structure and morphology of the different communities corresponds
to the differing, yet interactive, effects of water, nutrient, and
light availability. Because of the consistent hypersalinity and
desiccation, grazing effects on the mat communities are minimal.
One mat type, referred to as 'pie mounds' or 'ectoplasm' (Fig. 3),
to our best knowledge, has not been described previously for other
systems. These translucent, gelatinous communities form on the sediment,
and are characterized by an abundance of EPS. They may reach five
cm above the sediment, effectively extending the photic zone depth
from depths of millimeters (most common for laminated communities)
to centimeters. As a result, a significant proportion of the primary
production may be due to anoxygenic photosynthesis. Preliminary
sequence analysis of the dinitrogenase reductase gene, nifH, obtained
from the pie mounds has produced several unique cyanobacterial and
anaerobic bacterial sequences. The 'pie mounds' form exclusively
within the upper and lower bounds of lake level.
In deeper parts of the lake, there exists an extensive field of
mushroom- and club-shaped stromatolites. Such a large field is not
known to exist in other hypersaline Bahamian lakes. The stromatolites
are characterized by thin, high Mg-calcite, lithified laminae (2.0
to 3 mm thick). They may reach up to 1.0 m tall and 1.0 m wide.
A microbial mat covers the top lithified layer of the stromatolites
(Table 1). Accretion of the lithified layers is thought to be due
either to inorganic chemical precipitation or to biologically mediated
precipitation of CaCO3. The low hydrodynamic energy of Storr's Lake
precludes the possibility that formation and growth of the stromatolites
is due to trapping and binding of sediments. The stromatolites are
estimated to have initially formed approximately 2,000 years ago.
Microbial stromatolites are thought to be the first complex biological
communities that evolved on earth and the morphology of the Storr's
Lake stromatolites is reminiscent of Precambrian stromatolite fossils.
Geologically, fossilized stromatolites may indicate past presence
of shallow beach and lagoon environments. Because of its microbial
community structure and chemical composition, Storr's Lake may represent
a modern day analog of water-stressed Precambrian lagoon biomes.
In addition to its potential relevance to ancient biomes, Storr's
Lake is a unique system in the Bahamas, if not the world. The relative
isolation and unique biotic structure of San Salvador's lake communities,
therefore, greatly increase the likelihood of obtaining novel organisms
and discovering novel properties that allow them to survive and
actively grow in water-stressed environments characterized by intense
ultraviolet radiation and nutrient deprivation. Indeed, a unique
heterotrophic isolate from Storr's Lake potentially represents a
new genus. Other isolates, while possessing unique 16S sequences,
show a great deal of similarity to Salinivibrio, Flavobacterium,
Marinobacter and Rhodovulum.