Benthic diatoms in Arctic Seas - ecological functions and adaptions

Abstract

Arctic coasts exhibit an enormous area of shallow water regions, which are dominated by unicellular benthic diatoms. These microalgae form a community known as microphytobenthos. Microphytobenthic biofilms cover extensive sediment areas and are responsible for high rates of primary produc- tion. They stabilize sediment surfaces against erodibility under flow condi- tions by the excretion of extracellular polymeric substances, thereby affecting the exchange of oxygen and nutrients across the sediment-water interface. Consequently, this community represents a key component in the functioning of Arctic trophic webs, particularly as a major food source for benthic suspen- sion- or deposit-feeders. Arctic benthic diatoms are confronted with pronounced seasonal variations of solar radiation, low temperatures, hyposaline (meltwater) conditions in summer, as well as long periods of ice and snow cover. From the few data available, it seems that these organisms can easily cope with such environ- mental extremes. Diatoms show a rather unusual photosynthetic flexibility providing optimum photoprotection and rapid photoacclimation under fluc- tuating and highly variable radiation conditions. In addition, some benthic diatoms are capable to vertically migrate into the sediment-microphyto- benthos matrix, an efficient and energy-saving mechanism to avoid, for example, photodamage. These microalgae can survive the polar night, but the underlying processes are unknown. Although comprehensive data are still lacking, some studies indicate that Antarctic benthic diatoms are rather polar stenothermal and psychrophilic, while their Arctic pendants are more eurythermal and psychrotolerant. If these conspicuous differences in tempera- ture requirements of both benthic diatom floras are underlined by more ecophysiological investigations, the data could be related to the different cold- water histories of the Arctic and Antarctica. If environmental changes such as the observed Arctic warming are negatively affecting the dark survival poten- tial of benthic diatoms, their ecological function as important primary produ- cers in polar regions may be strongly reduced. Climate change in the Arctic predicts other multifactorial stressors, such as increase in precipitation and permafrost thawing with consequences for the shallow water regions. However, complex factor interactions, as well as the full genetic diversity and physiological plasticity of Arctic benthic diatoms are still not deeply studied. Zusammenfassung: Arktische Kusten weisen grose Flachwassergebiete auf, welche von einzelligen benthischen Diatomeen dominiert werden, die eine als Mikrophytobenthos benannte Lebensgemeinschaft ausbilden. Mikrophyto- benthische Biofilme bedecken riesige Sedimentflachen und tragen dort zu hohen Primarproduktionsraten bei. Sie stabilisieren die Sedimentoberflachen gegenuber stromungsbedingten Erosionsprozessen durch die Fahigkeit extra- zellulare polymere Substanzen zu exkretieren, wodurch der Austausch von Sauerstoff und Nahrstoffen an der Sediment-Wasser-Grenzschicht beeinflusst wird. Aufgrund dieser besonderen Fahigkeiten ubt diese Lebensgemeinschaft eine Schlusselrolle in den trophischen Nahrungsbeziehungen der Arktis aus, insbesondere als wichtige Nahrungsquelle fur benthische Suspensions- und Sedimentfresser. Benthische Diatomeen der Arktis sind mit ausgepragten saisonalen Variabi- litaten der Sonnenstrahlung, niedrigen Temperaturen, hyposalinen Bedin- gungen aufgrund von Schmelzwasser im Sommer sowie langen Perioden einer Eis- und Schneebedeckung konfrontiert. Die wenigen verfugbaren Daten zeigen jedoch, dass diese Organismen mit solchen extremen Umweltbedin- gungen gut klar kommen. Diatomeen zeigen eine ungewohnliche Flexibilitat der Photosynthese, welche sowohl einen optimalen Schutz als auch eine schnelle Anpassung an fluktuierende und hoch variable Strahlungsbedin- gungen ermoglicht. Zusatzlich sind einige benthische Diatomeen befahigt