Gunter O. Kirst

Grazing-activated chemical defence in a unicellular marine alga

Marine plankton use a variety of defences against predators, some of which affect trophic structure and biogeochemistry. We have previously shown that, during grazing by the protozoan Oxyrrhis marina on the alga Emiliania huxleyi, dimethylsulphoniopropionate (DMSP) from the prey is converted to dimethyl sulphide (DMS) when lysis of ingested prey cells initiates mixing of algal DMSP and the enzyme DMSP lyase. Such a mechanism is similar to macrophyte defence reactions,. Here we show that this reaction deters protozoan herbivores, presumably through the production of highly concentrated acrylate, which has antimicrobial activity. Protozoan predators differ in their ability to ingest and survive on prey with high-activity DMSP lyase, but all grazers preferentially select strains with low enzyme activity when offered prey mixtures. This defence system involves investment in a chemical precursor, DMSP, which is not self-toxic and has other useful metabolic functions. We believe this is the first report of grazing-activated chemical defence in unicellular microorganisms.

ALGAL PRODUCTION OF DIMETHYL SULFIDE AND ITS ATMOSPHERIC ROLE1

It has long been recognized that algae play a highly significant role in the global biogeochemical cycles of oxygen, carbon, nitrogen, phosphorus, and sulfw. The key chemical compounds driving these cycles are often low molecular weight and/or volatile species. In the case of sulfur, a dominant compound is dimethyl sulfide (DMS), which derives from dimethylsulfoniopropionate (DMSP) , an organic osmolyte that acts as a compatible solute in algal cells. High concentrations of DMS and DMSP can be found in areas of high algal productivity, such as marine phytoplankton blooms. In recent years research has unveiled greater detail concerning the biosynthesis and turnover of DMSP, DMS, and related compounds in the biosphere and with respect to the role of DMS in the global sulfur cycle and climate. In this minireview our aim is to concentrate mainly on these more recent discoveries.

The role of β-dimethylsulphoniopropionate, glycine betaine and homarine in the osmoacclimation of Platymonas subcordiformis

The tertiary sulphonium compound, β-dimethylsulphoniopropionate (DMSP) and the quaternary ammonium compounds glycine betaine and homarine are important osmotica in Platymonas subcordiformis cells. Following hypersaline stresses the compounds were accumulated after a lag period of 3 h and equilibrium concentrations were reached 6 h later. In contrast to these organic solutes, mannitol was synthesised immediately and equilibrium concentrations were reached within 90 min. Hyposaline stresses induced losses of the organic solutes from the cells. The ions K+, Na+, Cl- and the above organic solutes can account for the osmotic balance of the cells.

Dimethylsulphoniopropionate (DMSP) accumulation in green macioalgae from polar to temperate regions: interactive effects of light versus salinity and light versus temperature

SummaryThe effect of photon fluence rate on the ß-dimethylsulphoniopropionate (DMSP) content of salt-stressed eulittoral green macroalgae from different geographic regions was determined. At 55 μmol photons m−2s−1 DMSP increased continuously with increasing salinities up to 68‰ in Ulothrix implexa, Ulothrix subflaccida, Enteromorpha bulbosa and Acrosiphonia arcta from Antarctica, while the Subantarctic/cold-temperate Ulva rigida and the temperate Blidingia minima showed a large rise in intracellular DMSP concentration only under gentle hypersaline treatment (51‰). At the highest salinity tested the DMSP content of the latter species declined. In contrast, the capacity to form DMSP in the dark under hypersaline conditions was very low in all species. In addition, the DMSP content of the Antarctic species was determined after one year cultivation at 0°C under photon fluence rates of 2, 30 and 55 μmol m−2s−1. All isolates increased their DMSP concentration with increasing irradiance. In contrast to previous experiments done at 10°C, these species exhibited up to 5 fold higher DMSP values at 0°C under most photon fluence rates. The data support the idea of a light-dependent DMSP biosynthesis, and also demonstrate the stimulating effect of low water temperatures on the DMSP content of Antarctic green macroalgae. Apparently, in these plants DMSP may function as a cryoprotectant.

The effect of nitrogen deficiency, methionine and inhibitors of methionine metabolism on the DMSP contents of Tetraselmis subcordiformis (Stein)

The unicellular alga Tetraselmis (Platymonas) subcordiformis (Prasinophyceae) displayed a two-phase increase in DMSP (dimethylsulfoniopropionate) contents in response to nitrogen deficiency. In the first phase, DMSP pools increased by 75% within 24 h. The second increase in DMSP was observed after 14 d and was accompanied by cyst formation. Methionine metabolism was involved in DMSP accumulation. A 2.6-fold increase of the DMSP pool could be induced by supplementing a complete ASP (artifical seawater Provasoli) medium with 100 μM L-methionine. Inhibition of methionine utilization by T. subcordiformis caused by the addition of S-adenosylhomocystein, a competitive inhibitor of S-adenosylmethionine-dependent transmethylases, resulted in a 24% increase of DMSP contents after 7 h. DMSP accumulation after hyperosmotic shock was significantly delayed when methionine liberation from proteins was inhibited by a set of protease inhibitors. A model for the mechanism of DMSP accumulation is presented on the basis of these data and additional information from the literature. It is proposed that methionine availability in T. subcordiformis determines the rate of DMSP synthesis.

Osmotic acclimation and turgor pressure regulation in algae

Salinity, together with light, temperature, and nutrients, is one of the abiotic factors affecting algal growth and distribution. Changes in salinity result in immediate water fluxes according to the osmotic gradients. This rapid response is followed by osmotic acclimation through adjustment of the cellular solute concentrations. As a result of these processes, a more or less constant turgor pressure is maintained or, in the case of wall-less cells, the cell volume is regained. Ions, mainly Na +, K +, and C1-, and low-molecular-weight organic compounds are the major osmolytes. Possible mechanisms of sensing turgor pressure and the signal transduction during osmotic acclimation, which most likely involve Ca 2+ , are discussed for microalgae and charophytes as examples. What Are Osmotic Acclimation and Turgor Pressure Regulation?

The ß-dimethylsulphoniopropionate (DMSP) Content of Macroalgae from Antarctica and Southern Chile

It is of interest to determine the levels of DMSP in extreme cold water algae from polar regions such as Antarctica and the Southern Ocean, because low molecular weight organic compounds known to act as osmolytes under salinty stress may well be suitable as protectants under low temperatures stress. Comparison of the values obtained from polar species with those published for algae from temperate regions may point to the effect of temperature on DMSP metabolism

Growth pattern andβ-dimethylsulphoniopropionate (DMSP) content of green macroalgae at different irradiances

Growth rates and intracellularβ-dimethylsulphoniopropionate (DMSP) concentrations of five green algal species collected from different geographic regions in 1986 and 1989 were determined under four photon flux rates. InUlothrix implexa, U. subflaccida andAcrosiphonia arcta from Antarctica, growth was light-saturated at lower irradiances than in temperateUlva rigida from Southern Chile andBlidingia minima from Germany. The DMSP content ofUlothrix implexa, A. arcta andUlva rigida was directly correlated with the light factor: with increasing irradiance, algal DMSP level increased. In contrast, inUlothrix subflaccida andB. minima DMSP concentrations gradually decreased up to a photon flux rate of 30µmol m−2 s−1, then increased markedly under the highest photon flux rate tested. In non-growing, dark-incubatedA. arcta DMSP content was reduced by 35%, while the DMSP pool of all other species remained unchanged, at the level of pre-culture conditions. Under full darkness all plants exhibited a significantly higher DMSP concentration compared with algae grown at low photon flux rates of 2 to 30µmol m−2 s−1. These data show a correlation between growth pattern and DMSP biosynthesis, and may point to a species-specific minimum amount of light energy necessary for DMSP accumulation.

Physiological Responses of the Antarctic Green Alga Prasiola crispa ssp. antarctica to Salinity Stress

Summary The thalloid green alga Prasiola crispa ssp. antarctica (Kutzing) Knebel was subjected to salinity stress ranging from 0.35 ‰ (freshwater conditions) up to 175 ‰ (5-fold seawater). Hypoosmotic treatment did not affect growth rate, photosynthesis or dark respiration. The levels of Na+, K+, NH4+, CI- and PO43-were similar to control plants grown at 35 ‰. Sorbitol and sucrose decreased following decreases in external salinity. Increases in salinity above 35 ‰ (standard seawater) caused a reduction in growth rates, photosynthesis and dark respiration. The content of inorganic ions increased, exhibiting a maximum in 70 ‰ medium, whilst the highest contents of organic osmolytes such as sucrose and sorbitol were measured in 175 ‰ seawater. There was good recovery of physiological activity after returning the plants to normal seawater (35 ‰). This was indicated by restoration of normal growth rates, photosynthesis and dark respiration. The tolerance of P. crispa to a wide range of salinities is an important condition to its growth in the upper littoral of Antarctica. In this habitate the alga is frequently encountered with hypoosmotic milieus due to rain or meltwater. In contrast, hyperosmotic conditions may occur due to salt spray or desiccation.

D-Mannitol dehydrogenase and D-mannitol-1-phosphate dehydrogenase in Platymonas subcordiformis: some characteristics and their role in osmotic adaptation.

Abstractd-Mannitol-1-phosphate dehydrogenase (EC 1.1.1.17) and d-mannitol dehydrogenase (EC 1.1.1.67) were estimated in a cell-free extract of the unicellular alga Platymonas subcordiformis Hazen (Prasinophyceae), d-Mannitol dehydrogenase had two activity maxima at pH 7.0 and 9.5, and a substrate specifity for d-fructose and NADH or for d-mannitol and NAD+. The Km values were 43 mM for d-fructose and 10 mM for d-mannitol. d-Mannitol-1-phosphate dehydrogenase had a maximum activity at pH 7.5 and was specific for d-fructose 6-phosphate and NADH. The Km value for d-fructose 6-phosphate was 5.5 mM. The reverse reaction with d-mannitol 1-phosphate as substrate could not be detected in the extract. After the addition of NaCl (up to 800 mM) to the enzyme assay, the activity of d-mannitol dehydrogenase was strongly inhibited while the activity of d-mannitol-1-phosphate dehydrogenase was enhanced. Under salt stress the Km values of the d-mannitol dehydrogenase were shifted to higher values. The Km value for d-fructose 6-phosphate as substrate for d-mannitol-1-phosphate dehydrogenase remained constant. Hence, it is concluded that in Platymonas the d-mannitol pool is derectly regulated via alternative pathways with different activities dependent on the osmotic pressure.