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Reactive oxygen species production in marine microalgae
All living cells produce reactive oxygen species (ROS) as a byproduct of metabolism. ROS are reduced oxygen intermediates that include the superoxide radical (O2−) and the hydroxyl radical (OH•), as well as the non-radical species hydrogen peroxide (H2O2). These ROS are important in the normal functioning of cells, playing a role in signal transduction and the expression of transcription factors. However, when present in excess, ROS can cause damage to proteins, lipids and DNA by reacting with these biomolecules to modify or destroy their intended function. As an example, the occurrence of ROS have been linked to the aging process in humans, as well as several other diseases including Alzheimer's, rheumatoid arthritis, Parkinson's, and some cancers. Their potential for damage also makes reactive oxygen species useful in direct protection from invading pathogens, as a defense response to physical injury, and as a mechanism for stopping the spread of bacteria and viruses by inducing programmed cell death.
Reactive oxygen species are present in low concentrations in seawater and produced primarily through the photolysis of organic and inorganic matter. However, the biological production of ROS, generated through algal photosynthesis and subsequently 'leaked' to the environment, can contribute significantly to concentrations in the water column. Although there is very little information on the biological generation of ROS in marine surface waters, several species of marine phytoplankton have recently been shown to release significant amounts of ROS into the environment. This ROS has the potential to harm nearby organisms, and, in fact, has been implicated as the cause of massive fish, bacteria, and protist mortalities.
== Chemical background ==
In sea water, ROS can be generated through abiotic as well as biotic processes, among which are the radiolysis and photolysis of water molecules and cellular respiration. According to a model proposed by Fan for the prediction of ROS in surface waters, the biochemistry mediated by phytoplankton may be just as important for the production of ROS as photochemistry. Biological ROS is often synthesized in mitochondrial membranes, as well as the endoplasmic reticulum of animals, plants, and some bacteria. In addition, chloroplasts and the organelles peroxisomes and glyoxysomes are also sites for the generation of ROS.〔〔 The ROS most likely released to the environment are those produced at the cell surface as electrons get "leaked" from the respiratory chain and react with molecular oxygen, O2. The products of this subsequent reduction of molecular oxygen are what are referred to as reactive oxygen species. Thus, the production of ROS is in direct proportion to the concentration of O2 in the system, with increases of O2 leading to higher production of ROS. There are three main reactive oxygen species: the superoxide anion (O2−), hydrogen peroxide (H2O2), and the hydroxyl radical (OH•). The superoxide anion is formed directly from the one-electron reduction of molecular oxygen: O2↔ 2O2-. Hydrogen peroxide is then formed from the disproportionation of the superoxide anion: 2O2−↔2H2O2. This reaction occurs very quickly in seawater. Next, the reduction of hydrogen peroxide yields the hydroxyl radical, H2O2↔2OH•, which can then get reduced to the hydroxyl ion and water.〔 However, the presence of reactive oxygen species in marine systems is hard to detect and measure accurately, for a number of reasons. First, ROS concentrations are generally low (nanomoles) in seawater. Second, they may react with other hard to identify molecules that occur in low quantities, resulting in unknown products. Finally, they are (for the most part) transient intermediates, having lifetimes as little as microseconds.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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