102
In addition to their function as grazers, it seems probable that these organisms
are important in the regeneration of nutrients such as ammonia and phosphorus (Sherr
et ~. 1983).
They may also selectively facditate the breakdown of specific detrital
components such as polysaccharides (Sherr et ~. 1982) and maintain elevated heterotrophic bacterial metabolism by actively cropping bacteria (Sieburth and Davis 1982).
3) Our conceptions regarding the functions of bacteria in the marine ecosystem
have also undergone revision (Pomeroy 1980; Williams 1983).
In the upper waters of
the ocean most of the heterotrophic activity appears to be associated with single
rather than with clumped or attached bacteria.
The amount of carbon flux from algal
extracellular release of organic matter to the heterotrophic bacteria remains controversial (Sharp 1977), but at least in sone cases this may be a considerable portion
of the photosynthetic fixation (Smith 1982).
Despite some innovative methodological
approaches, in situ growth rates of bacteria have remained difficult to evaluate.
Since bacterial cell numbers seem to be rather constant in the upper ocean layers
(Williams 1983) what then is the turnover time of carbon at this trophic level?
Is
being eaten by a microprotozoan a likely fate for a marine bacterium?
4) Deep chlorophyll layers (DCl), usually located at about 1% of the incident
surface light are a common feature in many seas and their formation may be due to a
variety of causes (Cullen 1982).
A DCl may not necessarily represent a region of
greater phytoplankton biomass but may only reflect the increase of chlorophyll content
per cell due to the low level of irradiance.
Venrick (1982) has provided det~iled
evidence to show that, in the North Pacific Central Gyre, qualitatively different
algal populations were found in the DCl and in the upper phototrophic zone.
The
extent to which the phytoplankton are actively growing in DCl's and their contribution
to overall ecosystem and energy transfer remain unresolved.
5) The existence of microscale inhomogeneities of chemical concentrations in a
turbulent environment may have profound biological and ecological importance.
One
aspect of this is the suggestion that, in nutrient limited environments, the phytoplankton might derive some or all of their required nutrition in pulses from zooplankton excretions (McCarthy and Goldman 1979; lehman and Scavia 1982).
This conjures up the rather poignant picture of hungry algae in oligotrophic waters drifting
around waiting for a handout'
It also implies that the ability to assimilate a
nutrient rapidly confers a distinct competitive advantage.
Thus there may be two
kinds of algal strategists, those with uptake systems which maximize for substrate
affinity and those adapted for maximum uptake rate.
Bell ~ ~ (1974) proposed the idea of a 'phycosphere' zone of dissolved organic
compounds around algal cells in which heterotrophic bacteria could presumably develop
on the readily available substrates which are being released.
Recently Azam (1983)
has postulated that bacteria may in fact be actively attracted to such 'phycospheres'
by biochemical' signals' from compounds such as cyclic-AliP.
Experimental evidence is
still necessary to confirm these suggestions.
In addition to their function as grazers, it seems probable that these organisms
are important in the regeneration of nutrients such as ammonia and phosphorus (Sherr
et ~. 1983).
They may also selectively facditate the breakdown of specific detrital
components such as polysaccharides (Sherr et ~. 1982) and maintain elevated heterotrophic bacterial metabolism by actively cropping bacteria (Sieburth and Davis 1982).
3) Our conceptions regarding the functions of bacteria in the marine ecosystem
have also undergone revision (Pomeroy 1980; Williams 1983).
In the upper waters of
the ocean most of the heterotrophic activity appears to be associated with single
rather than with clumped or attached bacteria.
The amount of carbon flux from algal
extracellular release of organic matter to the heterotrophic bacteria remains controversial (Sharp 1977), but at least in sone cases this may be a considerable portion
of the photosynthetic fixation (Smith 1982).
Despite some innovative methodological
approaches, in situ growth rates of bacteria have remained difficult to evaluate.
Since bacterial cell numbers seem to be rather constant in the upper ocean layers
(Williams 1983) what then is the turnover time of carbon at this trophic level?
Is
being eaten by a microprotozoan a likely fate for a marine bacterium?
4) Deep chlorophyll layers (DCl), usually located at about 1% of the incident
surface light are a common feature in many seas and their formation may be due to a
variety of causes (Cullen 1982).
A DCl may not necessarily represent a region of
greater phytoplankton biomass but may only reflect the increase of chlorophyll content
per cell due to the low level of irradiance.
Venrick (1982) has provided det~iled
evidence to show that, in the North Pacific Central Gyre, qualitatively different
algal populations were found in the DCl and in the upper phototrophic zone.
The
extent to which the phytoplankton are actively growing in DCl's and their contribution
to overall ecosystem and energy transfer remain unresolved.
5) The existence of microscale inhomogeneities of chemical concentrations in a
turbulent environment may have profound biological and ecological importance.
One
aspect of this is the suggestion that, in nutrient limited environments, the phytoplankton might derive some or all of their required nutrition in pulses from zooplankton excretions (McCarthy and Goldman 1979; lehman and Scavia 1982).
This conjures up the rather poignant picture of hungry algae in oligotrophic waters drifting
around waiting for a handout'
It also implies that the ability to assimilate a
nutrient rapidly confers a distinct competitive advantage.
Thus there may be two
kinds of algal strategists, those with uptake systems which maximize for substrate
affinity and those adapted for maximum uptake rate.
Bell ~ ~ (1974) proposed the idea of a 'phycosphere' zone of dissolved organic
compounds around algal cells in which heterotrophic bacteria could presumably develop
on the readily available substrates which are being released.
Recently Azam (1983)
has postulated that bacteria may in fact be actively attracted to such 'phycospheres'
by biochemical' signals' from compounds such as cyclic-AliP.
Experimental evidence is
still necessary to confirm these suggestions.