The rise of terrestrial plants in the early Paleozoic or later Proterozoic precipitated a major ecological transition in Earth's history. Early embryophyte-dominated terrestrial communities provided habitat and food for colonizing animals and increased rates of atmospheric CO₂ sequestration into decay-resistant organic C, thereby influencing climate. Our previous studies of modern, early-diverging mosses as proxies of early plants suggested that the latter might have substantially influenced atmospheric chemistry prior to the rise of vascular plants (Graham et al. 2004a). Because fossil and molecular data indicate that earliest land plants resembled modern liverworts, we used the widespread, early-diverging liverwort Blasia pusilla to model UVA effects on resistant C production by earliest plants. B. pusilla was isolated from a wild population and replicate cultures of homogeneous age and size raised on agarized 1/3 Gamborg's medium. Plants were transferred to the surfaces of moist quartz sand, a substrate known to have occupied large areas of present day North America and Africa during the early Paleozoic. An experimental temperature range of 7-11ºC reflected conditions present in Ordovician North America, periglacial periods of the late Ordovician, and Proterozoic “snowball” environments. A starting biomass of 1 mg was estimated from mean dry weights of 20 plants. Replicates were illuminated with full spectrum lamps +/- a UVA source. Results/Conclusions

At the end of 4 weeks, mean dry masses were measured before and after acetolysis to estimate differences in resistant C biomass. Plants exposed to UVA displayed pink cell walls and produced 3.5 times the resistant C as those not exposed. Resistant B. pusilla biomass was a mean 1.5% of total dry biomass. This is an order of magnitude less than was estimated for the later-diverging Marchantia polymorpha, whose resistant lower epidermal tissues closely resemble enigmatic early Paleozoic plant remains (Graham et al. 2004b). Even so, acetolyzed UVA-treated B. pusilla also yielded sheet and tubular remains having phenolic-type autofluorescence that were similar to fossils attributed to early land plants. Plant productivity was estimated at 1.3g/cm²·yr. This quantity, multiplied by an estimated 10% substrate cover and potential mesic habitat estimate of 1.1·10⁶ km² suggests that very early land plants could have generated 8.6 · 10⁶ petagrams (Pg=a billion metric tons) of resistant, sequesterable C over a conservatively estimated 4·10⁷ yr time period before the rise of vascular plants, and 3·10⁷ Pg over a 14·10⁷ yr timespan suggested by Cambrian microfossils.