This task uses interdisciplinary field and laboratory work coupled with chemical, climate and ecosystem models to explore the impact of life on its environment, and how interactions between the biosphere, planet, and host star can result in remotely detectable biosignatures.
Past VPL accomplishments in these areas include modeling of marine biogenic gases in the Archean (Kharecha et al. 2005); simulation of the light environment for photosynthesis on planets orbiting different stars, and uncovering how photosynthetic pigments and atmospheric composition may have coevolved (Kiang et al., 2007a, 2007b, 2008); photoacoustic measurements of photon energy storage efficiency in a chlorophyll d-utilizing cyanobacterium (Mielke et al., 2011); genetic and biochemical studies of microbial mats at the Cuatro Cienegas field site as analogs for life on the Early Earth (Breitbart et al 2009, Desnues et al, 2008, Souza et al, 2008); coupling of dynamic land and ocean ecosystem models in GCMs in preparation for 3D extrasolar planet modeling; and pioneering work on alternative biosignatures for anoxic atmospheres (Domagal-Goldman et al., 2011).
In current and future projects, we plan to pay special attention to metabolism-environment combinations that have distinctive traits with regard to biogenic gases and/or surface spectral features, with accompanying process models. These projects span analogs for the Earth through time and extrasolar environments. In a broader context, our work will provide direct observational constraints on the possible path of biosphere-planet co-evolution as it relates to photosynthetic life, both on Earth and beyond.
Recent scientific progress and highlights include:
The Ent Dynamic Global Terrestrial Ecosystem Model
VPL researchers recently implemented a full carbon cycle in the Goddard Institute for Space Studies general circulation model (GISS GCM), including an ocean biogeochemistry model. We are continuing evaluation and testing of the coupled carbon dynamics to achieve a balanced carbon cycle with seasonal dynamics and prescribed land vegetation cover. Evaluation includes comparing equilibrium behavior of carbon stocks and identifying climate biases with observed meteorology for the 20th century versus coupled to the GISS GCM. To do this with realistic land vegetation cover, we have been developing a global vegetation structure dataset to utilize the latest MODIS products (land cover, leaf area index, albedo), as well as LiDAR from the Geoscience Laser Altimeter System aboard the Ice, Cloud, and Land Elevation Satellite (ICESat/GLAS) (vegetation height) to serve as both boundary conditions or as a performance evaluation dataset. This improved dataset performs corrections for areas of steep topography and uses MODIS and climate data to classify pixels not observed by ICESat/GLAS. Global tests with this dataset will help constrain the carbon budget of the Earth and processes in the Ent model, setting the stage for experiments for exoplanet biosphere modeling.
The Long Wavelength Limit for Oxygenic Photosynthesis
For planets orbiting M dwarfs, or for haze-covered planets like the early Earth, sunlight reaching the surface of the planet will be concentrated at longer wavelengths. To better understand the long-wavelength limit for photosynthesis, NAI postdoctoral fellow Steven Mielke is working on measurements of the efficiency of photon energy storage by wavelength in whole intact cells of the cyanobacterium Acaryochloris marina. A. marina is the only known organism to have chlorophyll d (Chl d) which uses photons at wavelengths in the far-red (713-715 nm) and near-infrared (740 m), whereas all other oxygenic photosynthetic organisms use chlorophyll a (Chl a) with absorbance peaks at 680 nm and 700 nm. Using pulsed, time-resolved photoacoustics (PTRPA or PA), we have obtained detailed measurements indicating that A. marina shows efficiencies higher than or comparable to those in Chl a-utilizing organisms. These results imply that oxygenic photosynthesis can operate quite effectively at far-red/near-infrared wavelengths without losses to back reactions (Mielke et al., 2011).
Figure: A. Marina growing in the laboratory