Polar microbioerosion patterns exemplified in Arctic and Antarctic barnacles

Abstract

Bioerosion is the degradation of hard substrates by living organisms, primarily in marine environments. The process is an important component of the carbon cycle, it attributes to biosedimentary processes, and it gains attention as the “other ocean acidification problem” acknowledging the acceleration of bioerosion as a consequence of the global climate change. Bioerosion leaves traces in the substrate, which serve as a useful tool to investigate palaeobathymetry or -temperature. Most bioerosion studies were conducted at low latitudes, with a focus on shallow water depths. Few studies were performed in the cold-temperate regions, but almost none at the highest latitudes in the polar seas, thus setting the scene for this doctoral thesis exploring traces of microbial bioerosion in the polar realm. Acorn barnacles from three polar study sites, spanning a wide bathymetrical range, were used as a hard substrate to visualize the microbioerosion traces in their shells by means of scanning electron microscopy of epoxy resin casts. Samples from the Arctic Svalbard archipelago were from the photic intertidal to aphotic water depths of 125 m; from the Frobisher Bay, Canadian Arctic, barnacles from 62 to 94 m were examined; from the Ross Sea, Antarctica, samples originated from 37 m to 1680 m. Each study area was investigated by semi-quantifying and statistically evaluating the ichnodiversity regarding a bathymetric trend and latitudinal gradient. In total, 29 different microendolithic traces formed by cyanobacteria (4), chlorophytes (2), rhodophytes (1), sponges (1), fungi (12), foraminifera (3), bacteria (1), unknown microorganisms (4), and cirripeds (1, macroboring) were recorded in more than 200 samples. Three traces were identified at all three sites, eight traces were found exclusively in Svalbard, one in Frobisher Bay, eight in the Ross Sea, and three were restricted to the Arctic. The inferred trace-makers were mainly organotrophs and expectedly dominated by fungi, as they are very robust and thrive even under harsh environmental conditions. The polar regions are characterized by extreme conditions such as cold temperatures, months of sea ice cover and the cycle of polar day and night. Especially the latter is reflected in a general impoverishment in phototrophic bioeroders, as statistically confirmed in Svalbard. The Svalbard study material enabled the identification and establishment of a previously unknown bioerosion trace that is interpreted as a potential key ichnotaxon for cool- to cold-water (palaeo)environments. Comparisons with studies from lower latitudes led to the conclusion that the ichnodiversity in aphotic water depths is nearly constant across all latitudes. A parallel comparison of all photic zones could not be accomplished due to scarcity of suitable samples from shallow waters caused by the sea-ice abrasion of barnacles and their comparatively limited distribution in shallow water depths in polar environments. The three comprehensive microbioerosion studies are an important step towards a better understanding of polar microbioerosion patterns and allow comparisons with lower latitudes and preliminary findings in a global context. Profound knowledge of bioerosion in polar environments is particularly important considering that the environment in the polar realm is responding to global climate change at an unprecedented pace.