Appendix A. Fossil pigment stability and preservation in lake sediments.
Lake sediments often preserve the chlorophylls (Chl), carotenoids and pigment derivatives (isomers, allomers, epimers, etc.) produced by phototrophic organisms within the lake, as well as other water- and lipid-soluble compounds including scytonemin, phycobiliproteins, mycosporine-like amino acids and flavenoids. In particular, five properties make carotenoids and chlorophylls important biomarkers for use by paleolimnologists including common initial biosynthetic pathways, diverse terminal chemical structures, broad distribution among related organisms (with some taxonomic specificity), functions essential to survival, and relative ease of analysis (Leavitt and Hodgson 2001). For example, ubiquitous pigments such as b-carotene, Chl a and pheophytin a have proven valuable indicators of total algal abundance, while taxon-specific carotenoids allow distinction among algal functional groups such as siliceous algae and dinoflagellates (fucoxanthin), cryptophytes (alloxanthin), chlorophytes (lutein, Chl b, pheophytin b), diatoms (diatoxanthin), dinoflagellates (peredinin), cyanobacteria (echinenone, zeaxanthin) and various other prokaryotic groups (e.g., colonial cyanobacteria, N2-fixing cyanobacteria, various sulfur bacteria, etc.). Similarly, derivatives of both Chls and carotenoids are valuable indicators of the occurrence of sedimentary and water-column processes that transform pigments (grazing, anoxia, stratification, light, etc.) and can be used as indicators of alterations in the physical, chemical and biotic characteristics of lakes (e.g., Hodgson et al. 1998).
Pigments in lake sediments are derived from planktonic and benthic algal communities, phototrophic bacteria and aquatic macrophytes (reviewed in Leavitt 1993), with only minor contributions of undegraded pigments from terrestrial plants. Instead water and sediments also contain pigments in detritus resuspended from bottom or littoral deposits or transported from the terrestrial environment. In all cases, pigments degrade both in the water column (reviewed in Cuddington and Leavitt 1999) and following deposition in the sediments (Hurley and Armstrong 1990, Hodgson et al. 1998). Pigment degradation in the water column is usually very rapid and extensive (>95% degraded; half-life of days) and includes rapid photo-oxidation by photosynthetically-active irradiance (PAR), oxidation by microbes or chemicals, grazing by invertebrates and decolourization or transformation by enzymes during cell senescence. The relative importance of these processes depends also on the type of phototrophic cell undergoing degradation and degradative processes can interact in complex manners to regulate the abundance and composition of pigments deposited in lake sediments (see Fig. 1 in Cuddington and Leavitt 1999). Pigment degradation in sediments is often less rapid than losses in the water column, particularly under anoxic conditions, but can be intensified by excessive exposure to light and burrowing invertebrates (Leavitt 1993).
The molecular mechanisms leading to pigment degradation differ among chlorophylls and carotenoids. In general, carotenoids are converted first into colored cis-isomers before complete cleavage of the conjugated double-bond system (chromophore) responsible for absorbance of visible light (see below; Leavitt 1993).
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In contrast, chlorophylls degrade first to pheophytins via the loss of Mg2+ atoms from the tetrapyrole ring, or to pheophorbides following loss of the phytol chain and various side groups. In both cases, molecules can be further transformed, or degraded to colorless derivatives.
Rates of degradation differ greatly among pigments (Leavitt 1993, Steenbergen et al. 1994) and between habitats within lakes (Hurley and Armstrong 1990). While most algal pigments are protected from net loss by biosynthesis within live cells, detrital pigments are rapidly and differentially degraded in most aquatic habitats. Losses are particularly great when compounds are exposed simultaneously to high irradiance, temperature and oxygen, such as commonly occurs within the epilimnion of most lakes. Further, grazing by invertebrates and chemical oxidation during algal sinking also influence pigment deposition rates at the lake bottom (Cuddington and Leavitt 1999). In general, compounds with complex oxygen-containing functional groups degrade more rapidly than less modified compounds (Leavitt and Carpenter 1990a, 1990b; Hurley and Armstrong 1990; Steenbergen et al. 1994), although rates of pigment loss also vary because of differences in biochemical characteristics of degrading cells or because of differences in habitat characteristics, selective grazing by invertebrates or other ecological factors (Leavitt and Carpenter 1989).
Pigment degradation continues following deposition of biomarkers in lake sediments. In general, rates of decolorization are greatest when sediments are exposed continuously to oxygen or burrowing in-fauna (Bianchi et al. 2000, 2002). Although high irradiance can also cause extensive photo-oxidation of sedimentary pigments associated with detritus, net loss is low for compounds protected in live algae (Leavitt and Carpenter 1989). Instead, losses are greatest for oxygen-rich pigments (e.g., highly substituted carotenoids, native Chl a) in detritus or sediment, particularly those containing epoxides and other intra-molecular rings containing oxygen (e.g., peredinin, fucoxanthin, violoxanthin, neoxanthin).
Mechanisms of carotenoid degradation and transformation differ substantially among individual compounds. For example, fucoxanthin, a carotenoid characteristic of diatoms, chrysophytes and some dinoflagellates, has an intramolecular functional group containing oxygen (-C=O) that can react with the central chromophore to decolorize the pigment beyond our ability to detect it by common spectrophotometric methods.
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When the central chromophore (light absorbing region of conjugated double bonds) is broken, the ability of the carotenoid to absorb visible light is eliminated, and the molecule disappears from the analytical detection window. In contrast, diatoxanthin (mainly diatoms, some chrysophytes) can be produced by inter-molecular rearrangement from diadinoxanthin (to eliminate an epoxide group), as follows:
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This transformation occurs within months of sedimentation to the lake bottom and results in a colored product (diatoxanthin) that is stable for thousands of years (e.g., Rusak et al. 2004). Similarly, experimental studies have demonstrated that myxoxanthophyll from colonial cyanobacteria is often less stable than other cyanobacterial biomarkers in lake sediments (Leavitt 1988). In this case, myxoxanthophyll has a sugar-containing functional group that makes it more attractive to microbial processing than more chemically-simple echinenone of zeaxanthin, both indicators of total cyanobacterial biomass.
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Despite extensive degradation of pigments both during sinking of phototrophs and following pigment deposition within the sediments, diverse studies including whole-lake mass balances (Leavitt and Carpenter 1990a), ecosystem experiments (Leavitt and Findlay 1994, Leavitt et al. 1999), simulation models (Cuddington and Leavitt 1999) and comparisons of coeval plankton and fossil records all suggest that algal production and pigment deposition are correlated well for historical reconstructions within single lakes (reviewed in Leavitt 1993, Cuddington and Leavitt 1999). In addition, simple ratios of labile:stable pigments (i.e., precursor:product ratios) can be used to identify how fossil preservation varies among lakes or through time at a single site. For example, a rapid decline in ratios of labile Chl a: stable pheophytin a can be used to identify periods of time in which post-depositional pigment transformations may have occurred (see Fig. A1 below).
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| FIG. A1. Temporal variations in the ration of chlorophyll a (labile) to pheophytin a (stable) concentration (nmol pigment / g organic matter) in sediment cores of the Qu'Appelle Valley lakes. Changes in ratios through time indicate variation in pigment preservation. | |
With improved understanding of the factors regulating pigment deposition and preservation of fossil pigments, sedimentary carotenoids and Chls have proven to be essential tools for the study of diverse ecological communities and processes, including analysis of algal and bacterial community composition (Züllig 1981), food-web interactions (Leavitt et al. 1989), lake acidification (Guilizzoni et al. 1992), changes in the physical structure of lakes (Hodgson et al. 1998), mass flux within lakes (Carpenter et al. 1988), past UV radiation environments (Leavitt et al. 1999) and analysis of the temporal scales of lake variation (Carpenter and Leavitt 1991). Further, fossil pigments have been used as indicators of a wide variety of human impacts on aquatic ecosystems, including eutrophication, acidification, fisheries management, land-use practices and climate change (reviewed in Leavitt and Hodgson 2001).
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