T. van der Heide

Glycine Betaine Transport in Lactococcus lactis Is Osmotically Regulated at the Level of Expression and Translocation Activity

Microorganisms react upon hyperosmotic stress by accumulating compatible solutes. Here we report that Lactococcus lactis uses a transport system for glycine betaine that, contrary to earlier observations (D. Molenaar et al., J. Bacteriol. 175:5438-5444, 1993), is osmotically regulated at the levels of both expression and transport activity.

Toxicity of reduced nitrogen in eelgrass (Zostera marina) is highly dependent on shoot density and pH

In sheltered, eutrophicated estuaries, reduced nitrogen (NHx), and pH levels in the water layer can be greatly enhanced. In laboratory experiments, we studied the interactive effects of NHx, pH, and shoot density on the physiology and survival of eelgrass (Zostera marina). We tested long-term tolerance to NHx at pH 8 in a 5-week experiment. Short-term tolerance was tested for two shoot densities at both pH 8 and 9 in a 5-day experiment. At pH 8, eelgrass accumulated nitrogen as free amino acids when exposed to high loads of NHx, but showed no signs of necrosis. Low shoot density treatments became necrotic within days when exposed to NHx at pH 9. Increased NH3 intrusion and carbon limitation seemed to be the cause of this, as intracellular NHx could no longer be assimilated. Remarkably, experiments with high shoot densities at pH 9 showed hardly any necrosis, as the plants seemed to be able to alleviate the toxic effects of high NHx loads through joint NHx uptake. Our results suggest that NHx toxicity can be important in worldwide observed seagrass mass mortalities. We argue that the mitigating effect of high seagrass biomass on NHx toxicity is a positive feedback mechanism, potentially leading to alternative stable states in field conditions.

Limited toxicity of NHx pulses on an early and late successional tropical seagrass species: Interactions with pH and light level

Seagrasses have declined at a global scale due to light reduction and toxicity events, caused by eutrophication and increased sediment loading. Although several studies have tested effects of light reduction and toxicants on seagrasses, there is at present no information available on their interacting effects. In a full-factorial 5-day laboratory experiment, we studied short-term interactive effects of light conditions, pH and reduced nitrogen (NH(x)) in the water layer, mimicking pulses of river discharge, on the tropical early successional species Halodule uninervis and the late successional species Thalassia hemprichii. In contrast to recent results reported for the temperate species Zostera marina, increased NH(x) supply did not affect leaf mortality or photochemical efficiency in H. uninervis and in 7 out of 8 treatments for T. hemprichii. However, both tropical species demonstrated striking differences in nitrogen accumulation, free amino acid composition and free NH₃ accumulation. The increase in tissue nitrogen content was two times higher for H. uninervis than for T. hemprichii. Nitrogen stored as free amino acids (especially asparagine) only increased in H. uninervis. High pH only affected T. hemprichii, but only when not shaded, by doubling its free NH₃ concentrations, concomitantly decreasing its photosynthetic efficiency. Our results indicate that the early successional H. uninervis has higher tolerance to high NH(x) loads as compared to the late successional T. hemprichii. H. uninervis was better able to avoid toxic internal NH(x) levels by further assimilating glutamine into asparagine in contrast to T. hemprichii. Moreover, both tropical species seem to cope much better with high NH(x) than the temperate Z. marina. The implications for the distribution and succession of seagrass species under high nutrient loads are discussed.

Expression of the gltP gene of Escherichia coli in a glutamate transport-deficient mutant of Rhodobacter sphaeroides restores chemotaxis to glutamate

Rhodobacter sphaeroides is chemotactic to glutamate and most other amino acids. In Escherichia coli, chemotaxis involves a membrane‐bound sensor that either binds the amino acid directly or interacts with the binding protein loaded with the amino acid. In R. sphaeroides, chemotaxis is thought to require both the uptake and the metabolism of the amino acid. Glutamate is accumulated by the cells via a binding protein‐dependent system. To determine the role of the binding protein and transport in glutamate taxis, mutants were created by Tn5 insertion mutagenesis and selected for growth in the presence of the toxic glutamine analogue γ‐glutamyl‐hydrazide. One of the mutants, R. sphaeroides MJ7, was defective in glutamate uptake but showed wild‐type levels of binding protein. The mutant showed no chemotactic response to glutamate. Both glutamate uptake and chemotaxis were recovered when the gltP gene, coding for the H+‐linked glutamate carrier of E. coli, was expressed in R. sphaeroides MJ7. It is concluded that the chemotactic response to glutamate strictly requires uptake of glutamate, supporting the view that intracellular metabolism is needed for chemotaxis in R. sphaeroides.

Interactive effects of pH, temperature and light during ammonia toxicity events in Elodea canadensis

Increased nutrient loading threatens many freshwater ecosystems. Elevated temperatures may increase the sensitivity to eutrophication in these ecosystems. Higher concentrations of possibly toxic reduced nitrogen (NH x ) in the water layer may be expected as production and anaerobic breakdown rates will increase. Apart from temperature, NH x and its effect on aquatic macrophytes will also depend on pH and light. We examined the interactive effects of NH x , temperature, pH and light on Elodea canadensis in a full factorial laboratory experiment. Results demonstrate that high NH x and high temperature together with low pH and low light causes the strongest toxic effects regarding relative growth rate and leaf tissue mortality. The adverse effects of high temperature and low light are most likely caused by increased metabolic activity and reduced photosynthesis, respectively. Severe toxicity at low pH compared to high pH can be ascribed to the ability of E. canadensis to induce a specialised bicarbonate-concentrating pathway at high pH, resulting in much higher carbon availability, needed for detoxification of NH x . We conclude that NH x toxicity will become more pronounced under higher temperatures, but that effects on aquatic macrophytes will strongly depend on pH of the water layer and specific metabolic adaptations of different species.