Collaborative Research: Production and Dynamics of DMSP and Related Compounds in Response to Oxidative Stress in Marine Phytoplankton

氧化应激对于进化氧气的植物来说是一个普遍存在的问题，对于生长在高可见和紫外线（UV）太阳辐射通量的营养贫乏水域的海洋浮游植物来说可能尤其重要。关于海藻如何应对氧化应激，我们所知甚少。在这项研究中，提出了一个新的假设，即二甲基磺酰丙酸（DMSP）是海洋中一种独特且非常重要的抗氧化剂，是对抗和缓解氧化应激的主要防御手段。它作为抗氧化剂的作用可能是因为DMSP是世界上许多海洋藻类中主要的细胞硫化合物，也是主要的有机成分。此外，初步结果表明，DMSP及其酶解产物DMS是细胞中有毒活性氧（如羟基自由基）的高效清除剂，在某些情况下，比文献记载的抗氧化剂、抗坏血酸和谷胱甘肽清除这些活性氧的速度更快。DMSP和DMS氧化生成二甲基亚砜（DMSO），它也是一种有效的氧化剂清除剂，与DMSP裂解的另一种产物丙烯酸酯一样。综上所述，这些相关化合物可作为一种多功能的、高度灵活的抗氧化系统在含dmsp的藻类中发挥作用。为了探究DMSP抗氧化系统，本课课组对以下相关假设进行了验证：1)浮游植物细胞内DMSP浓度在慢性氧化应激下升高，在急性氧化应激下降低。2)氧化应激诱导细胞DMSP的转换，导致推定的抗氧化剂的产生增加。3) DMSP含量高、DMSP裂解酶活性高的浮游植物比DMSP含量低、裂解酶活性低的浮游植物对氧化应激的抗性更强。为了验证这些假设，对生态重要的浮游植物物种和天然海水种群进行了无菌培养，使其暴露于各种形式的氧化应激，包括高光子通量的可见光（400-700 nm）和/或紫外线辐射（290-400 nm），营养限制（铁和氮），以及添加百草枯或铜。在这些暴露中，细胞DMSP和相关化合物的变化将与DMSP裂解酶活性一起测定。其他抗氧化防御系统的补充测量（例如，抗坏血酸、还原和氧化谷胱甘肽、抗坏血酸过氧化物酶、超氧化物歧化酶）将提供DMSP系统如何与这些已建立的抗氧化防御相关的信息。该项目将阐明控制海洋中DMSP水平及其转化为DMS的速率的机制，这是全球意义重大的，因为DMS在大气化学和可能的气候中发挥着重要作用。如果DMSP抗氧化假说是正确的，那么DMSP的一个重要的，迄今为止未知的细胞功能将被确定。了解DMSP的细胞生理学及其与其他鲜为人知的藻类抗氧化系统的关系，将增加我们对控制海洋中重要生态浮游植物分布的因素的理解。本研究亦将培养藻类生理生态学及海洋学的高级本科生及研究生。pi和学生将通过学术和公开演讲、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.