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

对于放氧植物来说，氧化应激是一个普遍存在的问题，对于在营养贫乏、可见光和紫外线(UV)太阳辐射通量较高的水域生长的海洋浮游植物来说，氧化应激尤其重要。人们对海藻如何应对氧化压力知之甚少。在这项研究中，提出了一个新的假设，即二甲基磺酸丙酸酯(DMSP)是海洋中一种独特的非常重要的抗氧化剂，在对抗和缓解氧化应激方面起着主要防御作用。它作为抗氧化剂的作用可能是因为DMSP是世界上许多海藻的主要细胞硫化合物和主要有机成分。此外，初步结果表明，DMSP及其酶裂解产物DMS是细胞内有毒活性氧物种(例如羟基自由基)的高效清除剂，在某些情况下，清除这些物种的速度比已有文献证明的抗氧化剂抗坏血酸和谷胱甘肽更快。DMSP和DMS的氧化生成二甲基亚砜(DMSO)，DMSO和DMSP裂解的另一产物丙烯酸酯一样，也是一种有效的氧化剂清除剂。总之，这些相关化合物应该在含有DMSP的藻类中作为一个多功能的、高度灵活的抗氧化系统。为了研究这一DMSP抗氧化系统，本研究小组正在检验以下相关假设：1)浮游植物细胞内的DMSP浓度会随着慢性氧化应激的增加而增加，而随着急性氧化应激的增加而降低。2)细胞DMSP的周转是由氧化应激诱导的，导致可能的抗氧化剂产生增加。3)高DMSP含量和高DMSP裂解酶活性的浮游植物比低DMSP和低DMSP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，具有生态重要性的浮游植物物种和自然海水种群的无菌培养正在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裂解酶活性一起被确定。对其他抗氧化防御系统(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)的补充测量将提供有关DMSP系统如何与这些公认的抗氧化防御系统相关的信息。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，这一点具有全球意义，因为DMS在大气化学中以及可能在气候中发挥重要作用。如果DMSP的抗氧化剂假设是正确的，那么DMSP的一个重要的、迄今尚不清楚的细胞功能将被鉴定出来。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级本科生和研究生在藻类生理、生态学和海洋学方面的跨学科培训。PIs和学生将通过学术和公开演讲、科学期刊、热门文章和免费访问的网页来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.