Collaborative Research: Production and dynamics of DMSP and related compounds in response to oxidative stress in marine phytoplankton

氧化胁迫是放氧植物普遍存在的问题，对于生长在营养贫乏的沃茨、具有高通量可见光和紫外线（UV）太阳辐射的海洋浮游植物可能特别重要。 人们对海藻如何科普氧化应激知之甚少。 在这项研究中，提出了一个新的假设，即二甲基磺基丙酸酯（DMSP）是一种独特的和非常重要的抗氧化剂在海洋中，作为一个主要的防御在对抗和减轻氧化应激。 它作为抗氧化剂的作用可能是因为DMSP是世界各地许多海藻中占主导地位的细胞硫化合物，也是许多海藻的主要有机成分。 此外，初步结果表明DMSP及其酶裂解产物DMS是毒性活性氧物质（例如，羟基自由基），在某些情况下，清除这些物质的速度比有据可查的抗氧化剂，抗坏血酸和谷胱甘肽更快。 DMSP和DMS氧化产生二甲基亚砜（DMSO），其也是有效的氧化剂清除剂，如丙烯酸酯，DMSP裂解的另一产物。 总之，这些相关的化合物应该作为一个多功能的，高度灵活的抗氧化剂系统在含DMSP的藻类。 为了研究DMSP抗氧化系统，本研究小组正在验证以下相关假设：1）浮游植物细胞内DMSP浓度会响应于慢性氧化应激而增加，并响应于急性氧化应激而降低。 2)氧化应激诱导细胞DMSP的周转，导致推定的抗氧化剂的产生增加。 3)高DMSP含量和高DMSP裂解酶活性的浮游植物比低DMSP或低裂解酶活性的浮游植物更能抵抗氧化胁迫。 为了验证这些假设，生态上重要的浮游植物物种和天然海水种群的纯培养物暴露于各种形式的氧化应激，包括可见光（400-700 nm）和/或紫外线辐射（290-400 nm）的高光子通量、营养限制（Fe和N）以及添加百草枯或铜。 在这些暴露期间，细胞DMSP和相关化合物的变化将与DMSP裂解酶活性一起沿着测定。 其他抗氧化防御系统的补充测量（例如，抗坏血酸、还原型和氧化型谷胱甘肽、抗坏血酸过氧化物酶、超氧化物歧化酶）将提供关于DMSP系统如何变化与这些良好建立的抗氧化防御有关的信息。 该项目将阐明控制海洋中二甲基磺酰脲含量及其转化为二甲基甲硫醚的速率的机制，由于二甲基甲硫醚在大气化学中的重要作用，并可能在气候中发挥重要作用，这一点具有全球意义。 如果DMSP抗氧化假说是正确的，那么一个重要的，迄今未知的，细胞功能的DMSP将被确定。 了解DMSP的细胞生理学及其与其他知之甚少的藻类抗氧化系统的关系，将增加我们对控制海洋中具有生态重要性的浮游植物分布的因素的理解。这项研究还将导致高年级本科生和研究生在藻类生理生态学和海洋学的跨学科培训。 PI和学生将通过学术和公共演讲，科学期刊，流行文章和免费访问的网页传播这项研究的结果。

Oxidative stress is a pervasive problem for oxygen-evolving plants, and is likely to be especially important for marine phytoplankton growing in nutrient-impoverished waters with high fluxes of visible and ultraviolet (UV) solar radiation. Very little is known about how marine algae cope with oxidative stress. In this study, a new hypothesis is presented that dimethylsulfoniopropionate (DMSP) is a unique and very important antioxidant in the ocean that serves as a primary defense in combatting and alleviating oxidative stress. Its role as an antioxidant is likely because DMSP is the dominant cellular sulfur compound in, and a major organic constituent of many marine algae worldwide. Furthermore, preliminary results indicate that DMSP and its enzymatic lysis product, DMS are highly effective scavengers of toxic reactive oxygen species (e.g., hydroxyl radicals) in cells, removing these species faster, in some cases, than the well documented antioxidants, ascorbic acid and glutathione. DMSP and DMS oxidation yield dimethylsulfoxide (DMSO), which is also an effective oxidant scavenger as is acrylate, the other product of DMSP lysis. Together, these related compounds should serve as a multifunctional, highly flexible antioxidant system in DMSP-containing algae. To investigate this DMSP antioxidant system, the following related hypotheses are being tested by this research team: 1) Intracellular DMSP concentrations in phytoplankton will increase in response to chronic oxidative stress, and will decrease in response to acute oxidative stress. 2) Turnover of cellular DMSP is induced by oxidative stress, resulting in increased production of putative antioxidants. 3) Phytoplankton with high DMSP content and high DMSP lyase activity will be more resistant to oxidative stress than phytoplankton with low DMSP or low lyase activity. To test these hypotheses, axenic cultures of ecologically-important phytoplankton species and natural seawater populations are being exposed to various forms of oxidative stress, including high photon fluxes of visible light (400-700 nm) and/or UV radiation (290-400 nm), nutrient limitation (Fe and N), and addition of paraquat or copper. During these exposures, changes in cellular DMSP and related compounds will be determined along with DMSP lyase activity. Complementary measurements of other antioxidant defense systems (e.g., ascorbate, reduced and oxidized glutathione, ascorbate peroxidase, superoxide dismutase) will provide information on how the DMSP system varies in relation to these well-established antioxidant defenses. This project will elucidate the mechanisms that control DMSP levels in the ocean and its rate of conversion to DMS, which is globally-significant because of the important role of DMS in atmospheric chemistry and, possibly, climate. If the DMSP antioxidant hypothesis is correct, then an important, hitherto unknown, cellular function of DMSP will have been identified. Understanding the cellular physiology of DMSP and its relationship to other poorly-understood algal antioxidant systems will increase our understanding of the factors that control the distribution of ecologically-important phytoplankton in the sea. This study will also result in the interdisciplinary training of advanced undergraduate and graduate students in algal physiological ecology and oceanography. The PIs and students will disseminate results of this study through scholarly and public presentations, scientific journals, popular articles and freely-accessible web pages.