PS 3-37 - Photosynthesis under rocks: Hypolith distribution across the Namib Desert rainfall gradient

Monday, August 7, 2017
Exhibit Hall, Oregon Convention Center
Brittney D. Monus, School of Earth and Space Exploration, Arizona State University, Phoenix, AZ, Elise Nghalipo, Namibia University of Science and Technology, Windhoek, Namibia, Vimbai Marufu, Natural Resources Management, Namibia University of Science and Techology, Windhoek, Namibia and Heather L. Throop, School of Life Sciences, Arizona State University, Tempe, AZ

In locations with low precipitation and high solar radiation, photosynthetic cyanobacteria can colonize the underside of quartz fragments, forming ‘hypoliths.’ The quartz provides protection against wind, reduces solar radiation, and slows the rate of evaporation following infrequent rain or fog events. In most desert systems, vascular plants are the main primary producers. However, hypoliths might play a key role in carbon fixation in hyperarid deserts that are mostly devoid of vegetation. We investigated hypolith distribution, carbon fixation, and carbon input into soil at six sites along a rainfall and fog gradient in the central Namib Desert in Namibia. We used line point intersect transects to assess ground cover (bare soil, colonized quartz fragment, non-colonized quartz fragment, non-quartz rock, grass, or lichen) at each site. We used digital image analysis to quantify percent cover by cyanobacteria on selected quartz fragments. To assess chlorophyll content and carbon inputs into soils from hypoliths, we analyzed cyanobacteria and soil samples from underneath hypoliths and non-quartz rocks, and in bare points. We used a portable gas exchange system to measure CO2flux of the hypoliths at each site.


Ground cover was fairly similar among sites, with bare ground > non-colonized quartz fragments > colonized quartz fragments > non-quartz rocks. Grass was present only at the highest MAP site where it accounted for 1% of ground cover. Lichens were present only at the lowest MAP site, where they accounted for 30% of ground cover. Both the proportion of quartz fragments colonized and the coverage of colonized quartz fragments by cyanobacteria generally increased with MAP. At the most coastal (lowest MAP) site, there was 5.9% coverage by colonized hypoliths, while colonized quartz accounted for 18.7% of ground cover at the most inland (highest MAP) site. There was CO2 uptake from most hypoliths measured, with net carbon uptake rates ranging from 0.3 and 6.4 μmol m-2 s-1 on well hydrated hypoliths. These carbon flux values are similar to previous work in the Mojave Desert. Our results suggest that hypoliths might play a key role in the fixation of organic carbon in hyperarid ecosystems where quartz fragments are abundant, with MAP constraining hypolith abundance.