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Georgia Journal of Science

Article Title

INVESTIGATING THE DEEP CHLOROPHYLL MAXIMUM IN LAKE LOUISE, GEORGIA, USA

Abstract

Lake Louise, a temperate lake in south-central Georgia is 6.5 meters deep at its deepest point and approximately 13 hectares in area. Despite being relatively shallow, the lake stratifies each year, typically stabilizing from mid-March until mid-November, with a thermocline located at a depth of 2 – 2.5 meters. High tannin concentrations create a steep light gradient, with less than 0.1% of incident light reaching a depth of 2.5 m. Spectrophotometric analysis of samples collected at one meter intervals from the surface to a depth of 5 meters, indicates the presence of a distinct deep chlorophyll maximum (DCM) with what appear to be relatively high concentration of chlorophyll b at an approximate depth of 3 m, well below the photic zone. The presence of chlorophyll b and the abundance of filamentous photosynthetic microorganisms at this depth suggested the presence of Prochlorothrix, a genus of cyanobacteria using chlorophyll as the main accessory pigment. We tested this hypothesis using molecular techniques. Water samples from depths of 1, 3, and 5 meters were collected during peak stratification, filtered onto 1.2 µm polycarbonate filters, and stored at -20°C. DNA was extracted from the filter using a phenol-chloroform extraction procedure. The 16s rRNA gene was amplified using PCR standard bacterial primers and primers specific for Prochlorothrix. The Prochlorothrix-specific primers did not produce any amplicons. Amplicons developed from the universal primers were cloned via the TOPO TA, then sequenced. Among the sequences generated, were some identified as belonging to the Chlorobiaceae, a group of photosynthetic bacteria that uses bacteriochlorophylls c and d as their major photosynthetic pigments; bacteriochlorophylls c and d have acetone spectra very similar to those of chlorophylls a and b, respectively. Reexamination of the micrographs of planktonic organisms present at a depth of 3 meters in light of this finding suggests that the filamentous form tentatively identified as Prochlorothrix, may actually be a member of the Chloroflexaceae, another group of photosynthetic bacteria using bacteriochlorophyll d. We are currently collecting more sequence data to verify this result. If true, this would indicate that the DCM can be attributed to anoxygenic photosynthetic bacteria.

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