Rienk H. Smittenberg

A hypersaline microbial mat from the Pacific Atoll Kiritimati: insights into composition and carbon fixation using biomarker analyses and a 13C-labeling approach

Modern microbial mats are widely recognized as useful analogs for the study of biogeochemical processes relevant to paleoenvironmental reconstruction in the Precambrian. We combined microscopic observations and investigations of biomarker composition to investigate community structure and function in the upper layers of a thick phototrophic microbial mat system from a hypersaline lake on Kiritimati (Christmas Island) in the Northern Line Islands, Republic of Kiribati. In particular, an exploratory incubation experiment with (13)C-labeled bicarbonate was conducted to pinpoint biomarkers from organisms actively fixing carbon. A high relative abundance of the cyanobacterial taxa Aphanocapsa and Aphanothece was revealed by microscopic observation, and cyanobacterial fatty acids and hydrocarbons showed (13)C-uptake in the labeling experiment. Microscopic observations also revealed purple sulfur bacteria (PSB) in the deeper layers. A cyclic C(19:0) fatty acid and farnesol were attributed to this group that was also actively fixing carbon. Background isotopic values indicate Calvin-Benson cycle-based autotrophy for cycC(19:0) and farnesol-producing PSBs. Biomarkers from sulfate-reducing bacteria (SRB) in the top layer of the mat and their (13)C-uptake patterns indicated a close coupling between SRBs and cyanobacteria. Archaeol, possibly from methanogens, was detected in all layers and was especially abundant near the surface where it contained substantial amounts of (13)C-label. Intact glycosidic tetraether lipids detected in the deepest layer indicated other archaea. Large amounts of ornithine and betaine bearing intact polar lipids could be an indicator of a phosphate-limited ecosystem, where organisms that are able to substitute these for phospholipids may have a competitive advantage.

A physico-chemical survey of inland lakes and saline ponds: Christmas Island (Kiritimati) and Washington (Teraina) Islands, Republic of Kiribati

The equatorial Pacific Ocean atoll islands of Kiritimati and Teraina encompass great physical, chemical and biological variability within extreme lacustrine environments. Surveys of lake chemistry and sediments revealed both intra- and inter-island variability. A survey of more than 100 lakes on Kiritimati found salinities from nearly fresh to 150 ppt with the highest values occurring within the isolated, inland portions of the island away from the influence of groundwater or extreme tides. Dissolved oxygen (DO) and pH values also showed considerable variability with a less regular spatial pattern, but were both generally inversely related to salinity. Series of lakes, progressively more isolated from marine communication, present a modern analog to the chemical and morphologic evolution of presently isolated basins. Sediments on both islands consist of interbedded red and green silt, possibly degraded bacterial mat, overlying white, mineralogenic silt precipitate. Variability may be indicative of shifts in climatological parameters such as the El Niño Southern Oscillation (ENSO) or the Pacific Intertropical Convergence Zone (ITCZ).