Hyperspectral imaging sediment core scanning tracks high-resolution Holocene variations in (an)oxygenic phototrophic communities at Lake Cadagno, Swiss Alps

Zander, Paul D.; Wirth, Stefanie B.; Gilli, Adrian; Peduzzi, Sandro; Grosjean, Martin (2023). Hyperspectral imaging sediment core scanning tracks high-resolution Holocene variations in (an)oxygenic phototrophic communities at Lake Cadagno, Swiss Alps. Biogeosciences, 20(12), pp. 2221-2235. European Geosciences Union 10.5194/bg-20-2221-2023

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Pigments produced by anoxygenic phototrophic
bacteria are valuable proxies of past anoxia in lacustrine
and marine environments. Pigment measurement typically
requires time-consuming and costly chemical extractions
and chromatographic analyses, which limits the temporal
resolution of paleoenvironmental reconstructions based on
sedimentary pigments. Here, we evaluate the potential of
in situ hyperspectral imaging (HSI) core scanning as a
rapid, non-destructive method to document high-resolution
changes in oxygenic and anoxygenic phototrophic communities
at meromictic Lake Cadagno, Switzerland. Three distinct
groups of pigments can be detected with the HSI
method in the sediments of Lake Cadagno; each pigment
group represents a different phototrophic community. Oxygenic
phototrophs are indicated by total chloropigments
(TChl; chlorophyll a, b, and derivatives). Two types of
anoxygenic phototrophs were distinguished – purple sulfur
bacteria (PSB), represented by bacteriochlorophyll a, and
green sulfur bacteria (GSB), represented by bacteriochlorophyll
c, d, and e. HSI pigment indices were validated by pigment
measurements performed on extracted samples using
spectrophotometer and high-performance liquid chromatography
(HPLC). Bacteriochlorophylls were present throughout
the past 10 kyr, confirming geochemical evidence of
nearly continuous stratification and sulfidic conditions at
Lake Cadagno. Major shifts in the anoxygenic phototropic
communities are recorded at decadal to millennial scales.
GSB and PSB communities coexisted from 10.2–8.8 kyr BP.
Dominance of PSB over GSB from 8.8–3.4 kyr BP indicates
strongly stratified conditions in the lake and strong
light radiation at the chemocline. From 3.4–1.3 kyr BP, PSB
were mostly absent, and GSB became dominant, implying
lower light intensity at the chemocline due to a combination
of factors including deforestation in the lake surroundings,
increased flood frequency, cooler climatic conditions,
and changes in groundwater solute concentrations. The highresolution
HSI data show that frequent flood events and mass
movements disturbed the chemocline and the anoxygenic
bacterial communities and that the PSB were particularly
sensitive and slow to recover following these disturbance
events. This study demonstrates for the first time that HSI can
detect GSB-related pigments, making the method uniquely
valuable as a rapid tool to study samples containing pigments
of both oxygenic and anoxygenic phototrophs.

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