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1、Page 91new bacterial group that contain bacteriochlorophyll, yet grow aerobically, has forced a revision ofthis view (Box 2.2).壬 Although they grow aerobically, these types do not produce O2 fromphotosynthesis. O2As noted in Section 5.2.2, the Roseobacter clade is well-represented in culture indepen
2、dentsurveys. Like the purple bacteria, the aerobic anoxygenic (AAnP) photobacteria containa range of carotenoid pigments.5.2.2 However, the photosynthetic apparatus is not as well structured andthe complex membrane invaginations typical of the anaerobic phototrophs are not seen in the aerobictypes.
3、This probably explains why the AAnP bacteria cannot use light as a sole source of energy andrely on various organic compounds as a source of carbon and energy.It is now known that the Roseobacter clade contains both culturable and nonculturable types, as wellas phototrophic and nonphototrophic repre
4、sentatives. These are widely distributed in coastal andoceanic plankton and occur in a wide range of associations with other marine organisms. In molecular studies, the Roseobacter clade emerges as the second most abundant 16S rRNA geneclone type (over 30% of clones).16S rRNAİ Their association with
5、 blooms of algae and dinoflagellates havebeen particularly studied, and they may play a role in the formation of dinoflagellate toxins (Box12.2). 涾(Box12.2Despite their obvious ubiquitous presence in ocean water, it is currently difficult to be sure ofthe role of this group in ecological processes.T
6、he diverse metabolic properties of the groupundoubtedly play a large part in nutrient cycling. For example, Roseobacter has a major role in thebreakdown of DMSP leading to the formation of dimethyl sulfide (DMS), which has great significancein the global climate (Box 8.1).磬DMSP5.3.3 Green sulfur bac
7、teriaThe group known as green sulfur bacteria forms a separate lineage distinct from the Proteobacteria,but resembles the purple sulfur bacteria in metabolism. However, sulfur is produced outside of thecell rather than as intracellular granules. Also, in addition to bacterio-chlorophyll a,
8、green sulfurbacteria contain bacteriochlorophyll c, d or e. These pigments are contained in a membrane-boundstructure known as the chlorosome.acde Members of the family Chromatiaceae are common on intertidalmudflats and as members of consortia in microbial mats and sediments.5.4 Oxygenic phototrophs
9、the Cyanobacteria-5.4.1 Nature of the CyanobacteriaThe Cyanobacteria is a large and diverse group, and members are characterized by their ability tocarry out photosynthesis in which O2 is evolved, although some anoxygenic Cyanobacteria haverecently been described.O2 Cyanobacteria contain chlorophyll
10、 a, together with accessory photosyntheticpigments called phycobilins.a嵨 This group was formerly known as the blue-green algae (due to thepresence of the blue pigment phycocyanin together with the green chlorophyll) and is still treated asa division of the algae by many marine biologists and phycolo
11、gists, although Cyanobacteria areclearly prokaryotes and form one of the major divisions of the domain Bacteria.塱 Furthermore, many marine genera contain phycoerythrins, which give the cells a red-orange, rather than blue-green,color. Fossil evidence, in the form of morphological structures and dist
12、inctive biomarkers typical ofthe group (hopanoids), suggests that organisms resembling Cyanobacteria may have evolved about 3billion years ago and the evolution of O2 from the photosynthetic activities of Cyanobacteria (or theirancestors) was probably responsible for changes in the Earths early atmo
13、sphere.hopanoids30仯 Cyanobacteria occupy very diverse habitats in terrestrial and aquatic environments, including extreme temperaturesand hypersaline conditions. In the marine environment, habitats include the plankton, sea ice andshallow sediments, as well as microbial mats on the surface of inanim
14、ate objects, algae or animaltissue.mats Some marine isolates require NaCl plus other marine salts for growth in culture, whilst otherstolerate a range of salt concentrations.Page 925.4.2 Morphology and taxonomyThe Cyanobacteria are morphologically very diverse, ranging from small undiffere
15、ntiated rods tolarge branching filaments showing cellular differentiation.İ Unicellular Cyanobacteria divide by binaryfission, whilst some filamentous forms multiply by fragmentation or release of chains of cells. Many types are surrounded by mucilaginous sheaths that bind cells together. The chloro
16、phyll a is contained within lamellae called phycobilosomes which are often complex and multilayered.aphycobilosomes Cyanobacteria show remarkable ability to adapt the arrangement of their photosynthetic membranes and the proportionof phycobilin proteins to maximize their ability to utilize light of
17、different wavelengths.嵨 Many Cyanobacteria form intracellular gas vesicles (Section 3.3), which enable cells to maintain themselvesin the photic zone. Gas vesicles, as well as mucilaginous sheaths and pigments, also protect cellsfrom extreme effects of solar radiation (Section 4.6.5).Gliding motilit
18、y is very important in Cyanobacteria that colonize surfaces. Gliding movement, up to 10m per second, occurs parallel to the cells long axis and involves the production of mucilaginouspolysaccharide slime. There are two possible mechanisms by which gliding occurs.10um One is the propagation of waves
19、moving from one end of the filament to the other, created by the contractionof protein fibrils in the cell wall. The other mechanism is secretion of mucus by a row of pores aroundthe septum of the cell. Some types, such as Nostoc, are only motile during certain stages of their lifecycle, when they p
20、roduce a gliding dispersal stage known as hormogonia. Synechococcus also seems able to swim in liquid media without using flagella.Until the use of 16S rRNA analysis established Cyanobacteria as a group within the Bacteria, theywere classified by botanists into about 150 genera and 1000 species base
21、d on morphological features(Table 5.1).16s rRNA1501000(Table 5.1). Subsequent phylogenetic analysis shows that these groupings are very unreliable andmany genera are polyphyletic. Since bacteriologists can now grow many of the Cyanobacteria in pureculture, analysis of biochemical characteristics and
22、 molecular features are taking over as a basis ofclassification, and a major revision of the group is in progress. Pure culture studies show that thephysiological properties of Cyanobacteria are more variable than previously thought. Many are capable of anaerobic growth, some can use H2S, H2 or redu
23、ced organic compounds as electrondonors and some can be photoheterotrophic.H2SH2岢 However, little is known about the significance of thesemodes of nutrition in natural marine environments.5.4.3 Nitrogen fixationAll major groups of marine Cyanobacteria contain members which fix atmospheric
24、N2. Figure 4.5shows an outline of the reactions involved in this process.N24.5漰Table 5.1 Examples of marine CyanobacteriaOrder Features Major marine generaChroococcales Unicellular or aggregates of single cells. May be motile ProchlorococcusSynechococcusSynechocystisPleurocapsales Aggregates of sing
25、le cells. Reproduce by small spherical glidingcells (baeocytes) formed by multiple fissionCyanocystisPleurocapsaOscillatorales Filamentous cells (trichome), often sheathed.Oscillatorales Intercalary binarydivision at right angles to long axis. MotileTrichodesmiumLyngbaNostocales Filamentous trichome
26、 with heterocysts NostocStigonematalesBranching clusters, filamentous with heterocysts Mainly freshwateror terrestrial飬The bond in molecular N2 is verystable, and its reduction to ammonia is an extremely energy-demanding process, requiring 16molecules of ATP for each molecule of N2 fixed.N216ATP The
27、 key enzyme, nitrogenase, consists of twoseparate protein components complexed with iron, sulfur and molybdenum. In the marineenvironment, N2 fixation is carried out by a wide range of heterotrophic and autotrophic bacteria andis of fundamental significance in primary production in the oceans (Chapt
28、er 9).塣 Most N2-fixers are anaerobic, but the Cyanobacteria are aerobic and, because the enzyme nitrogenase is highly O2-sensitive, N2 fixation is often restricted to the night when no O2 is generated. Many of the more efficient N2-fixers contain differentiated cells known as heterocysts wi
29、thin the filament. Because the heterocysts contain no photosytem II, they provide an O2-free environment which protects theenzyme nitrogenase.photosytem II However, one of the most prolific marine N2-fixers is Trichodesmium, whichdoes not contain heterocysts. Recently it has been shown that Trichode
30、smium is able to switch thetwo processes of the O2-producing photosynthetic system and the O2-sensitive N2-fixing system onand off over timescales of a few minutes. There also appears to be a spatial separation of O2evolution and N2 fixation, because N2 fixation is limited to certain parts of the ce
31、ll. Trichodesmiumforms dense filamentous masses which are responsible for large blooms, especially in tropical seas.仯İ5.4.4 Prochlorococcus and SynechococcusAlthough many types of Cyanobacteria are found in marine environments, two genera dominate thepicoplankton in large areas of the Earths oceans,
32、 namely Synechococcus and Prochlorococcus. These organisms are major contributors to the carbon cycle through photosynthetic CO2 fixation, accountingfor between 15 and 40% of carbon input to ocean food webs. Prochlorococcus is a very small (about0.6 m diameter) cyanobacterium which was not
33、 discovered until 1988 (following the use of FCM),despite the fact that it inhabits large parts of the oceans at a density between 105 and 106 ml?1,making it the most abundant photosynthetic organism on Earth. CO2嵽1988105106ml?1塣Prochlorococcus is most abundant inthe region from 40S to 40N,
34、 temperature range 10C to 33C, to a depth of about 200 m.Prochlorococcus contains modified forms of chlorophyll (divinyl chlorophylls a and b), but lacksphycobilins.40401030200嵨 The photosynthetic apparatus seems to be adapted to allow Prochlorococcus to grow atconsiderable depths, where the amount
35、of light is very low (below 1% that at the surface). The smallcell size gives a large SA:V ratio, which helps Prochlorococcus obtain scarce nutrients in oligotrophicocean waters. Other organisms with similar properties include Prochloron, which is an intracellularsymbiont of certain marine invertebr
36、ates. 塣These organisms were originally placed in a group calledthe prochlorophytes, but phylogenetic analysis shows that this is not a distinct lineage within theCyanobacteria. Prochlorophytes appear to have evolved divinyl chlorophylls a and b, which allowsthem to harvest longer wavelengths of blue
37、 light, which penetrate deeper waters. abIndeed, studies on Prochlorococcus cultures and populations from the field have shown that there are two distinctpopulations of ecotypes, which occupy different light niches. A high-light-adapted ecotype dominatesthe top 100 m of water, which is characterized
38、 by high light flux and very low nutrientconcentrations.100 The second ecotype thrives at depths of 80C200 m, which have low light intensitybut higher nutrient concentrations. 80-200The two ecotypes have different ratios of chlorophyll a2 to b2 anddiffer in their optimal irradiances for photosynthes
39、is. These significant differences in ecotype aredetermined by genetic differences of only about 2% in 16S rRNA sequences.a2b216srRNA They should probably be assigned to separate species, because the two types differ markedly in the number of genesencoding the light-harvesting complex. (See Box 5.1 f
40、or a discussion of the difficulties of speciesdefinition based on 16S rRNA studies.)16S rRNA5.116SThe discovery of Prochlorococcus is also highly significant for evolutionary theory. It has beenthought for many years that the chloroplasts present today in algae and plants evolved fromCyanobacteria i
41、n accordance with the endosymbiosis theory.塣 Phylogenetic analyses suggest that prochlorophytes, despite resembling chloroplasts without phycobilins, are not the immediate ancestralorigin of the chloroplast. It is possible that the prochlorophytes and the rest of the嵨壬Figure 5.3Stromatolites. (a) Co
42、lumnar build-ups in shallow water, Highborne Cay, Bahamas. (b) Vertical sectionshowing lamination; scale bar, 2 cm. From Reid et al. (2000), reproduced with permission, Nature-Macmillan Ltd.a) Highborne Cay, Bahamas.(b)2From Reid et al. (2000)Nature-Macmillan Ltd.Cyanobacteria may have evolved from
43、ancestors that contained phycobilins and more than one typeof chlorophyll. Prochlorophytes may have lost their phycobilins and the other Cyanobacteria may havelost their chlorophyll b during evolution, whilst the eukaryotic chloroplast evolved from thehypothetical ancestor of both groups. We do not
44、know when this divergence occurred.嵨嵨b5.4.5 Microbial mats and stromatolitesCyanobacteria are especially important in the formation of microbial mats in shallow water. Complexstratified communities of microorganisms develop at interfaces between sediments and the overlyingwater. Filamentous Cyanobac
45、teria such as Phormidium, Oscillatoria and Lyngbya are often dominantmembers of the biofilm in association with unicellular types such as Synechococcus andSynechocystis.徭 Steep concentration gradients of light, O2, H2S and other chemicals develop acrossthe biofilm. The mat becomes anoxic at night an
46、d H2S concentrations rise. Cyanobacteria (and othermotile bacteria in the biofilm) can migrate through the mat to find optimal conditions. Anoxygenicphototrophs as well as aerobic and anaerobic chemoheterotrophs are also present.O2H2SmatH2SchemoheterotrophsStromatolites are fossilized microbial mats
47、 of filamentous prokaryotes and trapped sediment. Theseancient structures were widespread in shallow marine seas over three billion years ago. Ancientstromatolites were probably formed by anoxygenic phototrophs, but modern stromatolites aredominated by a mixed community of Cyanobacteria and heterotr
48、ophic bacteria.3 Growth of modern marine stromatolites represents a dynamic balance between sedimentation and intermittentlithification of cyanobacterial mats (Figure 5.3). Rapid sediment accretion occurs when thestromatolite surfaces are dominated by pioneer communities of gliding filamentous Cyano
49、bacteria.During intermittent periods, surface films of exopolymer are decomposed by heterotrophic bacteria,forming thin crusts of microcrystalline calcium carbonate. Other types of Cyanobacteria modify thesediment, forming thicker stony plates.污塣5.5 The nitrifying bacteriaThis term describes bacteri
50、a that grow using reduced inorganic nitrogen compounds as electrondonors. Marine examples, which are present in suspended particles and in the upper layers ofsediments, include Nitrosomonas and Nitrosococcus (which oxidize ammonia to nitrite) andFigure 5.4Oxidation of ammonia by nitrifying bacteria.
51、塣 Examples of marine genera known to carry out theseprocesses are shown. These reactions are energetically unfavorable and oxidation of 35 ammoniamolecules or 15 nitrite molecules is required to produce fixation of one molecule of carbon dioxide.Nitrosobacter, Nitrobacter and Nitrococcus (which oxid
52、ize nitrite to nitrate). No organisms that cancarry out both reactions are known.纣3515 The ammonia-oxidizers are obligate chemolithoautotrophs andfix carbon via the Calvin cycle. The nitrite-oxidizers are usually chemolithoautotrophic, but can bemixotrophic using simple organic compounds heterotroph
53、ically. Because of these activities, nitrifyingbacteria play a major role in nitrogen cycling in the oceans, especially in shallow coastal sedimentsand beneath upwelling areas such as the Peruvian coast and the Arabian Sea. Previously, nitrifying bacteria were classified mainly on morpholog
54、ical characteristics. However, 16S rRNA analysis showsthat they occur in several branches of the Proteobacteria, and one type, Nitrospira, forms a distinctbacterial phylum. Members of the subdivision have been found only in marine environments. Likethe phototrophs, nitrifying bacteria have extensive
55、 internal structures in order to increase the surfacearea of the membrane.16s rRNAr档It is difficult to obtain estimates of the abundance and community structure of nitrifying bacteria.Although most can be cultivated in the laboratory; the energetics of this mode of chemolithotrophymean tha
56、t the bacteria grow slowly and are difficult to work with. 飩Immunofluorescence methods(Section 2.4) reveal that Nitrosococcus oceani and similar strains are widespread in many marineenvironments, with worldwide distribution, at concentrations between 103 and 104 cells ml?1. Thisorganism is thought t
57、o be responsible for significant oxidation of ammonia in the open ocean.Nitrospira also appears to be distributed worldwide.103-104ml?1 Study of their activities and contribution tonitrogen cycling is usually carried out using isotopic methods with 15 or 15 (Section 2.7.3) orby using vario
58、us inhibitors of nitrification enzymes (e.g. nitrapyrin inhibits ammonia monoxygenase).Nitrification is a strictly aerobic process and sufficient O2 usually only penetrates a few millimetersinto sediments.1515簱 Activity of burrowing worms can increase O2 availability to deeper levels ofsediments. Ni
59、trification rates are high in waters where plant photosynthesis releases O2 and therelease of nitrate stimulates plant growth. This is of great importance in the productivity of seagrassbeds. The overall reactions for oxidation of ammonia to nitrite and its subsequent oxidation to nitrateare shown in Figure 5.4.塣5.4.5.6 Sulfur-and iron-oxidizing chemolithotrophs5.6.1 Thiobacillus, Beggiatoa, Thiothrix, and ThiovulumA wide range of Proteobacteria can grow chemolithotrophically using reduced sulfur compounds as asource of electrons, leading to the formation of sulfate.