Morphology and Distribution of Bubble‐Supported Microbial Mats From Ice‐Covered Antarctic Lakes

Gas bubbles directly influence the macromorphology of benthic microbial mats resulting in preservable biosedimentary structures. This study characterizes the morphology and distribution of microbial mats growing in gas‐supersaturated, perennially ice‐covered lakes Fryxell, Joyce, and Hoare of the McMurdo Dry Valleys of Antarctica. Photosynthetic benthic mats within the gas‐supersaturated zone trap oxygen‐rich bubbles and become buoyant, tearing off the bottom as “liftoff mats.” These liftoff mats form a succession of morphologies starting with bubble‐induced deformation of flat mats into tent, ridge, and finger liftoff mat. With progressive deformation, mats tear, forming sheet liftoff, while multiple cycles of deformation and tearing transform sheet into strip liftoff. Some mats detach from the substrate and float to the underside of the ice. The depth range of the liftoff zone has varied over time at each lake. Downslope expansion of bubble formation brings previously bubble‐free, deep‐water pinnacle mats into the liftoff zone. When the liftoff zone shallows, liftoff mats at the deeper end deflate and can become scaffolding for additional mat growth. The superposition and relative orientation of liftoff and pinnacle mats can be used to track the maximum depth of the liftoff zone and changes in gas saturation state in these lakes through time. Our results demonstrate that gas bubbles, even when they are transitory, can exert a significant impact on the morphology of microbial mats at larger scales. This provides a way to identify similar structures and gas supersaturated environments in the biosedimentary record.

气泡直接影响底栖微生物席（benthic microbial mats）的宏观形态，并形成可保存的生物沉积构造（biosedimentary structures）。本研究对南极麦克默多干谷（McMurdo Dry Valleys）的弗莱克塞尔湖（Fryxell）、乔伊斯湖（Joyce）与霍尔湖（Hoare）这三处常年被冰覆盖的气体过饱和湖泊中生长的微生物席的形态与分布特征进行了分析表征。栖息于气体过饱和区域内的光合底栖微生物席会捕获富氧气泡并获得浮力，从湖底脱离形成脱离垫（liftoff mats）。这类脱离垫会经历一系列形态演化：最初由气泡诱导扁平垫变形，形成帐篷状、脊状与指状脱离垫；随着变形持续发展，微生物席发生破裂，形成片状脱离垫；而经过多轮变形与破裂循环后，片状脱离垫会进一步演变为条带状脱离垫。部分脱离垫会从基底完全脱离，漂浮至冰盖下表面。各湖泊中脱离垫区域的深度范围随时间发生变化。气泡形成区域沿坡向下扩张，会将此前无气泡分布的深水尖顶状微生物席（pinnacle mats）纳入脱离垫区域。当脱离垫区域向浅水区收缩时，位于较深处的脱离垫会逐渐排气收缩，并可作为新生微生物席生长的附着基底。通过脱离垫与尖顶状微生物席的叠置关系及相对方位，可反演各湖泊脱离垫区域的最大深度以及气体饱和度状态随时间的变化。本研究结果表明，即便只是短暂存在的气泡，也能在宏观尺度上对微生物席的形态产生显著影响。这一发现为在生物沉积记录（biosedimentary record）中识别同类构造及气体过饱和环境提供了可行途径。