ABSTRACT

Since the Industrial Revolution, increasing atmospheric CO2 has been causing a rise in the concentration of carbon dioxide dissolved in seawater. This process results in seawater acidification, which has a major impact on the physical and chemical parameters of the oceans, consequently affecting the numerous calcifying organisms in the marine environment. Calcifying organisms secrete calcium carbonate in their inner or outer skeleton and include plankton (e.g. coccolithophores and foraminifera), corals, mussels and some of the macroalgae. Calcifying macroalgae make a critical contribution to the structure and function of marine ecosystems in several coastal biotas, providing food and shelter to diverse organisms. The present review summarizes the current information about the brown alga Padina sp. and its ecophysiology, focusing on the environmental control of the calcification process; suggests possible benefits that seaweeds may derive from their calcium carbonate cover, and discuss different future Intergovernmental Panel on Climate Change scenarios of ocean acidification and their likely impact on calcifying algae and on the ecosystems in which they are a key component.

In: Israel Journal of Plant Sciences

The Gulf of Aqaba (Elat) is characterized by oligotrophic waters and low productivity conditions in summer, when the euphotic zone of the stratified water column is severely nutrient-depleted. Winter is moderately productive, when mixing injects nutrients into the lit upper waters. The annual phytoplankton cycle and the bathymetric distribution of biomass and photosynthetic activity in the Gulf are analyzed in conjunction with the harvesting and utilization of light by cells. The harvesting of light energy at any depth is shown as an interaction between the optical properties of cells, a*, and the intensity and spectral distribution of the underwater light field. The efficiency by which phytoplankton cells utilize the absorbed light energy is the quantum yield of the photosynthetic process, Φ. The efficiency of light utilization by phytoplankton depends on the nutrient status of cells, their photoacclimation to ambient light, and the irradiance to which they are exposed. We show that the seasonal and spatial changes in the quantum yield of photosynthesis control the wax and wane of the different phytoplankton assemblages in the Gulf, thereby ultimately determining the flux of energy fueling all of the Gulf's planktonic and benthic biota.

In: Israel Journal of Plant Sciences

In this study, we describe the seasonal patterns and the depth dependence of the underwater distribution of ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) in the Gulf of Eilat (Aqaba). Our data show a remarkable seasonal variation in the daily irradiance dose that reaches the benthos, including seaweeds, turf algae, seagrasses, and the endosymbiotic algae of corals. Corals experience 7 times less light in the winter than in summer, which is equivalent to an almost 30 m deeper location. This seasonal variation in irradiance to which high-latitude benthic phototrophs are exposed, is due to a synergetic effect of changes in solar elevation and in water transparency. Unlike the major seasonal difference in insolation in subtropical regions, low-latitude coral reefs and benthic macrophytes are exposed to only much subtler differences in irradiance. One of the northernmost coral reefs in the world, yet a very highly diverse one, is found at the northern tip of the Gulf of Eilat (Aqaba) at 29°N. Together with its geographical marginality, this reef is also characterized by low cloud cover, making it an appropriate model for demonstrating the dynamics of a "clean" seasonal light field, in contrast to coral reefs at high latitudes with much higher cloud cover. Based on these data, we present a model allowing calculation of the seasonal variation of a light field as a function of depth and latitude under clear-sky conditions, and variable water transparency.

In: Israel Journal of Plant Sciences

Two marine macroalgae have recently been identified using drift algal material collected in the Haifa Bay area, Israel. The first is the red seaweed Galaxaura rugosa, first seen south of Haifa Bay during November 2003 and observed thereafter both within seaweeds washed ashore and at 2-4 m deep within the subtidal zone. Galaxaura rugosa was previously described in the Lebanese coasts; therefore its distribution within the Israeli Mediterranean is expected to be far more extensive. The second is the green seaweed Codium adhaerens, which was collected along the northern shores of Haifa Bay during September-October 2007.

In: Israel Journal of Plant Sciences

We studied for the first time the effects of geological substrates on taxonomic composition of epilithic algal assemblages in Lake Kinneret and their seasonal fluctuations. It is shown that both species richness and cell densities increase during the winter. Among the fifty-four species identified, the major groups were Bacillariophyta, Chlorophyta, and Cyanobacteria (41%, 37%, and 16%, respectively). The dominant alga on stony substrates throughout the study period was the diatom Gomphonema sp., accompanied by Cymbella sp., Navicula spp., and Nitzschia spp. The Chlorophyta were represented by Cladophora glomerata (Linnaeus) Kutzing, Tetraedron minimum (A. Braun) Hansgirg, and the Cyanobacteria by Lyngbya sp.

The substrate effect on algal growth was also determined. The results show that 66%, 20%, and 14% of all epilithic algae were found on limestone, basalt, and flint, respectively.

In the depth profile, two peaks of epilithic algae were revealed. The first was found at 0.25-0.70 m, dominated by the Bacillariophyta Gomphonema sp., Nitzschia spp., Navicula spp., Cymbella sp., Cocconeis sp., Achnanthes sp., and Synedra ulna (Nitzsch) Ehrenberg. A second peak was found between 1.20-1.60 m, with Chlorophyta, mainly Cladophora glomerata, being dominant. The Cyanobacteria Lyngbya sp. and Chroococcus sp. were evenly distributed at all depths.

In: Israel Journal of Plant Sciences