60th Annual Report on Research 2015

Dr. Giulio Mariotti, a former postdoctoral scholar co-advised by Drs. Bosak and Perron, has established an experimental system to study the biogeochemical properties of and microbial growth on sandy sediments in the presence of bedforms and oscillatory flow. Dr. Mariotti has used this system to investigate the origin of some common patterns on the surface of sand beds. Recently, he identified interactions between oscillatory flow and cm-scale microbial aggregates that produce elongate trails on the surface of a sediment bed. Such trails abound in late Ediacaran and early Paleozoic sandstones and siltstones and are often attributed to early animals. Trails left by moving microbial aggregates share a number of characteristics with some presumed trace fossils of the earliest animals: elevated edges, zig-zag patterns, smooth curves, reversals, intersections with other trails, series of pits, and paths that terminate abruptly and restart nearby. Under the same flow conditions, millimetric microbial aggregates generate wrinkle structures. Thus, the interaction between flow and microbial aggregates on a sediment bed can produce a number of structures that are currently interpreted as evidence of early animal locomotion. Bosak presented this work at the GSA Annual Fall Meeting in Vancouver, 2014. An article describing these results is currently in review.

We used the same system to explore the colonization of ripples by photosynthetic mats in the presence of oscillatory flow. Specifically, we sought to determine how interactions between ripples and oscillatory flow influenced the growth rate, composition and spatial variability of microbial mats. The initial conditions and the chemistry of the growth medium give rise to different spatial patterns of microbial growth (TOC figures). In experiments initiated with a nutrient-rich medium, the microbial mat grow faster on ripple troughs than on ripple crests and uniformly cover the bed surface after two months. Analysis of bacterial diversity in the mats by Illumina sequencing of 16S rRNA sequences revealed similar microbial composition in crests and troughs. We hypothesize that the faster growth in the troughs is due to the “strainer” effect: the suspended bacteria from the inoculum are preferentially delivered to troughs by the wave-induced porewater flow. When microbial growth and colonization start in a recycled medium, the mats grow preferentially on the ripple crests, the enhanced growth on the crests persists for up to two years, and the composition of microbial communities differs between troughs and crests. We speculate that the organic degradation in the sediments releases nutrients, which are then delivered to the ripple crests by porewater upwelling, stimulating microbial growth. The same nutrients are depleted from the recycled medium and limit the growth in the downwelling areas (ripple troughs). Thus, the initial delivery of microbes strongly influences the development of microbial mats and porewater pumping stimulates the growth of microbial mats on sand in nutrient depleted waters. Similar macroscopic patterns of mat growth on sand ripples might be used to infer water biochemistry, the ages of microbial cover and may lead to differential patterns of microbially-induced lithification in porous sediments. Dr. Mariotti is currently preparing a manuscript describing these findings and will present them at the upcoming Ocean Sciences Meeting in 2016.

Sharon Newman, a graduate student in the Bosak lab is studying how cyanobacteria, major microbial builders of photosynthetic microbial mats, interact with sediments and flow in siliciclastic environments. She tested the influence of: (1) silica concentrations in seawater, (2) the abundance of fine particles in sandy substrate, and (3) agitation on the extent and composition of coating around sheaths of filamentous cyanobacteria. She used dialysis bags to physically separate the cyanobacteria from the sand and determine the relative contributions of mineral precipitation and biological trapping of suspended mineral grains in agitated samples. She found that cyanobacteria trapped suspended sediment within days when in the presence of moderate to high suspended sediment loads and agitation which did not mobilize sand grains, but mobilized clay. Localized precipitation of minerals around cyanobacterial filaments occurred over longer timescales (>1 month), and was facilitated by high silica concentrations (0.1-0.4 mM). These experiments show that microbial precipitation and trapping of clays, elevated concentrations of dissolved silica and a steady delivery of clay-sized, suspended mineral grains are most conducive to microbial fossilization and the preservation of organic matter in sandstones, siltstones and shales. The microbial contribution to mineral accumulation depends on the microbial species, suggesting that environmental conditions which select for thin, filamentous cyanobacteria may promote mineral accumulation. She is preparing two manuscripts and she presented these results at AbSciCon and Goldschmidt Conference in 2015.