IntroductionTypical of many Bahamian Islands, San Salvador Island (24o05’N, 74o30’W) contains numerous hypersaline (45 to 200‰) lakes and ponds. The lakes are subject to intense irradiance (>2100 µE m-2 s-1), high temperatures (> 30° C) and chronic nutrient depletion. Highly productive microbial mats blanket the shallow sediments in many of these lakes. In addition to the osmotic stress induced by the hypersaline conditions, the mats are subjected to an additional water stress, desiccation, as lake levels recede during the dry season (December to April). Many of these organisms are found in mats throughout the world. However, the relative isolation of San Salvador, its extreme climatic conditions, and diversity of organisms increases the likelihood that novel organisms possessing unique adaptive mechanisms inhabit the mats and lakes. One of our primary goals is to isolate and characterize anhydrophilic microorganisms from the mats and lakes. Field work is based out of the Bahamian Field Station (BFS) on San Salvador Island. Every year, the BFS hosts students and researchers from many institutions in the United Sates and abroad. In cooperation with the BFS staff, a prime focus for this project will be the establishment of educational and research opportunities for students wishing to better understand the structure and function of halotolerant microbial mat communities of San Salvador.  

Tim Steppe & UNC-CH Institute of Marine Sciences tim_steppe@unc.eduHans Paerl hans_paerl@unc.eduAlan Decho USC-Columbia Dept. of Environmental adecho@sph.sc.edu

Health SciencesJay Pinckney Texas A&M Dept. of Oceanography pinckney@ocean.tamu.edu

Objectives The overall research objective of this study is to assess the influence water availability has on structural diversification, community composition, production, and carbon sequestration in microbial mats. The specific goals for this observatory are to:

1) Isolate and characterize desiccation tolerant organisms.2) Describe the structural and microbial diversity of the mat communities in relation to water

availability.3) Assess the influence water availability has on primary production extracellular polymeric substances (EPS) production, and EPS degradation.4) Develop a conceptual model linking climate and water budget data, water availability, andprimary production.

Significance & ImpactWater is indispensable for life. The proposed work addresses a fundamental mechanism that is essential to all life on earth, the ability to conserve water and balance ions. Anhydrophilic communities exist under ‘feast or famine’ conditions with regards to water availability. The mechanisms of cell survival as it enters the air-dried state, remains desiccated for extended periods (months to years), and then rehydrates, are poorly understood. Monitoring the physiological responses of pristine, anhydrophilic communities over time and under different conditions of water availability will provide information necessary to understanding how long and short term climatic oscillations impact critical biogeochemical cycles.

Frankia alniHighborne Cay Culture #06 Synechococcus sp.

Phormidium sp.S.L. Ecto 04 AD Culture 01 S.L Ecto 14

Nostoc muscorumAnabaena oscillariodes

Exuma Heterocyst. #01

Xenococcus sp.S.L. Ecto 09

BS CyanoS.L. Ecto 05

S.L. Ecto 01Plectonema boryanum

Lyngbya lagerhaemiiS.L. Ecto 17Highborne Cay Culture #02

Highborne Cay Culture #05Symploca chtonoplastes

GM26Azotobacter vinelandii

BS LM 51Marichromatium purpurtum

Klebsiella pneumoniaeS.L Ecto 16

B.S. UM 01

S.L. Ecto 03S.L. Ecto 08

BS LM 29Desulfovibrio gigas

S.L. Ecto 24Halodule wrightii #02

S.L. Ecto 20BS UM 113

S.L. Ecto 07BS LM J7

Desulfovibrio salexigensHB Lactate Enrichment 01

BS Um 413BS UM 913

Clostridium pasteurianum 01Clostridium pasteurianum 02

Desulfotomaculum sp.Clostridium cellobioparum

Clostridium pasteurianum 03Rhodobacter capsulatus

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Two lakes have been chosen as our main study sites: Storr’s Lake (b) & Salt Pond (j). Storr’s Lake possesses many mat types including ~1.0 m high stromatolites in the deeper portions and the ‘pie mound mats’ (e). The mat (g) in Salt Pond continuously covers the bottom of the shallow lake from the eastern edge to ~35 m from the edge. There are a great diversity of microorganisms on Sal Salvador. We are in the process of culturing both cyanobacteria and heterotrophic bacteria, which we will screen for the ability to withstand desiccation. Organisms arranged clockwise from the top include: mix of organisms from Salt Pond Mat (a), Aphanothece sp. (major planktonic organism in Salt Pond) (c), Lyngbya sp. (cosmopolitan diazotrophic mat organism) (d), Johannesbaptista sp.(f), benthic diatom (although eukaryotic, diatoms are significant phototrophs in the mats) (h), and Microcoleus sp. (another cosmopolitan mat cyanobacterium) (i).

Photosynthesis and Nitrogenase Activity

Diel patterns of CO2 fixation and nitrogenaseacitivity. Similar to many mats, oxygen sensitive nitrogen fixation is temporally separated from photosynthesis. Salt Pondmat.

Salinity effects on CO2 fixation and nitrogenase activity. Salt Pond mat pieces were incubated for 2-3 days in seawater. Rates were compared to mat pieces similarly incubated in Salt Pond water.

Water Chemistry(Salt Pond)

A phylogenetic tree created from aminoacid translations of the nifH gene. The tree shows some of the sequences obtainedfrom the ‘pie mound’ or ectoplasm mats.Similar to other mats the ectoplasm containsa diverse assemblage of diazotrophs. In addition to nifH, we will characterize cyano-bacteria using 16S rRNA genes and sulfate-reducing bacteria using the dissimilatoryreductase gene (dsr). This will allow us to characterize the organisms involved in the major biogeochemical processes. Initially, the major effort will be to characterize theorganisms along a desiccation gradient (see below) established in Salt Pond.

Anhydrophilic, Halotolerant Microbial Mats of San Salvador, BahamasAnhydrophilic, Halotolerant Microbial Mats of San Salvador, Bahamas

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Ecophysiology along a desiccation gradient

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Three transects across a desiccation gradient, 26 m in length and 15 m apart, have been established in Salt Pond. These transects cover areas of the mat that range from infrequently hydrated to those that are permanently submerged. While the phototrophic community composition did not vary, the carotenoid pigment ratios shifted from being primarily photoprotective in the more desiccated mats to ones associated with photoacclimation responses in the more submerged mats. Oxygen microelectrode measurements of photosynthesis (not shown) showed that the highest rates of oxygenic photosynthesis occurred in the middle part of the transect. The results suggest that changes in the ecophysiology of the mats are good indicators of the degree and frequency of desiccation.

Virginia Tech University of NC-Wilmington

University of Miami

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