Abstract

1. Anhydrophilic, Halotolerant Microbial Mats of San Salvador, Bahamas O2 Concentration (µM) Depth in Sediment (µm) Tim Steppe Hans Paerl Lou Anne Cheshire Melissa Leonard Alan Decho Jay Pinckney Photosynthesis and Nitrogenase Activity UNC-CH Institute of Marine Sciences USC-Columbia Dept. of Environmental Health Sciences Texas A&M Dept. of Oceanography tim_steppe@unc.edu hans_paerl@unc.edu awdecho@gwm.sc.edu pinckney@ocean.tamu.edu Mat & Water Chemistry (Salt Pond) Sites and Design Abstract Like many Bahamian Islands, San Salvador Island (24o05' N, 74o30' W) contains numerous shallow, hypersaline (45 to 322 ‰) lakes. The lakes are subjected to intense irradiance (> 2100 μE m-2 s-1), high temperatures (> 35o C) and chronic nutrient depletion. Highly productive microbial mats blanket the shallow sediments in many of the lakes. 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. Three transects, 26 meters in length, have been established along a natural desiccation gradient in one of the hypersaline lakes, Salt Pond. Samples for community composition, extracellular polymeric substances (EPS) content, C & N content, and microscopic documentation are collected during each site visit (two to three times a year). Rates of key C, O, and N cycling processes (photosynthesis and N2 fixation) are obtained. In cooperation with the staff from the Gerace Research Center, Salt Pond’s salinity and temperature are being measured every 10-21 days. From March to July, Salt Pond’s salinity increased from ~ 110‰ to over 320‰. Light and dark vertical O2 distribution profiles of the mat’s upper 5 mm indicate that, under dark conditions, anoxia reaches the mat surface. When exposed to light (1,500 µmol m-2 s-1, 10 min), O2 was detected as deep as 5 mm with concentrations (ca. 800% O2 saturation) peaking at 1 mm depth. Light and dark cycles create a dynamic chemical environment that changes from anoxic to hyperoxic conditions within minutes. How EPS may buffer against drastic changes in redox conditions is being examined. Nutrient addition bioassays (e.g., NH4+, NO3-, and PO42-) indicate salinity levels and not nutrient availability has the greatest impact on these crucial biogeochemical processes. Sequencing surveys of cyanobacterial 16S (primary producers), dsr (sulfate reducers/carbon mineralizers), and nifH (diazotrophs) genes show that diverse assemblages comprise the key functional groups of microorganisms. We are currently analyzing the sequence distributions to determine if there are any differences along the gradient. Carbohydrate analyses have led to the discovery of “amadori products" (APs) in the Salt Pond mats. APs are unique protein-carbohydrate linkages that form when basic amino acids cross-link with carbohydrate carboxyl groups. This is the first report of APs being found in natural systems. The potential for amadori products to act as a further defense (e.g., scytonemins, mycosporine amino acids, etc) against UV is being investigated. 23m 11m 7m 3m 0m Site Abundances of Extracellular Polymeric Secretions (EPS) in three different layers of the Salt Pond Microbial Mat: (1) An “orange” surface “ layer (L1); a “green” cyanobacterial layer (L2); and a “purple” Chromatium sp. Layer (L3). Significantly higher abundances of EPS occur in the surface L1 layer, and at sites where water-cover occurs most often. Light and dark profiles of dissolved oxygen concentration in hypersaline microbial mats. Oxygen gradients change from anoxic under dark conditions to ca. 10 times O2 saturation under sunlight. EPS may provide a buffering mechanism to prevent oxidative damage to photosynthetic enzymes. Combined results of short-term nutrient bioassays from March 2002 and 2003. Mat pieces were collected and incubted in Salt Pond water or seawater ammended with nutrients (NH4+ 20 μM; NO3- 20 μM; and/or PO42- 5 μM). We observed no significant stimulation of photosynthesis or nitrogenase activity (N2 Fixation) due to nutrient additions. Both forms of nitrogen repressed nitrogenase activity, while phosphorus appeared to ameliorate any N repression. Salinity appeared to affect 14CO2 upatke more than it did NA. These observations suggest water availability and salinity, in particular, have the largest impact on production and cycling in the mats. EPS Characterization Polymer Towers 3-D Reconstruction of Tower L1(U) L2(M) Salt Pond salinity exhibits both inter- and intra-annual variation. Salinity and temperature measurements contributed by Elyse Voegeli. L3(L) Surface of Mat X-Section of Mat X-Section of Mat Diversity of Key Biogeochemical Functional Groups The surface layer microbial communities of Salt Pond mats form crenulated “polymer towers” that extend upward during water cover (see X-section). When examined using confocal scanning laser microscopy (CSLM), these polymer towers contain dense arrays of cyanobacteria and heterotrophic bacteria enveloped in a dense gel matrix of extracellular polymers (EPS). Dense colonies of cells suggest chemical signaling may occur in these towers. Also, clusters of cells contained within amphiphilic (hydrophobic/hydrophilic) EPS. 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: dsrA sulfate-reducers NifH diazotrophs Cyanobacterial 16S primary producers T323U08 NifH transect meter mark upper, middle, or lower portion of mat • 1) Describe the structural and microbial diversity of the mat communities in relation to water availability. • Assess the influence water availability has on primary production extracellular polymeric substances (EPS) production, and EPS degradation. • Isolate and characterize desiccation tolerant organisms • 4) Develop a conceptual model linking climate and water budget data, water availability, and primary production. clone number www.SanSalMO.net EPS Participants Collaborators Virginia Tech University of NC-Wilmington University of Miami