faccio un copia e incolla di un pezzo che è lungo 1 km
se qualcuno gli va di tradurlo lo metto per intero
Implications for Reefkeeping: Calcium Concentration
Reefkeeping hobby lore has it that boosting the calcium concentration above natural levels of 410 ppm does little to enhance calcification in most corals. That idea is supported by experiments on Stylophora pistillata where calcification becomes limited by calcium at levels below natural levels, but is not increased above about 360 ppm.4 The relationship between external calcium concentration and calcification rate displays exactly the behavior to be expected if an active transport process were limiting the calcification rate, and that this transport process is saturated with calcium at concentrations above 360 ppm.
Using some of the information provided in previous sections, we can understand why this may be the case. Again, for Stylophora pistillata, as the calcium level is increased in an artificial seawater medium from 0 to 800 ppm, the calcium uptake by the coelenteron increases in a linear fashion.4 The uptake by most of the tissues other than the calicoblastic epithelium also increases in a linear fashion. There is no data specific to the calicoblastic epithelium, but the data show that calcification does not increase above 360 ppm calcium.
If the calcium is let into the calicoblastic epithelium by a calcium channel, then the influx of calcium is dependent on the concentration in the coelenteron, and the proportion of time that the calcium channels are open. Since the cells themselves control the gating of the calcium channels, they presumably can control their internal calcium levels at will UNLESS there is not enough calcium outside of the cells to go through the gate, cross the calicoblastic epithelium cells, and get to the active transporter that sends it into the ECF. Consequently, one interpretation is that at external calcium concentrations below 360 ppm, the calcium flux into the calicoblastic cells becomes the rate-limiting step in calcification.
There is a second interpretation that is also possible, however. In this scenario, calcium enters the calicoblastic epithelium through the gated channels, but is not controlled very well in the cell. As the calcium concentration in the coelenteron drops, the concentration inside of the cell drops (regardless of whether there is a large efflux or not), making it harder for the active transport to pump the calcium into the ECF, and thereby decreasing the rate of calcification.
The difference between these two scenarios is rather esoteric, and probably not of interest to most reefkeepers, but it is intellectually stimulating nevertheless. The difficulty in distinguishing these two scenarios comes about because the nature of the control of the calcium level in these cells is unknown. How exactly the large influx is regulated in relation to the large efflux is not understood and has apparently never been investigated.2 Consequently, we cannot yet know whether calcification drops primarily because the influx through the gates cannot keep up with the efflux rate when calcium concentrations in the coelenteron are low, or whether it drops primarily because the active transport of the calcium into the ECF cannot keep up when the calcium concentration in the calicoblastic cell is low.
Modalità lineare
