The development and release of gas-filled, aggregates formed by diatoms immersed in a marine mucoid-sediment layer were observed. They consisted of a gas-filled aggregate stalked structure (5.4-5.6 mm in diameter and 1-3 cm height including the stalk) (Fig. 1A, 1B). The microscopic assessment of collected aggregates, allowed us to identify a mucoid-sediment layer colonized by living diatoms (Fig. 1C) and the occurrence of bacteria.
The structures were composed mainly of pennate diatoms with occasional centric diatoms. A general lower abundance of diatoms was observed in the stalk compared to the gas-filled part (Fig. 1D). The most frequently occurring and by far dominant genus within the layer belonged to Pleurosigma sp. (Fig. 1C, 1D). Other occurring pennate taxa were Achnanthes, Amphora, Cocconeis, Fragilaria, Gyrosigma, Licmophora, Nitzschia and naviculoids.
Aggregates stayed attached to the sediments by their elongated stalks for up to 10 days until the buoyant force was sufficient to release them from the bottom. We were not able to characterize the gas entrapped in the head of the structure, nor the origin of it. We assume that buoyancy was caused by the accumulation of photosynthetically produced gases (probably oxygen) according with,,.
In coincidence with and, these structures developed and released in stable bed sediments, in the absence of currents and under a presumably high light intensity. Our system was characterized also by the absence of mesograzers. High light intensity after sea ice melting in early summer or resuspension of bottom sediment have been ascribed as main drivers of increased abundances of benthic diatoms in Antarctic nearshore waters. We recognized these three as potential drivers of our finding. Light intensity seems to be a key factor involved in the occurrence of stalked gas-aggregates formation. Photosynthetically active radiation (PAR) in Potter Cove during summer can be 734 (+291) µmol m-2 s-1 below the surface (10 cm) which is much higher than 0.4 to 1.2 µmol m-2 s-1 reported by. While such high radiation may have a potentially detrimental effect, Antarctic marine benthic diatoms are highly resistant to UV radiation. Sediments were artificially resuspended in the tanks and were let to settle in stable bed sediments exposed to high radiation. Resuspension may be relevant before stabilization of the sediment bed, by making nutrients confined in sediments available for diatom growth. As the tanks were set outdoor and there were snowfall and light rain in some days, we assume that some change in salinity may have occurred. However, we dismiss any potential negative effect on diatoms, since low salinity (even artificially extreme values) on marine pennate benthic diatoms has no evident effect on growth or photosynthetic activity.