Antarctic benthic marine diatoms from the Potter Cove region, King George Island were studied in samples collected during the austral summer 2003. A floristic list was made to add information on the Antarctic benthic diatom distribution. A total of 84 species was identified from four localities in Potter Cove, the majority of which are of cosmopolitan distribution. The most common taxa encountered were Cocconeis spp., Gyrosigma fasciola, Navicula cf. cancellata, N. cf. perminuta, Petroneis plagiostoma and Pleurosigma obscurum. Both G. fasciola and P. obscurum are recorded for the first time from Antarctica with such common occurrence. The overall diatom population in Potter Cove appeared rather different from other diatom populations observed in Antarctic marine habitats.

Polar algae have a striking ability to photosynthesize and grow under very low light and temperatures. In seaweeds, minimum light demands for photosynthetic saturation and compensation can be as low as 10 and 2 mu mol photons m(-2) s(-1), respectively. For benthic microalgae, these values can be even lower because of the limited irradiance reaching deep sea floors. The extreme shade adaptation of these organisms sets their distributional limits at depths close to 40 m and enables them to tolerate long periods of extended darkness. In addition to their capability for efficient photosynthesis at extremely low light levels, polar algae possess metabolic adaptations to persist at low temperatures, which permit them to complete their life cycles at year-round temperatures close to 0 degrees C. Seaweeds with the lowest temperature demands are the species endemic to the Antarctic while Arctic algae are comparatively less cold-adapted. These adaptive characteristics allow benthic marine algae to make high contributions to high latitude coastal primary productivity and energy fluxes, exceeding or equaling the production of primary producers in more temperate systems. The studies summarized here give important insights into the major physiological adaptations allowing marine benthic microalgae and seaweeds to colonize these extreme habitats.

Stratospheric ozone depletion and the concomitant increase in ultraviolet (UV) B radiation at the earth's surface represent major threats to polar marine ecosystems. Whereas in coastal rocky shore environments macroalgae constitute an assemblage of particular significance to ecosystem function, benthic diatoms dominate micro-phytobenthic assemblages, which typically grow on shallow-water sediments as highly productive and stabilising phototrophic biofilms. This review summarises present knowledge on how UV radiation affects the physiology of polar benthic algae with an emphasis on cell biological and structural changes, molecular targets and repair mechanisms, induction of reactive oxygen species and antioxidative strategies, photosynthesis and growth, photoprotective mechanisms, interactive effects between UV radiation and other abiotic factors, and finally ecological consequences. Although available data indicate that there are specific characteristics and adaptations in polar benthic micro- and macroalgae that explain their ecological success and limits under environmentally extreme conditions, much more research is needed to understand the underlying mechanisms. In particular, more ecosystem approaches and studies on interactive effects, as well as modern genomic, proteomic and metabolomic approaches could help address all open questions and depict a more holistic picture.

Information on succession in marine benthic primary producers in polar regions is very scarce, particularly with regard to effects of abiotic and biotic drivers of community structure. Primary succession begins with rapid colonizers, such as diatoms and ephemeral macroalgae, whereas slow, highly seasonal recruitment and growth are characteristic of annual or perennial seaweed species. Colonization of intertidal and subtidal assemblages on polar rocky shores is severely affected by physical disturbance and by seasonal changes in abiotic conditions. Biotic factors, such as grazing, can strongly affect colonization patterns and also alter competitive interactions among benthic algae. Ambient UV radiation affects the diversity of macroalgal communities during early and later stages of succession. In contrast, microalgal assemblages have high tolerance to UV stress. Climate warming could alter algal latitudinal distribution and favor invasion of polar regions by cold-temperate species. Reduced sea ice cover and retreating glaciers could expand colonization areas but alter light, salinity, sedimentation and disturbance processes. Although the key role of macroalgae in coastal systems and, to a much reduced extent, the importance of microphytobenthos have been documented for polar regions, information on the successional process is incomplete and will benefit from further ecological studies.

This paper reviews the composition, biogeography and zonation of benthic algae in Arctic and Antarctic polar regions. There is a marked contrast in the literature between the amount of information on microalgae vs. macroalgae. Perhaps not surprising in view of their size and conspicuous nature, the macroalgae are better known than the microalgae and they have been studied more intensively. Macroalgal biodiversity is greater in Antarctica than in the Arctic, as is the number of endemic species. Both these characteristics of the Antarctic marine macroalgal flora can be explained by the biogeographical histories of the regions. In contrast, endemism amongst Arctic and Antarctic benthic microalgae is generally considered to be low; however, there is very little evidence to support this and further molecular research is needed to document and clarify the biodiversity of marine benthic microalgae of both polar regions. The zonation or local distribution of polar macroalgae and microalgae is influenced by physiological, morphological, chemical and ecological characteristics that determine responses to a range of environmental factors, including the ability to resist and survive algal grazing. Typically, the lower depth distribution limit elevates with increasing latitude.

Due to different oceanographic and geological characteristics, benthic algal communities of Antarctica and the Arctic differ strongly. Antarctica is characterized by high endemism, whereas in the Arctic only a few endemic species occur. In contrast to the Antarctic region, where nutrient levels never limit algal growth, nutrient levels in the Arctic region are depleted during the summer season. Both regions have a strongly seasonally changing light regime, fortified by an ice covering throughout the winter months. After months of darkness, algae are suddenly exposed to high light caused by the breaking up of sea ice. Simultaneously, harmful ultraviolet radiation (UVR) enters the water column and can significantly affect algal growth and community structure. In the intertidal zone, fluctuations of temperature and salinity can be very large. Ice scours can further influence growth and settlement of intertidal algae. The subtidal zone offers a more stable habitat than the intertidal, permitting the growth of larger perennial algae and microbial mats. Polar regions are the areas most affected by global climate change, i.e., glacier retreat, increasing temperature and sedimentation, with as yet unknown consequences for the polar ecosystem.