Chemistry Reference
In-Depth Information
The enzyme has a higher affinity for Ca
2+
than for Sr
2+
(Yu and Inesi 1995), fractionating
the Sr/Ca ratio between fluid in the calicoblastic cells and fluid in the calcifying space.
Because the pump is activated by exposure of the polyp to light (Al-Horani et al. 2003),
active transport is likely the dominant pathway for Sr
2+
and Ca
2+
entry during daytime.
Thus, skeleton accreted during the daytime is likely to be Sr-depleted. On the contrary,
passive transport is likely to dominate at night (or in darkness) when the Ca
2+
-ATPase
pump is slow or inactive. At night, the Sr/Ca ratio of the calcifying fluid should be close
to that of seawater and skeleton accreted at night will have a Sr/Ca ratio equivalent to an
inorganic aragonite precipitated from seawater.
Figure 7 shows how the diurnal shift between passive transport-dominated and active
transport-dominated results in an ontogenetic change in Sr/Ca ratio of the sclerodermite
as it grows out from the nucleation site to fill the calcifying space. The Sr/Ca analyses
made by SIMS ion microprobe start at the calcification center and follow the growth axis
of the fasciculus up to the edge of the skeletal spine. The Sr/Ca content of the crystals is
high in the calcification centers but low in the aragonite fibers. Along the length of the
fibers, Sr/Ca shows a progressive decline as the crystals elongate away from the
calcification center. The average Sr/Ca ratio of this aragonite fiber bundle, accreted
during the summertime, is ~8.6 mmol/mol and comparable with values obtained from
bulk coral skeletal samples analyzed by thermal ionization mass spectrometry (TIMS)
(Alibert and McCulloch 1997). Because the aragonite fibers bundles contribute 99% of
the skeletal mass of all corals, Sr/Ca ratios in bulk skeletal samples are depleted relative
to inorganic aragonite precipitates (Weber 1973, Smith et al. 1979). Although kinetic
effects are considered responsible for this depletion (de Villiers et al. 1995), the observed
drop-off in Sr/Ca content of the fasciculus is unlikely to be the result of solution
boundary layer-related processes (Rimstidt et al. 1998). The reason is that once Sr
2+
and
Ca
2+
traverse the calicoblastic ectoderm and enter the micron-sized calcifying space, their
rate of diffusion to the crystal growth surface through a solution with the viscosity of
seawater must be extremely rapid. Therefore, a depletion of either ion at the solution-
mineral interface relative to the bulk calcifying solution is unlikely and is not the cause of
Sr depletion in the crystal.
A more probable scenario is that changes in the Sr/Ca ratio of the bulk calcifying
solution, modulated by the daytime activity of the transport enzyme Ca
2+
-ATPase, are
responsible for the low (and declining) Sr/Ca content of the aragonite fiber bundles.
When the pump is active, the proportion of Ca
2+
entering the calcifying space is large
compared with Sr
2+
, causing a corresponding decline in the Sr/Ca ratio of both the
calcifying fluid (Ferrier-Pages et al. 2003) and of course, the crystals growing into it.
According to this model, diurnal, seasonal and interannual changes in the activity of the
pump—which is linked to zooxanthellate photosynthesis and is sensitive to various
factors including temperature, nutrient availability, cloudiness—will cause corresponding
changes in both skeletal calcification rate and the Sr/Ca content of the aragonite crystals.
The model predicts that, amongst corals in general, the skeletal Sr/Ca of rapid calcifiers
will be lower than that of slow calcifiers, consistent with the observations of Weber
(1973), de Villiers et al (1995), Cohen et al. (2002a) and others.
Cohen et al. (2002a) found that symbiotic colonies of
Astrangia poculata
incorporate
progressively less Sr as they grow, especially during the summertime (Fig. 8a). In
contrast, non-photosynthetic
Astrangia
colonies experiencing the same environmental
conditions did not exhibit such a progressive downward drift in skeletal Sr/Ca, and their
Sr/Ca ratios exhibited about the same temperature sensitivity as inorganically precipitated
aragonite (Fig. 8b). Non-symbiotic
Astrangia
thus resembled the nighttime skeletal
crystals from symbiotic colonies of
Porites
in this regard (Cohen et al. 2001).
