BIOGEOCHEMISTRY OF MICROBIAL MATS IN COASTAL PONDS OF THE MCMURDO DRY VALLEYS, ANTARCTICA

Numerous small coastal ponds containing photosynthetic microbial mat communities appear to be a significant source of labile organic carbon within the McMurdo Dry Valleys, Antarctica. Understanding the biogeochemistry of these mat communities will help improve our knowledge of the carbon dynamics within the Dry Valleys ecosystem. Bulk organic C- and N- isotopic composition and compound-specific C-isotopic composition and abundances of phospholipid fatty acid (PLFA) of the mat communities were investigated at the individual layer, whole mat, pond, and ecosystem scales. Five representative ponds were examined at Hjorth Hill (HH) and four at Garwood Valley (GV) in both the 2003-2004 and 2004-2005 austral summer seasons.

The bulk C-isotopic composition of the coastal pond organic matter was consistent with values obtained for cyanobacterial mats dominated by Phormidium and Nostoc found in ice-fee moats around the large ice-covered Dry Valley lakes (Lawson et al., 2004). Mats in both environments exhibit high summer growth rates and potentially experience CO2-limiting conditions during winter as the entire water column freezes in the moats and ponds. These mats are enriched in 13C relative to other mat communities found elsewhere in the Dry Valleys. GV mats appear to contain an abundance of nitrogen fixing microbes, such as Nostoc, as supported by the bulk nitrogen isotopic values, which range from –0.2‰ to –3.4‰.

Concentrations of PLFAs in all ponds indicated the mats are dominated by aerobic prokaryotes and sulfate-reducing bacteria. The low trans/cis ratios (< 0.1) for 16:1&omega7 and 18:1&omega7 indicate the presence of healthy microbial communities (Guckert et al., 1986). Mats from GV have higher percentages of C and N, and greater concentrations of PLFAs than HH, possibly owing to greater productivity. High productivity was evident from thicker, more laminated mats at GV and higher dissolved oxygen (DO) values for GV ponds. Based on visual observations of the mats, increased DO values, and relatively enriched C-isotopic compositions, mats collected during the 2004-2005 season were more productive than mats collected the previous season. This may be attributed to sample collection later in the 2004-2005 season, after the ponds had thawed and mats were exposed to increased temperatures, light and CO2 concentrations.