| Summary: | Microbial mat aggradation and morphology can be strongly influenced by sedimentation and light in ice-covered lakes of the McMurdo Dry Valleys, Antarctica. In Lake Joyce, mats transitioned from prostrate with widely spaced pinnacles to having densely spaced pinnacles with complex webs and ornamentation at greater distances from inflowing melt water streams. This transition is interpreted to result from decreasing mud sedimentation, which compacted delicate microbial structures such as pinnacle tips, webs, and surface ornamentation. Mud sedimentation also changed through time at sites adjacent to inflowing streams on one of the Lake Joyce deltas; sedimentation likely increased from 1947 through 2009 as lake levels rose. Although mud sedimentation demonstrably affected mat morphology in Lake Joyce, changes in sand and mud sedimentation associated with overhanging rocks in Lake Vanda were not sufficient to dramatically change mat morphology. Instead, microbial mat pinnacles and ridges had a variety of morphological responses to their light environment. Microbial mats growing with oblique directional light both grew down from overhanging rocks with pinnacle orientation independent from the directional light and grew up from the rock-sheltered mat with pinnacles and ridges oriented relative to incident light: asymmetrical pinnacles were inclined toward and flattened perpendicular to the incident light, and ridges were oriented parallel to the incident light. Changes in mat morphology and microbial processes are also preserved in Lake Joyce stromatolites that grew over decades. Stromatolites contain ?13Ccalcite records of variable photosynthetic fractionation of local DIC under lower lake levels, followed by decades of DIC pool 13C enrichment with lower rates of photosynthesis during lake level rise. These results demonstrate that microbial responses to their environments are complex and under the right conditions can be preserved in the rock record.
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