Manja Hethke

A complete Holocene record of trematode–bivalve infection and implications for the response of parasitism to climate change

Significance There is growing concern about how parasites will respond to climate change. Previous studies were based on data from short time scales (10−1 to 103 y), but here we present, to our knowledge, the first analysis of the response of parasites to global change on a longer time scale (104 y), utilizing the subfossil record. A 9,600-y record of clams and flatworm parasites from the Pearl River Delta exhibits a significant spike in parasite prevalence during the initial phase of sea-level rise. This increase is not related to changes in salinity or intermediate host availability. Temperature and productivity cannot be quantified and tested as driving factors. These results suggest stark implications for macrobenthos, fisheries, and human health in the context of climate change. Increasing global temperature and sea-level rise have led to concern about expansions in the distribution and prevalence of complex-lifecycle parasites (CLPs). Indeed, numerous environmental variables can influence the infectivity and reproductive output of many pathogens. Digenean trematodes are CLPs with intermediate invertebrate and definitive vertebrate hosts. Global warming and sea level rise may affect these hosts to varying degrees, and the effect of increasing temperature on parasite prevalence has proven to be nonlinear and difficult to predict. Projecting the response of parasites to anthropogenic climate change is vital for human health, and a longer term perspective (104 y) offered by the subfossil record is necessary to complement the experimental and historical approaches of shorter temporal duration (10−1 to 103 y). We demonstrate, using a high-resolution 9,600-y record of trematode parasite traces in bivalve hosts from the Holocene Pearl River Delta, that prevalence was significantly higher during the earliest stages of sea level rise, significantly lower during the maximum transgression, and statistically indistinguishable in the other stages of sea-level rise and delta progradation. This stratigraphic paleobiological pattern represents the only long-term high-resolution record of pathogen response to global change, is consistent with fossil and recent data from other marine basins, and is instructive regarding the future of disease. We predict an increase in trematode prevalence concurrent with anthropogenic warming and marine transgression, with negative implications for estuarine macrobenthos, marine fisheries, and human health.

Oxygen deficiency in Lake Sihetun; formation of the Lower Cretaceous Liaoning Fossillagerstätte (China)

The redox state of Lake Sihetun, represented by the Lower Cretaceous Yixian Formation (western Liaoning, China), is evaluated to understand the formation of this Konservat-Lagerstätte. Lake evolution is subdivided into four phases, of which Phases 2 and 3 exhibit excellent fossil preservation. Exceptional preservation and mass mortality events within Phase 2 were previously attributed to synsedimentary volcanism and oxygen deficiency. However, the volcanic trigger for mass mortality events remains enigmatic and distinction between anoxia and dysoxia has not been put forward so far. To resolve the redox state of the lake during Phase 2, 5394 diameters of pyrite framboid pseudomorphs in 29 thin sections from three localities have been measured. Framboid size distributions reveal that Lake Sihetun was governed by dysoxic bottom waters with spells of anoxia, recording pronounced environmental stress. Hence, holomictic, eutrophic conditions were episodically replaced by meromictic interludes. Spatial variations in redox state were common and oxic conditions were detected in two of the three studied localities. Concentrated iron sulphide layers resulted from biofilms at the lake floor or the development of microenvironments around organic remains, which preserved highly reactive organic compounds. In contrast, sediments of Phase 3 record oxic conditions and an entirely holomictic lake.