Tracking high-resolution variations of phototrophic communities during the Holocene using hyperspectral imaging core-scanning, Lake Cadagno, Swiss Alps

Pigments produced by anoxygenic phototrophic bacteria are amongst the most reliable indicators of water column anoxia preserved in sediments [1], yet conventional pigment measurement techniques are labor intensive and prohibit high-resolution analyses of these biomarkers. In this study, we assess the potential of hyperspectral imaging (HSI) core-scanning to reconstruct high-resolution variations in anoxygenic phototrophic communities using sediment cores from meromictic Lake Cadagno, Switzerland. Three different pigment groups were detected, and each pigment group is diagnostic of different phototrophic communities. Mixolimnetic aerobic primary production is recorded by chlorophyll a (and derivatives), whereas anoxygenic phototrophs are split into two groups – purple sulfur bacteria (PSB) represented by bacteriochlorophyll a, and green sulfur bacteria (GSB) represented by bacteriochlorophylls c, d, e. Spectrophotometer and high-performance liquid chromatography (HPLC) pigment analyses were used to validate the HSI-inferred pigment data. Near-continuous presence of bacteriochlorophylls confirms previously published geochemical evidence for persistent anoxic/sulfidic conditions at Lake Cadagno throughout the past 9.8 ka [2]. Furthermore, major shifts in the anoxygenic phototrophic communities appear to be related to environmental factors that affect lake stratification and light penetration at the chemocline. PSB and GSB became established after ca. 9.8 ka BP, and high abundances of both groups are inferred from 9.8-8.8 ka BP. PSB became dominant during the mid-Holocene from 8.8-3.4 ka BP, likely indicating a stable and shallow chemocline with high light penetration to the sulfidic chemocline. From 3.4-1.4 ka BP, PSB were substantially reduced and GSB became more dominant, likely due to a combination of cooler temperatures and land cover changes in the catchment that led to weakened lake stratification and more frequent turbid underflows. The high-resolution data show GSB established prior to PSB during the initial development of sulfidic conditions at 9.8 ka BP, and show that GSB tend to re-establish more quickly following mass movements and floods. This study demonstrates for the first time the capability of hyperspectral imaging to detect GSB related pigments, and shows the strong potential for high-resolution reconstruction of anoxygenic phototrophic bacteria communities using this technique.

1. Sinninghe Damsté, J. S. and Schouten, S.: Biological markers for anoxia in the photic zone of the water column, in: Handbook of Environmental Chemistry, Volume 2: Reactions and Processes, Springer-Verlag, Berlin, Heidelberg, 127–163, https://doi.org/10.1007/698_2_005, 2006.

2. Wirth, S. B., Gilli, A., Niemann, H., Dahl, T. W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M. F., and Anselmetti, F. S.: Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps), Geochim. Cosmochim. Acta, 120, 220–238, https://doi.org/10.1016/j.gca.2013.06.017, 2013.