30th International Conference of the Polish Phycological Society “The past, present and future of phycological research. Its significance for man and environment protection” WROCŁAW, 19–21 MAY 2011, POLAND Fossilization of modern benthic cyanobacteria exemplified by stromatolites from alkaline volcanic lakes of Niuafo’ou Island (Tonga, South Pacific) M. Łukomska-Kowalczyk1, B. Kremer2, J. Kaźmierczak2, S. Kempe3 1
Faculty of Biology, Warsaw University, Poland,
[email protected]; 2 Institute of Paleobiology PAS, Twarda 51/55, 00-818 Warsaw, Poland,
[email protected]; 3 TU Darmstadt, Institut für Angewandte Geowissenschaften, Schnittspahnstr. 9, 64287 Darmstadt, Germany Niuafo’ou is a round volcanic island with 5-km-diameter caldera, that contains two larger lakes with actively growing stromatolites: Vai Lahi (121 m deep) and Vai Si’i (31 m deep). These low salinity lakes are autonomous hydrological system, without direct connection to the ocean. The water of both lakes is characterized by increased alkalinity comparing with seawater. This results in relatively high saturation with respect to calcium carbonate minerals (aragonite and calcite), much higher than in modern seawater. Temporal volcanic activities are probably responsible for oscillations of hydrochemical conditions in both lakes. They are reflected in the variety of textures observed in stromatolites accreting along the shores of the lakes. The crustal and brain-like domal stromatolites are growing in the lakes since c. 15,000 years ago. Some of the stromatolites are subaerially exposed on the shore, and some occur permanently underwater. The largest are over a meter in diameter and up to 80 cm tall. Coccoid and filamentous cyanobacteria participate apparently in the origin of the stromatolites by in vivo precipitation of calcium carbonates (mostly aragonite) in their mucilage sheaths. A large morphological diversity of filamentous and coccoid cyanobacteria was noticed both in the living mats and their subfossil remains preserved in the mineral matrix. Taxonomically they have been assigned to the genera Rivularia, Calothrix, Myxosarcina and Pleurocapsa. Apart from cyanobacteria, fungi, heterotrophic bacteria and diatoms have also been found. The cyanobacteria are most frequently preserved as mineralized mucilage sheaths. Generally, the microorganisms in the studied stromatolites are fossilized due to two mineralization processes: (i) in vivo calcification, associated often with transformation of aragonite into calcite, a process which, as a rule, almost completely destructs morphology of cyanobacteria, and (ii) post mortem silicification. The preservation of the silicified cyanobacteria depends on the degree of their post mortem decomposition at the time of silica permineralization. The silicified parts of stromatolites may enclose either nearly perfectly fossilized remains of cyanobacteria, or be free of their any recognizable traces.
