A Passive or Active Biogenic Origin of the Whitings Phenomenon by Synechococcus bacillaris
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Whitings are a natural phenomenon of calcium carbonate (CaCO3) precipitation that occurs in both fresh and tropical marine waters. Previous studies have associated whitings with Synechococcus bacillaris and have suggested that these bacteria have an active role in whitings development including acting as nucleation sites and actively alkalinizing the surrounding microenvironment. CaCO3 minerals that have been influenced by bacteria in their formation have been found to have different crystal structures than inorganically precipitated CaCO3. What is unknown is whether the carbonate (CO32-) that is incorporated into the whitings comes from the external environment or is produced by the bacteria through photosynthesis. Oligotrophic S. bacillaris was grown in the laboratory in L1 medium. Microcosm experiments were conducted under closed conditions. pH in the experimental microcosms rose to 9.5 resulting in the precipitation of CaCO3, but then re-acidified as the bacteria entered death phase resulting in the precipitate re-dissolving. Uncontrolled temperature shocks caused decreased cell counts, clumping and acidification. Inorganic CaCO3, experimental precipitate, and pure cells were examined under scanning electron microscopy (SEM) and electron dispersive x-ray spectrometry (EDS). The inorganic CaCO3 had crystalline structures consistent with previous studies (mostly aragonite and amorphous calcium carbonate[ACC]). Extracellular polysaccharide (EPS) was observed in the experimental precipitate, but no CaCO3 precipitates were found, and no bacteria or CaCO3 could be observed in the pure cell sample. Improvements that could be made include stable isotope analysis, more stringent temperature control, variation of conditions and the use of ESEM or gold-coating SEM.