Oren Aharon
Department of Plant and Environmental Sciences, Hebrew University of Jerusalem, Jerusalem, Israel.
Saline Syst. 2008 Apr 15;4:2. doi: 10.1186/1746-1448-4-2.
Halophiles are found in all three domains of life. Within the Bacteria we know halophiles within the phyla Cyanobacteria, Proteobacteria, Firmicutes, Actinobacteria, Spirochaetes, and Bacteroidetes. Within the Archaea the most salt-requiring microorganisms are found in the class Halobacteria. Halobacterium and most of its relatives require over 100-150 g/l salt for growth and structural stability. Also within the order Methanococci we encounter halophilic species. Halophiles and non-halophilic relatives are often found together in the phylogenetic tree, and many genera, families and orders have representatives with greatly different salt requirement and tolerance. A few phylogenetically coherent groups consist of halophiles only: the order Halobacteriales, family Halobacteriaceae (Euryarchaeota) and the anaerobic fermentative bacteria of the order Halanaerobiales (Firmicutes). The family Halomonadaceae (Gammaproteobacteria) almost exclusively contains halophiles. Halophilic microorganisms use two strategies to balance their cytoplasm osmotically with their medium. The first involves accumulation of molar concentrations of KCl. This strategy requires adaptation of the intracellular enzymatic machinery, as proteins should maintain their proper conformation and activity at near-saturating salt concentrations. The proteome of such organisms is highly acidic, and most proteins denature when suspended in low salt. Such microorganisms generally cannot survive in low salt media. The second strategy is to exclude salt from the cytoplasm and to synthesize and/or accumulate organic 'compatible' solutes that do not interfere with enzymatic activity. Few adaptations of the cells' proteome are needed, and organisms using the 'organic-solutes-in strategy' often adapt to a surprisingly broad salt concentration range. Most halophilic Bacteria, but also the halophilic methanogenic Archaea use such organic solutes. A variety of such solutes are known, including glycine betaine, ectoine and other amino acid derivatives, sugars and sugar alcohols. The 'high-salt-in strategy' is not limited to the Halobacteriaceae. The Halanaerobiales (Firmicutes) also accumulate salt rather than organic solutes. A third, phylogenetically unrelated organism accumulates KCl: the red extremely halophilic Salinibacter (Bacteroidetes), recently isolated from saltern crystallizer brines. Analysis of its genome showed many points of resemblance with the Halobacteriaceae, probably resulting from extensive horizontal gene transfer. The case of Salinibacter shows that more unusual types of halophiles may be waiting to be discovered.
嗜盐菌存在于生命的所有三个域中。在细菌域内，我们已知蓝细菌门、变形菌门、厚壁菌门、放线菌门、螺旋体门和拟杆菌门中的嗜盐菌。在古菌域中，最需要盐的微生物存在于嗜盐杆菌纲中。盐杆菌及其大多数亲缘种需要超过100 - 150克/升的盐才能生长并保持结构稳定。在甲烷球菌目中也有嗜盐物种。嗜盐菌和非嗜盐亲缘种在系统发育树中常常共存，许多属、科和目都有盐需求和耐受性差异极大的代表物种。有几个系统发育上连贯的类群仅由嗜盐菌组成：嗜盐杆菌目、嗜盐杆菌科（广古菌门）以及嗜盐厌氧菌目（厚壁菌门）的厌氧发酵细菌。嗜盐单胞菌科（γ - 变形菌纲）几乎只包含嗜盐菌。嗜盐微生物采用两种策略来使其细胞质与周围环境在渗透压上达到平衡。第一种策略涉及积累摩尔浓度的氯化钾。这种策略需要对细胞内的酶机制进行适应性调整，因为蛋白质需要在接近饱和的盐浓度下维持其正确的构象和活性。这类生物的蛋白质组高度酸性，大多数蛋白质在低盐溶液中会变性。这类微生物通常无法在低盐培养基中存活。第二种策略是将盐排除在细胞质之外，并合成和/或积累不干扰酶活性的有机“相容性”溶质。细胞蛋白质组几乎不需要进行调整，采用“有机溶质摄入策略”的生物通常能适应惊人的广泛盐浓度范围。大多数嗜盐细菌以及嗜盐产甲烷古菌都使用这类有机溶质。已知多种这类溶质，包括甘氨酸甜菜碱、四氢嘧啶和其他氨基酸衍生物、糖类和糖醇类。“高盐摄入策略”并不局限于嗜盐杆菌科。嗜盐厌氧菌目（厚壁菌门）也积累盐而非有机溶质。还有第三种在系统发育上无关的生物积累氯化钾：最近从盐田结晶卤水中分离出的红色极端嗜盐菌盐红菌（拟杆菌门）。对其基因组的分析显示出与嗜盐杆菌科有许多相似之处，这可能是广泛水平基因转移的结果。盐红菌的例子表明可能还有更多不寻常类型的嗜盐菌有待发现。
