Ralf Rautenberger

The abiotic environment of polar marine benthic algae.

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 few endemic seaweeds occur. In contrast to the Antarctic region, where nutrient levels never limit algal growth, nutrient levels in the Arctic regions are depleted during the summer season. Both regions have a strong 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) entersthe 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 yet unknown consequences for the polar ecosystem.

Saturating light and not increased carbon dioxide under ocean acidification drives photosynthesis and growth in Ulva rigida (Chlorophyta).

Carbon physiology of a genetically identified Ulva rigida was investigated under different CO2(aq) and light levels. The study was designed to answer whether (1) light or exogenous inorganic carbon (Ci) pool is driving growth; and (2) elevated CO2(aq) concentration under ocean acidification (OA) will downregulate CAext-mediated dehydration and alter the stable carbon isotope (δ13C) signatures toward more CO2 use to support higher growth rate. At pHT 9.0 where CO2(aq) is <1 μmol L−1, inhibition of the known use mechanisms, that is, direct uptake through the AE port and CAext-mediated dehydration decreased net photosynthesis (NPS) by only 56–83%, leaving the carbon uptake mechanism for the remaining 17–44% of the NPS unaccounted. An in silico search for carbon-concentrating mechanism elements in expressed sequence tag libraries of Ulva found putative light-dependent transporters to which the remaining NPS can be attributed. The shift in δ13C signatures from –22‰ toward –10‰ under saturating light but not under elevated CO2(aq) suggest preference and substantial use to support photosynthesis and growth. U. rigida is Ci saturated, and growth was primarily controlled by light. Therefore, increased levels of CO2(aq) predicted for the future will not, in isolation, stimulate Ulva blooms.

Seaweed Responses to Environmental Stress: Reactive Oxygen and Antioxidative Strategies

Oxygen radicals are inevitably produced in the metabolism. In plants, as seaweeds, a major source for oxygen radical formation is photosynthesis, in particular pseudo-cyclic electron transport, also referred to as Mehler reaction. The related transfer of electrons to molecular oxygen yields the aggressive superoxide radical, which may react further to even more reactive oxygen species (ROS). This process is typically enhanced under environmental conditions resulting in restricted electron flow in photosynthesis, such as under light, temperature, salt stress, malnutrition, etc. In particular, seaweeds populating the intertidal and shallow subtidal are exposed to large amplitudes of variation of environmental conditions and may thus rely on strategies to either suppress the generation of oxygen radicals or scavenge them as fast as possible. One important process to scavenge the superoxide radical is its cleavage by superoxide dismutase (SOD), a central enzyme involved in stress response of photosynthetic and non-photosynthetic organisms. This chapter summarizes some common features of generation and scavenging of photosynthetically formed ROS in seaweeds under environmental constraints with emphasis on the spatial and temporal variability of SOD activity.