLB), for phagotrophic flagellates and ciliates isolated from coastal northwest Mediterranean waters. Accumulation of FLB in protozoan food vacuoles was followed until a plateau of FLB per cell occurred; then after a 1:10 dilution of FLB with unlabeled bacteria, disappearance of FLB in food vacuoles was monitored. For both 3- to 5-mum flagellates and 10- to 40-mum ciliates, the absolute linear slopes of FLB uptake and disappearance were nearly identical in individual experiments over a temperature range of 12 to 22 degrees C. We inferred from these results that the leveling off of the uptake curves resulted when equilibrium between ingestion and digestion of bacteria was attained. The time to leveling off then represented the average time needed for complete digestion of the bacteria ingested at the start of the experiment, and the inverse of this time represented a bacterial digestion rate. The digestion rate increased exponentially from 12 to 22 degrees C for both a mixed flagellate assemblage and the oligotrichous ciliate Strombidium sulcatum, with a Q(10) of 2.8 for the flagellates and 2.0 for the ciliate. Although bacterial ingestion rates varied greatly, depending on protozoan cell size, total bacterial abundance, and temperature, digestion times appeared to be significantly influenced only by protozoan cell size (or type of protozoan) and by temperature.