4-7 Solar ponds are bodies of water in which circulation is

4-7
Solar ponds are bodies of water in which circulation is
incomplete and there is a very high salt concentration that
increases with depth. This vertical change in salinity serves to
trap heat because concentrated brine in the lowest water level acts
as a collector and storage area for solar heat, while the less saline,
lighter water at the upper levels provides insulation. Heat is thus
retained in the depths.

An artificial pond of this type has been constructed on the
western shore of the Dead Sea in Israel in order to test its
suitability as a source of low-grade heat for conversion into
electricity. An immediate threat to the success of the venture was
the growth of algae. Water in solar ponds must be kept
maximally transparent to allow penetration of light to the deep
storage area. Therefore, any particles of matter in the water,
such as algae cells, that scatter or absorb light will interfere with
the collection of heat.

One proposed method of controlling the algae was the
application of an algicide. However, the Dead Sea is a closed
body of water without any outlet and as such is very easily
contaminated. Extensive use of chemicals in numerous future
full-scale solar ponds would lead to such contamination of the
Dead Sea, which now enjoys a lucrative tourist trade.

A recent experiment has supplied a more promising method for
controlling the algae. To repress the algae cells' capacity for
accommodating themselves to environmental changes, the water
in the solar pond was first made more saline through evaporation
and then diluted by a rapid inflow of fresh water. This shock
reduced the cells' ability to regulate the movement of water
through their membranes. They rapidly absorbed water,
resulting in distortions of shape, increase in volume, and
impairment to motility. Their buoyancy adversely affected, the
cells sank to the bottom of the pond, where they encountered the
hot waters of the storage layer and were destroyed. This method
allows for effective control of nuisance algae while leaving solar
ponds as one of the cleanest technologies providing energy for
human use.
4.
The primary purpose of the passage is to

(A) discuss ways of solving a problem that threatens to limit the usefulness of an energy source
(B) explain the mechanisms by which solar heat may be converted into energy
(C) detail the processes by which algae cells colonize highly saline bodies of water
(D) report the results of an experiment designed to clean contaminated bodies of water
(E) describe the unique properties of a solar pond on the edge of the Dead Sea