Storr's Lake

The relative isolation and climate regime of San Salvador may have led to the development of some unique biomes such as Storr's Lake. Storr's Lake basin formed approximately 4300 years ago, when Holocene sea level rise caused the flooding of interdune depressions. Salinities in the lake vary greatly on a seasonal basis, ranging from 60 to over 100 ppt. The water exhibits a relatively constant pH, from 8.3 to 9.0. It is rimmed by mangrove stands. Several distinct mat communities may be found in Storr's Lake. Within a horizontal transect of 35 m, as many as five morphologically distinct mat communities, as characterized by their extent of calcification, content of exopolymeric substance (EPS), and/or structure may be observed (Fig. 2) (See Table 1 for description, location, and major phototrophic constituents.).
 
Location
Calcification
Major Phototrophic Constituents
Water Column
-
-
Aphanothece sp., Entophysalis sp., diatoms, large heterotrophic bacteria
Lyngbya Crusts
above water level
none
Lyngbya sp., Microcoleus sp., Oscillatoria, sp., Arthropsira-like sp.
Calcareous Mat
from highest water mark to 1 meter deep
calcified crust layer on surface, but no accretion of laminae
Aphanothece, sp., Entophysalis sp., Microcoleus sp., Phormidium sp., Pleurocapsales spp., Schizothrix sp.
Pie Mounds
from highest water mark
some CaCO3 precipitates
Microcoleus sp., Phormidium, green and purple bacteria
Scytonema Heads
between 1 to 10 cm deep, tops become exposed on occasion
accretion of calcite laminae up to 15 cm high
Entophysalis sp., Gloeocapsa sp., Schizothriz sp., Scytonema, purple bacteria
Stromatolites
> 1 meter deep
accretion and compaction of lithified calcite laminae up to 1m high
Aphanothece sp., Entophysalis sp., Microcoleus sp., Schizotrhix sp.,
Table 1. Microbial mat communities of Storr's Lake, San Salvador, Bahamas.



Figure 2.

Approximate distribution of Storr's Lake mat communities in relation to water level.
 
                                                                                   Figure 3.
Microbial mat communities of Storr's Lake. A) Lyngybya crust. B) Calcareous mat. C) Cross-section of Ectoplasm mats showing EPS content. D) Ectoplasm mats in situ.
The 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.