Spotlight on macronutrients: Stressed-out sulphur…

Amongst its many roles in plants, sulphur (S) is found in two of the 20 standard amino acids that form proteins, namely cysteine and methionine, and is therefore important in crucial cell components such as membranes and enzymes. Sulphur is also present in the organic compounds that give plants such as onion, garlic and mustard their characteristic odours.  Sulphur is generally taken up from the environment by plants as the sulphate ion (SO₄^2–), which is frequently produced by bacterial activity in the soil.  Well, as much as plants need sufficient amounts of S to maintain growth, development and ‘health’, some forms of S in the environment can be damaging. Take for example H₂S – hydrogen sulphide, a gas with the ‘characteristic foul odor of rotten eggs’ – which is found naturally in oxygen-poor areas as bacteria metabolise SO₄^2–. Sediment-derived H₂S can impact deleteriously on the growth and health of seagrasses  – flowering plants that live a submerged existence and that provide important marine habitats often covering large areas (up to 600 000 km² of the oceans), which, because of their similarity to terrestrial meadows, are termed seagrass meadows.  In view of the inter-relatedness of marine ecosystems, damage to seagrass stands can have knock-on effects upon such iconic habitats as coral reefs.  Monitoring seagrass health is therefore important. And an important diagnostic technique to assess seagrasses’ well-being has been developed by Kieryn Kilminster et al., and has a S dimension. Outwardly, seagrass that is ‘compromised’ may look healthy, so an internal diagnostic test is needed to indicate its state of health. Such a test was provided when the Dano-Australian team discovered that elemental S accumulated in tissues of the seagrass Halophila ovalis when their environment was stressful. The incorporated sulphur resulted from the plant’s uptake of H₂S from the sediment, whose microbial production was in turn an indication that the sediment had become anaerobic, which is a stressful state of affairs for the aerobic seagrasses… Another marine–sulphur–stress dimension has been revealed by Melissa Garren et al. (The ISME Journal in press) for hard corals – those mutualistic symbiotic organisms that comprise an animal coral polyp and an internalised microalga, a zooxanthella. When the coral Pocillopora damicornis was heat-stressed (to 31 ^oC), concentrations of DMSP (dimethylsulphoniopropionate) in its mucus increased 5-fold and the chemotactic response of the pathogenic bacterium Vibrio coralliilyticus was enhanced. The bacterium appears to be using the DMSP as an ‘infochemical’ to home in on stressed coral hosts, which it subsequently attacks. Vibrio coralliilyticus is associated with many coral diseases and infects them at temperatures above 27 ^oC. (Nikole Kimes et al., The ISME Journal 6: 835–846, 2012). And what is the relevance of all of this? Think heat-stress, think global warming. Interestingly, DMSP – which is produced by a wide range of marine algae when variously ‘damaged’, not just heat-stressed corals – is the precursor for DMS (dimethylsulphide), which ultimately acts as a nucleating agent for cloud formation in the atmosphere. The clouds can act as reflectors of some of the incoming solar radiation, which would otherwise serve to increase the temperature of the Earth (global warming). Thus, DMS might actually contribute to global cooling (and features in the CLAW Hypothesis), and which DMS may have been formed from DMSP produced by corals as a response to global warming… Nature: she’s complicated(!). For more information on the range of S-compounds in plant biology, see Katharina Gläser et al.’s paper that explores the so-called ‘sulphur metabolome’ of arabidopsis.

 

[By way of fuelling the debate, Mr Cuttings says that he will continue to spell sulphur with a ‘ph’. He knows the ‘f-spelling’  is the standard form of spelling for this element in ‘chemistry and other technical uses’, but he prefers consistency of spelling, so SULPHUR (NOT sulfur…), please. And anyway, our cousins across the Atlantic pond have won the war and got us all to use ‘program’ for those computer programme things…, so let’s make a principled stand; there’s no ‘f’ in sulphur! – Ed.]