Actively growing siliceous oncoids in the Waiotapu geothermal area, North Island, New Zealand

Oncoids that are actively growing in some of the shallow-water pools around Champagne Pool, Waiotapu, New Zealand, are formed of amorphous silica (opal-A) with minor amounts of native sulphur. The growth of these oncoids is being mediated by a high-diversity microbial biota that achieves optimal growth in waters that have a temperature of 35-42 degrees C and pH of 6.6-6.9. Although this biota is dominated by coccoid, bacilliform, and filamentous cyanobacteria, sulphur-oxidizing bacteria are common in the cortical laminae of some oncoids. In addition, diatoms and silicified pollen grains are present in some cortical laminae. The silicified microbes are superbly preserved with their three-dimensional form and some of their internal structures being apparent, including the sites of sulphur globules in the sulphur-oxidizing bacteria. The microbes probably acted as templates for silica precipitation and thereby mediated the growth of the oncoids. Some cortical laminae are formed entirely of silicified pollen grains that came from the Pinus radiata that grow in the exotic pine plantations around Champagne Pool. By using these laminae as a record of the annual pollination event (typically in September-November), it can be shown that many of the oncoids are <15 years old and that the cortical laminae grew at 0.2-1.0 mm (average 0.35 mm) per year. The average daily rate of silica precipitation (0.5-2.75 mu m, average 1 mu m) is high compared to the average size of the microbes (commonly <1 mu m in length and diameter). This rapid rate of silica precipitation accounts for the superb preservation of the microbes and is consistent with silicification of the microbes within days of their demise.