172
R. Casotti et aL
remineralized by bacteria or else excreted by heterotrophs, therefore contributing to regenerated
production. Indeed, thanks to their high surfaceto-volume ratio, they can tolerate strong nutrient
limitation and grow in conditions otherwise prohibitive for other algae (Vaulot et al. 1995). As a
matter of fact, ultra-autotrophs are dominant in
oligotrophic areas of every ocean and also in
areas where iron and not nitrogen is the limiting
element (e.g. Binder et al. 1996). Thanks to their
particular pigment, divinyl chlorophyll a,
Prochlorophytes can live at the very low light irradiances of the ocean's depths, but thanks to their
exceptional photo acclimation properties, they
span from the surface to the bottom of the
euphotic zone and have been found also in the
surface at very high light intensities. This exceptional photoacclimation ability is made possible
by the physiological characteristics of prochlorophytes (Partensky et al. 1993), but also by the
presence of different populations with different
growth optima replacing one the other along the
water column (Moore et al. 1995; 1998). Much less
information is available on small eukaryotes, due
to the heterogeneity of taxonomic groups represented and to the technical difficulties of their
study, since they are very delicate cells and very
difficult to culture (Andersen et al.1993; Simon et
al. 1994). The very few species cultured are probably not very representative of in situ species
(Campbell et al. 1994).
Heterotrophic bacteria are also a very important component of microbial food webs and contribute significantly to biogeochemical cycles in
the ocean. Their study has also very much
improved in precision, speed and ease of analysis since flow cytometry has been used (Li et al.
1995). Much of the information is available from
coastal systems and from species easy to grow on
plates, but little is known about the importance
of free-living planktonic bacteria in pelagic environments. Some authors observed that in oligotrophic areas bacterial biomass often exceeds
phytoplankton biomass (Cho and Azam 1990),
and termed the phenomenon «the inverted pyramid". The constancy of such a feature is under
debate (Li et al. 1992; Robarts et al. 1996). Also in
the Mediterranean Sea cyanobacteria and
prochlorophytes dominate oligotrophic areas.
averaging 71 % of total primary production (e.g.
Decembrini and Magazzu 1990; Maugeri et al.
1992; Magazzu and Decembrini 1995), but they
are very abundant and growing actively also in
neritic areas (Vaulot and Partensky 1992).
This study aims at describing and quantifying ultraplankton by flow cytometry in order to
describe its general features and assess its contribution to total biomass in a coastal area of the
Mediterranean Sea, the Gulf of Naples. The Gulf
of Naples is a coastal eutrophic area open to the
general circulation of the oligotrophic
Tyrrhenian Sea (e.g. Scotto di Carlo et al. 1985
and references therein). From a biological point
of view, phytoplankton production shows a surface peak in the spring, and a second development in the autumn, distributed in a thicker layer
of the water column (Scotto di Carlo et al. 1985).
When meteorological conditions are favourable,
a minor peak occurs around mid-November
(Zingone et al. 1995). The ultraplankton of the
Gulf has been poorly studied. Modigh et al.
(1996) investigated cyanobacteria distribution
for one year at a coastal station, concluding that
their concentrations are maintained stable by
grazers control. No data are available on
prochlorophytes, neither on planktonic heterotrophic bacteria.
Materials and Methods
The inner ~art of the Gulf has been sampled on
9 th and 10 November 1995 along three coastoffshore transects (Fig. 1), in order to assess general distribution of ultraplankton as a function
of the water masses and physical structures present. Successively, several aspects of ultraplankton features have been investigated through the
weekly sampling of a fixed coastal station
(named "MareChiara", Fig. 1), from which preliminary results will be presented. During the
November 1995 cruise, temperature and salinity
have been measured by using a CTD probe
(Seabird SBE 16). Discrete samples have been
taken at 2, 10, 20,40 and 70 m by Niskin bottles
for analysis of different parameters. Chlorophyll
a concentrations have been estimated by HPLC
(Beckman Gold System), using the protocol
described in Brunet et al. (1993).
Flow cytometric parameters (FALS, RALS,
red from chlorophyll and orange from phycoerythrin fluorescence) have been measured by
a FACScalibur instrument (Becton-Dickinson)
equipped with an Argon laser emitting at 488 nm
and 15 mW. Fluorescence beads of 1 p.m
(Polyscience, USA) were used as an internal stan-
R. Casotti et aL
remineralized by bacteria or else excreted by heterotrophs, therefore contributing to regenerated
production. Indeed, thanks to their high surfaceto-volume ratio, they can tolerate strong nutrient
limitation and grow in conditions otherwise prohibitive for other algae (Vaulot et al. 1995). As a
matter of fact, ultra-autotrophs are dominant in
oligotrophic areas of every ocean and also in
areas where iron and not nitrogen is the limiting
element (e.g. Binder et al. 1996). Thanks to their
particular pigment, divinyl chlorophyll a,
Prochlorophytes can live at the very low light irradiances of the ocean's depths, but thanks to their
exceptional photo acclimation properties, they
span from the surface to the bottom of the
euphotic zone and have been found also in the
surface at very high light intensities. This exceptional photoacclimation ability is made possible
by the physiological characteristics of prochlorophytes (Partensky et al. 1993), but also by the
presence of different populations with different
growth optima replacing one the other along the
water column (Moore et al. 1995; 1998). Much less
information is available on small eukaryotes, due
to the heterogeneity of taxonomic groups represented and to the technical difficulties of their
study, since they are very delicate cells and very
difficult to culture (Andersen et al.1993; Simon et
al. 1994). The very few species cultured are probably not very representative of in situ species
(Campbell et al. 1994).
Heterotrophic bacteria are also a very important component of microbial food webs and contribute significantly to biogeochemical cycles in
the ocean. Their study has also very much
improved in precision, speed and ease of analysis since flow cytometry has been used (Li et al.
1995). Much of the information is available from
coastal systems and from species easy to grow on
plates, but little is known about the importance
of free-living planktonic bacteria in pelagic environments. Some authors observed that in oligotrophic areas bacterial biomass often exceeds
phytoplankton biomass (Cho and Azam 1990),
and termed the phenomenon «the inverted pyramid". The constancy of such a feature is under
debate (Li et al. 1992; Robarts et al. 1996). Also in
the Mediterranean Sea cyanobacteria and
prochlorophytes dominate oligotrophic areas.
averaging 71 % of total primary production (e.g.
Decembrini and Magazzu 1990; Maugeri et al.
1992; Magazzu and Decembrini 1995), but they
are very abundant and growing actively also in
neritic areas (Vaulot and Partensky 1992).
This study aims at describing and quantifying ultraplankton by flow cytometry in order to
describe its general features and assess its contribution to total biomass in a coastal area of the
Mediterranean Sea, the Gulf of Naples. The Gulf
of Naples is a coastal eutrophic area open to the
general circulation of the oligotrophic
Tyrrhenian Sea (e.g. Scotto di Carlo et al. 1985
and references therein). From a biological point
of view, phytoplankton production shows a surface peak in the spring, and a second development in the autumn, distributed in a thicker layer
of the water column (Scotto di Carlo et al. 1985).
When meteorological conditions are favourable,
a minor peak occurs around mid-November
(Zingone et al. 1995). The ultraplankton of the
Gulf has been poorly studied. Modigh et al.
(1996) investigated cyanobacteria distribution
for one year at a coastal station, concluding that
their concentrations are maintained stable by
grazers control. No data are available on
prochlorophytes, neither on planktonic heterotrophic bacteria.
Materials and Methods
The inner ~art of the Gulf has been sampled on
9 th and 10 November 1995 along three coastoffshore transects (Fig. 1), in order to assess general distribution of ultraplankton as a function
of the water masses and physical structures present. Successively, several aspects of ultraplankton features have been investigated through the
weekly sampling of a fixed coastal station
(named "MareChiara", Fig. 1), from which preliminary results will be presented. During the
November 1995 cruise, temperature and salinity
have been measured by using a CTD probe
(Seabird SBE 16). Discrete samples have been
taken at 2, 10, 20,40 and 70 m by Niskin bottles
for analysis of different parameters. Chlorophyll
a concentrations have been estimated by HPLC
(Beckman Gold System), using the protocol
described in Brunet et al. (1993).
Flow cytometric parameters (FALS, RALS,
red from chlorophyll and orange from phycoerythrin fluorescence) have been measured by
a FACScalibur instrument (Becton-Dickinson)
equipped with an Argon laser emitting at 488 nm
and 15 mW. Fluorescence beads of 1 p.m
(Polyscience, USA) were used as an internal stan-