Pigment biomarkers
A large component of my research involves the analysis of pigment and pigment-derived biomarkers. Pigments, specifically chlorophylls, and carotenoids, are ubiquitous in photosynthetic organisms, and play important roles in harnessing light, as well as shielding organisms from the deleterious effects of UV radiation. Specific pigments and their derivatives can be linked to specific classes of organisms, and therefore can serve as biomarkers and environmental indicators in the sedimentary record, as well as in complex microbial communities. Pigment-derived biomarkers can also be used to trace cyanobacterial blooms, ecological hazards that have been increasing in frequency due to climate warming and nutrient runoff.
The pigments shown in figure are A: beta-Carotene; B: Lutein; C: Fucoxanthin; D: Diadinoxanthin; E: Chlorophyll d; F: Chlorophyll b; G: Chlorophyll a epimer; H: Hydroxy-Pheophytin a; I: Pheophytin a; J: Pyropheophytin a; K: Scytonemin
Environmental controls on pigment distributions in the freshwater microbialites of Fayetteville Green Lake. Organic Geochemistry https://doi.org/10.1016/j.orggeochem.2018.08.012
Microbial ecology in freshwater microbialites
Organic geochemical analyses of modern microbialites provide context from which we can better understand past environments. Ancient microbialites typically do not preserve the type of detailed genetic data that allows us to investigate microbial communities and the diversity of metabolisms that exist in modern microbialites. However, lipid and pigment biomarkers have a comparatively higher potential for preservation and can provide data about the makeup of ancient communities where organic biomarkers are preserved. Investigating modern systems and the range of factors that influence pigment and lipid biomarker moieties provides this necessary context.
Analysis of the pigment distributions within the microbialites of Fayetteville Green Lake, (Fayetteville, NY) revealed a diverse assemblage of cyanobacteria, diatoms, and green algae communities. The concentrations of the photoprotective pigment scytonemin show a distinct correlation with water depth and intensity of UV A light. This, coupled with a known increase in cyanobacterial diversity with depth in this system, highlights the enormous selective pressure that UV radiation can impose on shallow dwelling microbialite communities. The observed distribution of scytonemin and its correlation with UV A light may also provide a proxy for determining the relative water depth of ancient microbialites in which pigments or their derivatives are preserved.
Environmental controls on pigment distributions in the freshwater microbialites of Fayetteville Green Lake. Organic Geochemistry https://doi.org/10.1016/j.orggeochem.2018.08.012
The microbial ecology of chemocline collapse
Lake Kivu is a 482-meter-deep meromictic lake situated within the Western branch of the East African rift system, which straddles the border of the Democratic Republic of Congo and the Republic of Rwanda. Previous studies have described past water column mixing events in Lake Kivu driven either by hyperpycnal flows or subaquatic volcanism that destabilized the stratification and chemocline in the lake, and allowed upward mix-ing of CO2 enriched waters to the surface. Immediately above these hyperpycnites are diatomite layers that grade into sapropels, with elevated TOC concentrations. The spikes in both diatom frustules and TOC are indicative of massive diatom blooms associated with these chemocline destabilizations. Following the initial upwelling of nutrient-enriched deep water, the diatom blooms likely depleted the available dissolved silica to the point of limitation at a higher rate than other limiting nutrients, which then allowed for an ecological shift to dominance by other phytoplankton. The carotenoid pigments lutein and zeaxanthin then show that the system was overtaken by cyanobacteria which continued to supply an enhanced flux of biomass to the sediments.
Overall, pigment biomarker ratios and diatom frustules record an ecological signature associated with mixing and shallowing of the euphotic zone, and provide an additional metric for characterizing more ancient chemocline destabilizations.
Chemocline collapse in Lake Kivu as an analogue for nitrogen cycling during Oceanic Anoxic events. Earth and Planetary Science Letters https://doi.org/10.1016/j.epsl.2020.116459
