Volume 52, Issue 6 - Special Issue: Biology of Polar Benthic Algae

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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.

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.

The present study consolidates knowledge of Antarctic marine Rhodophyta, incorporating new observations from the Antarctic Peninsula. Descriptions of four new genera and five new species are included: Rhodokrambe laingioides (Delesseriaceae), Leniea lubrica , Austropugetia crassa , Varimenia macropustulosa R.L. Moe gens. et spp. nov., and Nereoginkgo populifolia R.L. Moe sp. nov. (Kallymeniaceae). The new combination Trematocarpus antarcticus (Hariot) Fredericq et R.L. Moe is proposed for Kallymenia antarctica Hariot (Sarcodiaceae). Phylogenetic relationships are suggested based primarily on unpublished molecular analyses of rbc L sequences provided by S.-M. Lin, S. Fredericq, P.W. Gabrielson, D.W. Freshwater and M. Volovsek. Habitat and environmental conditions are incorporated from the field studies of R.L. Moe and C.D. Amsler. A preliminary assessment is made of the biogeographic relationships between Antarctic red algae and those found in related temperate regions based on a summary of tectonic and climatic reconstructions of the Cenozoic Southern Ocean and the unpublished phylogenetic studies of Hommersand and his associates.

Chemical defenses against herbivores are common in macroalgae from Antarctica, particularly in those species that typically dominate benthic communities. Conversely, although little studied, chemical defenses do not appear to be as important in the ecological relationships of Arctic macroalgae. Mesoherbivory may be particularly heavy in Antarctic macroalgal communities, but Antarctic macroalgae produce chemical defenses against both mesoherbivores and macroherbivores. Antarctic macroalgae have been useful in testing and extending chemical defense theories. While physical defenses may also be important in some Antarctic macroalgal-herbivore relationships, particularly with respect to mesoherbivores, physical defenses do not appear to have wide-spread importance against macroherbivores. In Arctic macroalgae, physical defenses may be of more overall importance in reducing mesoherbivory. Antarctic macroalgae also produce compounds with the potential to control biofouling by sympatric microalgae.

Antiherbivore effects of phlorotannins from eight common Antarctic brown algae were tested with three omnivorous sympatric grazers, the fish Notothenia coriiceps , the sea star Odontaster validus, and the amphipod Gondogeneia antarctica . Incidence of grazing deterrence overall was relatively low, with only a few algal species deterring each of the grazers, and with phlorotannins from different algae active against different grazers. Most activity was found in Desmarestia menziesii and Cysto-sphaera jacquinotii , which deterred two out of the three grazers, while D . anceps and Himantothallus grandifolius deterred only one grazer type each. Only weak antimicrobial effects were detected in all phlorotannin extracts against four Antarctic bacterial strains, with phlorotannins of D. anceps and Phaeurus antarcticus having the strongest effects. Phlorotannin extracts of all brown algal species tested caused strong in vitro mortality in a fouling diatom. The ecological significance of this activity remains to be tested. While the bulk tissue phlorotannin concentrations likely overestimate naturally occurring surface concentrations encountered by foulers in situ , fouler settlement may be deterred at much lower concentrations than those eliciting mortality. Overall, Antarctic phlorotannin deterrent properties were specific both to the algal species as well as the grazer, bacteria and diatom taxa tested. For grazer deterrence, we propose that, for each of the tested grazers, complex interactions between phlorotannin chemical properties and grazer digestive characteristics may determine the effectiveness of specific phlorotannins. Larger generalizations of phlorotannin-grazer patterns such as distinction between meso- and macrograzers may not be valid, at least not for Antarctic brown algal phlorotannins.

The current database on benthic microalgal production in Arctic waters comprises 10 peer-reviewed and three unpublished studies. Here, we compile and discuss these datasets, along with the applied measurement approaches used. The latter is essential for robust comparative analysis and to clarify the often very confusing terminology in the existing literature. Our compilation demonstrates that i) benthic microalgae contribute significantly to coastal ecosystem production in the Arctic, and ii) benthic microalgal production on average exceeds pelagic productivity by a factor of 1.5 for water depths down to 30 m. We have established relationships between irradiance, water depth and benthic microalgal productivity that can be used to extrapolate results from quantitative experimental studies to the entire Arctic region. Two different approaches estimated that current benthic microalgal production in the Arctic is between 1.1 and 1.6×10 7 tons C year -1 . Climate change is expected to increase the overall primary production and affect the balance between pelagic and benthic productivity in the Arctic. It is therefore imperative to get better quantitative understanding of the relationship between increased freshwater run-off, shrinking sea-ice cover, light availability and benthic primary production to assess future impact on the Arctic food web and trophic coupling.

In contrast to numerous studies on the biomass of marine microphytobenthos from temperate coastal ecosystems, little is known from polar regions. Therefore, microphytobenthos biomass was measured at several coastal sites in Arctic Kongsfjorden (Spitsbergen) during the polar summer (June–August 2006). On sandy sediments, chl a varied between 8 and 200 mg m -2 and was related to water depth, current/wave exposure and geographical location. Biomass was rather independent of abiotic parameters such as sediment properties, salinity, temperature or light availability. At three stations, sediments at water depths of 3–4, 10, 15, 20 and 30 m were investigated to evaluate the effect of light availability on microalgae. Significant differences in distribution patterns of biomass in relation to deeper waters >10 m were found. The productive periods were not as distinct as phytoplankton blooms. Only at 3–4 m water depth at all three stations were two- to threefold increases of biomass measured during the investigation period. Hydrodynamic conditions seemed to be the driving force for differences in sediment colonisation by benthic microalgae. In spite of the extreme Arctic environmental conditions for algal growth, microphytobenthos biomass was comparable to marine temperate waters.

The phenology of polar seaweeds is strongly tuned to the strong seasonal changes in underwater radiation. Daylength triggers the onset of reproduction and growth during winter in season anticipators, particularly in endemic species. More widely distributed species are often season responders growing predominantly in summer in direct response to light, temperature and nutrient conditions. The physiology of polar seaweeds is strongly linked to the life strategy of the individual species. For season anticipators, photosynthetic rates are often highest in late winter-spring when sunlight penetrates deep into the clear waters. Since Antarctic species are seldom nutrient-limited, even during summer, they mostly incorporate photosynthetically fixed carbon directly into biomass. In contrast, Arctic kelps, such as Laminaria solidungula utilise the carbon fixed during summer for synthesis of storage compounds, which are used to fuel growth during the dark winter period when nutrients are sufficient for new tissue formation. This growth pattern reflects a strategy for species optimally adapted to the seasonal changes of nutrient concentrations, which in the Arctic are low (or nearly undetectable) in summer, and high in winter. This review concludes with a discussion of possible implications of global climate changes on the phenology and productivity of polar seaweed communities.

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 μmol 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°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.

Organisms populating benthic shallow water systems of both polar regions are adapted to a particularly harsh environment. We studied effects of freezing and the combination of high light intensities and low water temperatures on photosynthesis of key macroalgal species from the Arctic intertidal ( Fucus distichus ) and Antarctic subtidal ( Palmaria decipiens ). Photosynthetic activity of F. distichus specimens was monitored during the freezing process; there was a marked decrease in quantum yield with decreasing temperatures, and a rapid recovery as soon as temperatures increased again. Thus, under the experimental conditions tested, no indication of photodamage was found. Specimens of Palmaria were exposed to a combination of high light intensities and low water temperatures. A persistent impairment of photosynthetic activity occurred at 0°C at light intensities of 400 μmol photons m -2 s -1 . In all treatments, there was a decreasing ratio of phycobiliproteins to chlorophyll a . Overall, the two studies provide baseline data for interpreting physiological responses of two important macroalgal species in an extreme environment, the polar coastal ecosystem.

Temperature is one of the most important factors controlling the biogeographic distribution of seaweeds and is expected to increase due to the rise in anthropogenic greenhouse gas concentrations, especially in polar and cold-temperate regions. To estimate prospective distributional shifts in cold-water key structural seaweeds from both hemispheres, we related temperature requirements and recent distributions of seaweeds to observed mean sea surface temperature (SST) isotherms for the periods 1980–1999 (Meteorological Office Hadley Centre's SST data set; HadISST) and to modelled temperatures for 2080–2099 [Coupled Model Intercomparison Project 3 (CMIP3) database prepared for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) report] based on moderate greenhouse gas emissions Special Report on Emission Scenarios – Scenario B1 (SRESA1B). Under this scenario, North Atlantic polar to cold-temperate seaweeds investigated will extend their distribution into the High Arctic until the end of the 21st century, but retreat along the northeastern Atlantic coastline. In contrast, selected Antarctic seaweeds will probably not significantly alter their latitudinal distributions, as deduced from our presently incomplete knowledge of their temperature requirements. We identified several cold-temperate regions where seaweed composition and abundance will certainly change with elevated temperatures. The results are discussed in the context of local temperature conditions, effects of multifactorial abiotic and biotic interactions and expected ecological consequences for seaweed-dominated ecosystems.

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 microphytobenthic 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.

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