Publikationsdatum:
2015-08-06
Beschreibung:
The biological response to extreme temperatures and
salinities is investigated in the laboratory for seven
species of planktonic foraminifera: Globigerinoides
sacculi/er (Brady), Globigerinoides ruber (d'Orbigny),
Globigerinoides conglobatus (Brady), Globigerine/la siphonifera
(d'Orbigny), Orbulina universa d'Orbigny,
Neogloboquadrina dutertrei (d'Orbigny) and Globorotalia
menardii (d'Orbigny). When one of the vital processes,
food acceptance, growth or reproduction is inhibited
by a culture variable, the absolute survival limit
is reached. The measured in vitro temperature ranges
compare well with the global temperature distribution
patterns of these species, suggesting that this parameter
plays a major role in their biogeographical distribution.
The salinity ranges that are tolerated in laboratory
cultures exceed the range encountered in modern oceans.
Thus salinity does not limit the distribution of the
species investigated herein.
In general, larger mean final shell sizes are attained
and the total shell length increase is larger at optimum
temperatures and salinities than at extreme culture conditions,
but the differences were not always statistically
significant. Marginal temperature and salinity conditions
do not induce contained growth in expatriated
specimens.
Under extreme culture conditions, the relative frequency
of the different shell morphologies is altered
relative to normal conditions. "Abnormal" phenotypes
are more frequent under normal conditions and the
"normal" morphology is found more often under extreme
conditions. As opposed to previous reports, the
frequency of kummerform chambers generally decreases
toward extreme temperature and salinity culture
conditions, indicating that kummerform phenotypes
are not indicative of environmental stress. The
incidence of sac-like chambers in G. sacculi/er and the
formation of spherical chambers in adult 0. universa
decrease toward extreme temperature and salinity culture
conditions, demonstrating that maturation is suppressed
in stress situations.
SEM investigations show that changes in shell porosity
are correlated with treatment variables in culture.
The highest porosities are attained at higher temperatures
and lower salinities. Generally, an increase
in total porosity is achieved by an increase of the pore
area accompanied by a reduction of the pore density.
The in vitro experiments explain the changes that
occurred in the Pleistocene foraminiferal assemblages
from the Red Sea around 18 thousand years ago and
earlier. During glacial periods, salinity approximated
or even exceeded the upper thresholds that were tolerated
under laboratory conditions. Under these circumstances,
species disappeared from the water column.
The order of disappearance as recorded in the
sediments may be explained with the upper salinity
limits found in this study. Also, the recurrent shifts of
dominance between G. sacculi/er and G. ruber are well
documented for this fossil assemblage. The present experiments
support the conclusion that salinity is the
driving mechanism behind this phenomenon. Observations
in modern oceans suggest that the fertility of
the water mass is probably also an important factor
behind the shifts of dominance between G. sacculi/er
and G. ruber.
Materialart:
Article
,
PeerReviewed
Format:
text
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