The emission of volatile halocarbons by seaweeds and their response towards environmental changes

Volatile halocarbons can deplete the protective stratospheric ozone layer contributing to global climate change and may even affect local climate through aerosol production. These compounds are produced through anthropogenic and biogenic processes. Biogenic halocarbons may be produced as defence compounds, anti-oxidants or by-products of metabolic processes. These compounds include very short-lived halocarbons (VSLH), e.g. bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I), diiodomethane (CH2I2). Efforts to quantify the biogenic sources of these compounds, especially those of marine origin, e.g. seaweeds, phytoplankton and seagrass meadows, are often complicated by inherent biological variability as well as spatial and temporal changes in emissions. The contribution of the coastal region and the oceans to the stratospheric load of halocarbons has been widely debated. This highlights the need to understand the factors affecting the release of these compounds from marine sources for which data for modelling purposes are generally lacking. Seaweeds are important sources of biogenic halocarbons subjected to changing environmental conditions. Huge uncertainties in the prediction of current and future global halocarbon pool exist due to the lack of spatial and temporal data input from coastal and oceanic sources. Therefore, investigating the effect of changing environmental conditions on the emission of VSLH by the seaweeds could help towards better estimations of halocarbon emissions. This is especially important in light of global changes in both climate and the environment, the expansion of seaweed cultivation industry and the interactions between halocarbon emission and their environment. In this paper, we review current knowledge of seaweed halocarbon emissions, how environmental factors affect these emissions and identify gaps in understanding. Our aim is to direct much needed research to improve understanding of the contribution of marine biogenic sources of halocarbons and their impact on the environment

Emission of selected halocarbons by seaweeds inhabiting a coral reef / Fiona Keng Seh Lin

Since the discovery of the Antarctic stratospheric ozone hole in 1985 there has been increasing scientific interest in the halocarbon species that can cause ozone destruction. Although an important region for halocarbons in terms of atmospheric circulation, the tropics are underrepresented in terms of halocarbon measurements, especially those biogenic short-lived halocarbon compounds. A fringing coral reef flat at Cape Rachado, west coast Peninsular Malaysia was selected for a study on the emissions of halocarbons by seaweeds. A portable, automated gas chromatograph with electron capture detector was used to measure a suite of halocarbon species trimonthly over a 15-month period at the study site. The measurements of the halocarbon atmospheric mixing ratios were then correlated to the seaweed standing biomass to investigate its influence on the halocarbon mixing ratios at the survey site. Although it was found that the atmospheric mixing ratio for the biogenic halocarbon compounds were poorly correlated (ρ < 0.5) to some of the important seaweed species at the sampling site, there was no significant correlation between the total seaweed standing biomass with the atmospheric concentration of biogenic halocarbon compounds. This may be due to many contributing factors such as localized emissions, wind direction and speed that might influence the halocarbon contents in the atmosphere. To better understand the halocarbon emissions by the seaweeds, a laboratory-based incubation study was conducted to observe if the halocarbon emissions by the seaweeds varied with irradiance. Three selected seaweed species, Sargassum binderi Sonder ex J. P a g e | i v Agardh, Padina australis Hauck, and Turbinaria conoides (J. Agardh) Kützing were collected from the sampling site and exposed to a range of irradiance in the laboratory. The halocarbon contents in the seawater were then analyzed using a purge-and-trap system attached to a gas chromatograph with mass selective detector. Release of halocarbons especially dibromochloromethane, CHBr2Cl (r= 0.79; p< 0.01) was found to be influenced by irradiance. Correlations were also observed between emission of certain halocarbons with photosynthetic activity, especially bromoiodomethane, CH2BrI (r = 0.85; p< 0.01) and bromoform, CHBr3 (r = 0.79; p< 0.01) suggesting that environmental factors such as light can affect the release of these volatile halogenated compounds by the seaweeds into the atmosphere. From this study, it was also found that upon comparison with temperate and polar brown seaweeds, tropical species, such as Turbinaria conoides, may emit higher levels of bromoform, CHBr3 and other halocarbons. It is therefore important to investigate the contribution of tropical seaweeds towards the local atmospheric composition of halocarbons

Halocarbon emissions by selected tropical seaweeds exposed to different temperatures

Four tropical seaweeds, Gracilaria manilaensis Yamamoto & Trono, Ulva reticulata Forsskål, Kappaphycus alvarezii (Doty) L.M.Liao and Turbinaria conoides (J.Agardh) Kützing, collected from various habitats throughout Malaysia, were subjected to temperatures of 40, 35, 30, 25 and 20 ◦C in the laboratory. An exposure range of 21–38 ◦C is reported for Malaysian waters. The effect of the temperature exposures on the halocarbon emissions of the seaweeds were determined 4 and 28 h after treatment. The emission rates for a suite of six halocarbons commonly emitted by seaweeds, bromoform (CHBr3), dibromomethane (CH2Br2), diiodomethane (CH2I2), iodomethane (CH3I), dibromochloromethane (CHBr2Cl) and dichlorobromomethane (CHBrCl2), were measured using a cryogenic purge-and-trap sample preparation system coupled to a gas chromatography–mass spectrometry. The emission rate of CHBr3 was the highest of the six halocarbons for all the seaweeds under all the temperatures tested, followed by CH2Br2, and CH2I2. The emission rates were affected by temperature change and exposure duration, but overall responses were unique to each seaweed species. Larger decreases in the emissions of CHBr3, CH2Br2, CH2I2 and CHBr2Cl were found for K. alvarezii and T. conoides after 4 h at 40 ◦C. In both cases there was a >90% (p < 0.05) reduction in the Fv/Fm value suggesting that photosynthetic actitivity was severely compromised. After a 28 h exposure period, strong negative correlations (r = -0.69 to -0.95; p < 0.01) were observed between temperature and the emission of CHBr3, CH2Br2 and CH2I2 for U. reticulata, K. alvarezii and T. conoides. This suggests a potential decrease in the halocarbon emissions from these tropical seaweeds, especially where the temperature increase is a prolonged event. Strong correlations were also seen between seaweed chlorophyll and carotenoid pigment contents and the emission rates for CHBr3, CH2Br2 and CH2I2 (r = 0.48 to 0.96 and -0.49 to -0.96; p < 0.05). These results suggest that the regulation of halocarbon production versus reactive oxygen species production in seaweeds is an area worthy of further exploration

Can seaweed farming in the tropics contribute to climate change through emission of short-lived halocarbons?

Volatile halocarbons form a major source of halogen radicals in the atmosphere, which are in- volved in the catalytic destruction of ozone. Studies show that marine algae release halocarbons, with 70% of global bromoform produced by marine algae (Carpenter et al., 2000). The role of halocarbons in algae is linked to their use as defense against epiphytes and grazing as well as scavengers of strong oxidants (Nightingale et al., 1995). Halo- carbon release rates are higher for tropical algae than temperate species (Abrahamsson et al., 1995). The Maritime Continent is a major contributor to emissions of short-lived halocarbons and their transport to the stratosphere due to deep convection. The Coral Triangle situated in the Maritime Continent, is a centre for seaweed farming. The fol- lowing discusses the potential impact of tropical seaweed emissions of halogenated compounds to climate change

Volatile halocarbon emissions by three tropical brown seaweeds under different irradiances

The emission rates of eight volatile halogenated compounds by three tropical brown seaweed species collected from Cape Rachado, west coast Peninsular Malaysia, under different irradiances have been determined. A purge-and-trap sample preparation system with a gas chromatograph and mass-selective detector was used to measure a suite of halocarbons released by Sargassum binderi Sonder ex J. Agardh, Padina australis Hauck, and Turbinaria conoides (J. Agardh) Kützing. All species are widely distributed in Peninsular Malaysia, with S. binderi a dominant seaweed species at our survey site. Release of few halocarbons was found to be influenced by irradiance. Correlations were also observed between emission of certain halocarbons with photosynthetic activity, especially bromo-and iodinated compounds (0.6 < r <0.9; p < 0.01) suggesting that environmental factors such as light can affect the release of these volatile halogenated compounds by the seaweeds into the atmosphere. Compared with temperate and polar brown seaweeds, tropical species, such as T. conoides, may emit higher levels of bromoform, CHBr3, and other halocarbons. It is therefore important to investigate the contribution of tropical seaweeds towards the local atmospheric composition of halocarbons