模拟亚热带常绿阔叶林土壤温度与土壤异养呼吸对气候变暖的响应

为把握森林土壤温度及土壤异养呼吸对气候变暖的响应,利用1986—2013年哀牢山亚热带常绿阔叶林土壤温度观测数据模拟土壤温度未来上升2℃需要的时间,采用2011—2013年人工控制土壤增温试验中切根处理(NR)与切根增温处理(SW)的观测数据,结合WNMM模型及SRES情景下A2与B2未来气候数据模拟哀牢山森林土壤异养呼吸对气候变暖的响应。结果表明:5 cm土壤温度增加速率为0.224℃·10 a~(-1),自然增温2℃需要90 a;NR与SW处理下土壤Q_(10)值分别为5.17和4.50,根据Q_(10)值进行计算,NR处理在土温升高2℃后土壤异养呼吸较SW处理实测值升高14.6%;经过校正、验证后WNMM模型可以模拟土壤水分(P0.001)与土壤温度的变化(P0.001);A2、B2情景下,NR处理土壤异养呼吸较SW处理分别升高10.2%和9.8%;A2情景下土壤异养呼吸较B2情景下土壤异养呼吸,在NR、SW处理下分别升高7.0%和6.6%。本研究中数学模拟会高估土壤异养呼吸,表明野外的实测试验是不可替代的评估土壤异养呼吸对气候变暖响应的方法。     

蚂蚁筑巢对高檐蒲桃热带森林群落土壤呼吸的影响

蚂蚁筑巢能够改变热带森林土壤微生物与土壤理化性质的状况,从而对土壤呼吸时间动态产生重要影响。本研究以西双版纳高檐蒲桃热带森林群落为研究对象,采用Li-6400-09便携式土壤呼吸测定仪对蚂蚁筑巢地与非筑巢地土壤呼吸进行测定。研究结果表明:(1)高檐蒲桃群落土壤呼吸呈明显的单峰型季节变化趋势,且土壤呼吸速率蚂蚁筑巢地(4.96μmol CO_2m~(-2)s~(-1))高于非筑巢地(4.42μmol CO_2m~(-2)s~(-1))。(2)土壤温度和土壤水分显著影响土壤呼吸的时间动态(P0.01);蚂蚁筑巢显著改变巢内温度与水分(P0.05),进而影响土壤呼吸动态。土壤温度对土壤呼吸动态的贡献:蚁巢(83.8%—91.8%)大于非巢地(81.2%—83.1%),但由于筑巢地土壤湿度低于非巢地,土壤水分对土壤呼吸动态的贡献率表现为蚁巢低于非筑巢地。(3)蚂蚁筑巢显著增加土壤微生物生物量(P0.05),从而对土壤呼吸速率产生极显著的影响(P0.01)。蚂蚁筑巢引起微生物生物量碳的增加能够解释76.9%—71.1%的土壤呼吸变化。(4)蚂蚁筑巢引起土壤理化性质变化对土壤呼吸产生一定的影响。土壤容重与土壤呼吸速率呈显著负相关;土壤呼吸速率与土壤微生物量碳、有机质、易氧化有机碳、全氮、硝氮和铵氮显著正相关(P0.05或P0.01)。因此,蚂蚁筑巢显著改变土壤微生物(如微生物生物量碳)、土壤物理性质(如土壤温度与水分)、土壤化学性质(如碳和氮养分),进而对热带森林土壤呼吸产生重要影响。     

太岳山油松人工林土壤呼吸对模拟氮沉降的短期响应

通过对山西太岳山油松人工林进行模拟氮沉降实验,探究土壤呼吸对模拟氮沉降刺激的短期响应动态。2015年7—8月,分3次分别对同一样地进行模拟氮沉降处理,水平皆为100 kg hm~(-2)a~(-1),同时采用LI-COR8150土壤碳通量自动观测系统全天候连续监测土壤呼吸动态,探究土壤施氮前后呼吸速率的动态变化以及呼吸速率与土壤温度和湿度的关联。结果表明:3次氮沉降处理均呈现出相同规律,土壤呼吸值在施氮后1 d内达到最大值,随即下降,在施氮后第3天土壤呼吸趋于稳定;第一、二次氮沉降处理3 d后土壤呼吸恢复到处理前的状态,并未表现出显著差异(P0.05)。第三次氮沉降处理后土壤呼吸并未恢复到施氮前的状态,土壤呼吸均值由1.99μmol m~(-2)s~(-1)显著上升到3.39μmol m~(-2)s~(-1)(P0.05)。这表明,氮处理对土壤呼吸产生了持续效应。施氮后土壤呼吸与土壤温度呈极显著(P0.001)指数相关(R_s=ae~(bT)),随着时间的推移,施氮处理解释土壤呼吸的相对贡献值由60%—69%下降到14%—59%。施氮提高了土壤温度敏感系数Q_(10)值;土壤温度和湿度(R_s=ae~(bT)W~c)能更好的解释土壤呼吸变化,解释率达到49%—91%。在全球变化的背景下,研究模拟氮沉降对土壤呼吸、Q_(10)的影响,可以对进一步模拟、预测全球暖温带地区森林碳循环和碳储量提供理论基础。     

亚热带毛竹人工林土壤呼吸组分动态变化及其影响因素

利用Li-8100土壤碳通量测量系统,研究了2013年4月—2014年3月浙江临安市毛竹人工林土壤呼吸、异养呼吸和自养呼吸速率的动态变化规律.结果表明：毛竹人工林土壤总呼吸速率、异养呼吸速率和自养呼吸速率均呈现出明显的季节变化特征,最高值出现在7月,最低值出现在1月,年平均值分别为2.93、1.92和1.01 μmol CO₂·m^-2·s^-1.毛竹林土壤总呼吸、异养呼吸和自养呼吸年累积CO₂排放量分别为37.25、24.61和12.64 t CO₂·hm^-2·a^-1.土壤呼吸各组分均与土壤5 cm温度呈显著指数相关,土壤总呼吸、异养呼吸和自养呼吸的温度敏感系数Q₁₀值分别为2.05、1.95和2.34.土壤总呼吸速率、异养呼吸速率与土壤水溶性有机碳(WSOC)含量均呈显著相关,而自养呼吸与WSOC无显著相关性;土壤呼吸各组分与土壤含水〖JP2〗量以及微生物生物量碳均无显著相关性.土壤温度是影响毛竹人工林土壤呼吸及其组分季节变化的主要驱动因子,土壤WSOC含量是影响土壤总呼吸和异养呼吸的重要环境因子.     

森林土壤呼吸及其对全球变化的响应

森林土壤呼吸是全球碳循环的重要流通途径之一 ,其动态变化将直接影响全球 C平衡。森林土壤呼吸由自养呼吸和异养呼吸组成 ,不同森林类型、测定季节和测定方法等直接影响其所占比例。土壤温度和湿度是影响森林土壤呼吸的最主要因素 ,共同解释了森林土壤呼吸变化的大部分。因树种组成、生产力和枯落物数量等不同而使不同森林类型土壤呼吸速率表现出明显差异。采伐对森林土壤呼吸的影响结果有增加、降低或无影响 ,因采伐方式、森林类型、采伐迹地上植被恢复进程和气候条件等而异。火烧一般导致土壤呼吸速率降低。因肥料种类、施用剂量和立地条件不同 ,施肥对森林土壤呼吸的影响出现增加、降低或无影响等不同结果。大气 CO2 浓度升高和升温均可促进森林土壤呼吸。 N沉降有可能刺激了土壤呼吸 ,而酸沉降则可能降低了土壤呼吸。臭氧浓度和 UV-B辐射强度亦会在一定程度上影响森林土壤呼吸。但目前全球变化对森林土壤呼吸的综合影响尚不清楚 ,深入探讨森林土壤呼吸的调控因素及其对全球变化和营林措施的响应等仍是今后努力的主要方向。     

神农架海拔梯度上4种典型森林的土壤呼吸组分及其对温度的敏感性

量化森林土壤呼吸及其组分对温度的响应对准确评估未来气候变化背景下陆地生态系统的碳平衡极其重要。该文通过对神农架海拔梯度上常绿阔叶林、常绿落叶阔叶混交林、落叶阔叶林以及亚高山针叶林4种典型森林土壤呼吸的研究发现: 4种森林类型的年平均土壤呼吸速率和年平均异养呼吸速率分别为1.63、1.79、1.74、1.35 μmol CO₂·m^-2·s^-1和1.13、1.12、1.12、0.80 μmol CO₂·m^-2·s^-1。该地区的土壤呼吸及其组分呈现出明显的季节动态, 夏季最高, 冬季最低。4种森林类型中, 阔叶林的土壤呼吸显著高于针叶林, 但阔叶林之间的土壤呼吸差异不显著。土壤温度是影响土壤呼吸及其组分的主要因素, 二者呈显著的指数关系; 土壤含水量与土壤呼吸之间没有显著的相关关系。4种典型森林土壤呼吸的Q₁₀值分别为2.38、2.68、2.99和4.24, 随海拔的升高土壤呼吸对温度的敏感性增强, Q₁₀值随海拔的升高而增加。     

土壤异养呼吸的测定及其温度敏感性影响因子

土壤异养呼吸主要指土壤中微生物分解有机质释放CO2的过程,是陆地生态系统中土壤碳的主要净输出途径,土壤异养呼吸与净初级生产力的差值是决定生态系统碳源/汇的关键.本文介绍了土壤异养呼吸的测定方法--室内培养的去根土壤样品培养和原状土柱培养,以及野外原位测定的根排除法、环割法和同位素法等操作方法的优缺点以及适用范围.在土壤异养呼吸的研究方面,土壤异养呼吸温度敏感性(Q10)是碳循环研究的重要方面之一,温度、水分以及土壤呼吸底物是影响Q10的主要因子,一般情况下,随着温度的升高,Q10下降;土壤含水量过低或过高时,Q10降低;土壤有机碳的有效性影响着土壤异养呼吸对温度变化的响应程度,当有效性降低时Q10下降,不同周转时间的有机碳的温度敏感性也不相同,活性有机碳的温度敏感性较惰性有机碳的温度敏感性低.     

亚热带沟叶结缕草草坪土壤呼吸

随城市化进程加速,城市草坪生态系统释放CO2将对区域碳循环产生重要影响。采用LI-8100开路式土壤碳通量测量系统对亚热带沟叶结缕草草坪（Zoysia matrella）土壤呼吸进行为期1a的定位研究,结果表明：草坪土壤呼吸季节动态呈现为单峰曲线,全年土壤呼吸速率的变化范围在38.99—368.50 mg C?m-2?h-1之间,年通量为1684 g C?m-2?a-1。土壤温度、总生物量、以及二者的交互作用对土壤呼吸季节变化的解释程度接近,分别为89%、88%和90%,但仅二者的交互作用进入土壤呼吸的逐步回归方程,表明草坪土壤呼吸的季节变化主要受土壤温度与总生物量共同驱动。春末修剪草坪对土壤呼吸速率没有显著影响。在秋末无雨时期,浇水后1—2d土壤湿度对土壤呼吸的促进作用可掩盖同期降温的影响,使土壤呼吸速率显著升高。     

米槠和杉木人工林土壤呼吸及其组分分析

区分森林土壤呼吸组分是了解生态系统碳循环的重要环节。该文以福建省三明市格氏栲自然保护区米槠(Castanopsis carlesii)人工林和邻近的杉木(Cunninghamia lanceolata)人工林为研究对象, 于2012年8月至2013年7月, 采用LI-8100开路式土壤碳通量系统, 通过挖壕沟方法, 测定了土壤呼吸及异养呼吸的速率, 同时测定了5 cm深处的土壤温度和0-12 cm深处的土壤含水量。利用指数模型和双因素模型, 分析土壤呼吸及其组分与土壤温度和土壤含水量的关系, 同时计算了土壤呼吸各组分在土壤呼吸中所占的比例, 并分析了不同森林类型对土壤呼吸及其组分的影响。结果表明: 米槠人工林和杉木人工林土壤呼吸及其组分的季节变化显著, 均呈单峰型曲线, 与5 cm深处的土壤温度呈极显著正相关关系。土壤温度可以分别解释米槠人工林土壤呼吸、自养呼吸和异养呼吸变化的70.3%、73.4%和58.2%, 可以解释杉木人工林土壤呼吸、自养呼吸和异养呼吸变化的77.9%、65.7%和79.2%。土壤呼吸及其组分与土壤含水量没有相关关系。米槠和杉木人工林自养呼吸的年通量分别为4.00和2.18 t C·hm^-2·a^-1, 占土壤呼吸年通量的32.5%和24.1%; 异养呼吸年通量分别为8.32和6.88 t C·hm^-2·a^-1, 分别占土壤呼吸年通量的67.5%和75.9%, 米槠人工林土壤呼吸及其组分的年通量都大于杉木人工林。     

鼎湖山三种主要植被类型土壤碳释放研究

土壤呼吸是土壤微生物活性和土壤肥力一个重要指标 ,是土壤碳流通的一个主要过程 ,也是陆地生态系统碳循环的一个关键部分 ,对研究全球变化非常重要。国内土壤呼吸的研究主要集中在北京山地温带林区、尖峰岭热带森林及东北羊草草原和中亚热带等地 ,南亚热带地区森林土壤呼吸尚无报道。选取南亚热带鼎湖山自然保护区森林演替系列中的 3种主要植被类型 (季风常绿阔叶林 ,针阔叶混交林和马尾松林 )为研究对象 ,研究了土壤呼吸和与之相关的土壤微生物生物量、土壤温度和土壤含水量以及他们之间的关系。结果表明 ,季风常绿阔叶林、针阔叶混交林和马尾松林年均土壤呼吸速率依次是 477.9,435 .4,42 9.5 mg CO2 · m- 2 ·h- 1,土壤呼吸速率与土壤温度的季节变化规律接近 ;3种植被类型土壤微生物生物量变化规律与土壤呼吸变化规律一致 ,季风常绿阔叶林最高 ,马尾松林最低 ,土壤微生物量高的土壤中碳周转量较大 ,碳素周转还带动了其他营养元素周转 ,有利于生态系统生存和持续发展 ;季风常绿阔叶林、针阔叶混交林和针叶林代谢熵依次是 0 .5 8～ 0 .60 ,0 .92～ 1 .0 0 ,1 .30～ 1 .35 ,表明 3种植被类型土壤中土壤微生物对土壤碳的利用效率依次降低。     

Continuous Processing for Production of Biopharmaceuticals

The merits of continuous processing over batch processing are well known in the manufacturing industry. Continuous operation results in shorter process times due to omission of hold steps, higher productivity due to reduced shutdown costs, and lowers labor requirement. Over the past decade, there has been an increasing interest in continuous processing within the bioprocessing community, specifically those involved in production of biotherapeutics. Continuous operations in upstream processing (perfusion) have been performed for decades. However, recent development of continuous downstream operations has led the industry to envisage an integrated bioprocessing platform for efficient production. The regulators, key players in the biotherapeutic industry, have also expressed their interest and willingness in this migration from the traditional batch processing. This paper aims to review major developments in continuous bioprocessing in the past decade. A discussion of pros and cons of the different proposed approaches has also been presented.     

Validation and optimization of viral clearance in a downstream continuous chromatography setting

Continuous bioprocessing holds the potential to improve product consistency, accelerate productivity, and lower cost of production. However, switching a bioprocess from traditional batch to continuous mode requires surmounting business and regulatory challenges. A key regulatory requirement for all biopharmaceuticals is virus safety, which is assured through a combination of testing and virus clearance through purification unit operations. For continuous processing, unit operations such as capture chromatography have aspects that could be impacted by a change to continuous multicolumn operation, for example, do they clear viruses as well as a traditional batch single column. In this study we evaluate how modifying chromatographic parameters including the linear velocity and resin capacity utilization could impact virus clearance in the context of moving from a single column to multicolumn operation. A Design of Experiment (DoE) approach was taken with two model monoclonal antibodies (mAbs) and two bacteriophages used as mammalian virus surrogates. The DoE enabled the identification of best and worst-case scenario for virus clearance overall. Using these best and worst-case conditions, virus clearance was tested in single column and multicolumn modes and found to be similar as measured by Log Reduction Values (LRV). The parameters identified as impactful for viral clearance in single column mode were predictive of multicolumn modes. Thus, these results support the hypothesis that the viral clearance capabilities of a multicolumn continuous Protein A system may be evaluated using an appropriately scaled-down single mode column and equipment.     

Integrated continuous production of recombinant therapeutic proteins

In the current environment of diverse product pipelines, rapidly fluctuating market demands and growing competition from biosimilars, biotechnology companies are increasingly driven to develop innovative solutions for highly flexible and cost‐effective manufacturing. To address these challenging demands, integrated continuous processing, comprised of high‐density perfusion cell culture and a directly coupled continuous capture step, can be used as a universal biomanufacturing platform. This study reports the first successful demonstration of the integration of a perfusion bioreactor and a four‐column periodic counter‐current chromatography (PCC) system for the continuous capture of candidate protein therapeutics. Two examples are presented: (1) a monoclonal antibody (model of a stable protein) and (2) a recombinant human enzyme (model of a highly complex, less stable protein). In both cases, high‐density perfusion CHO cell cultures were operated at a quasi‐steady state of 50–60 × 10⁶ cells/mL for more than 60 days, achieving volumetric productivities much higher than current perfusion or fed‐batch processes. The directly integrated and automated PCC system ran uninterrupted for 30 days without indications of time‐based performance decline. The product quality observed for the continuous capture process was comparable to that for a batch‐column operation. Furthermore, the integration of perfusion cell culture and PCC led to a dramatic decrease in the equipment footprint and elimination of several non‐value‐added unit operations, such as clarification and intermediate hold steps. These findings demonstrate the potential of integrated continuous bioprocessing as a universal platform for the manufacture of various kinds of therapeutic proteins. Biotechnol. Bioeng. 2012; 109: 3018–3029. © 2012 Wiley Periodicals, Inc.     

Studies on the Bacterial Formation of Peptide Antibiotic,Colistin

The biosynthetic mechanism of 6-methyloctanoic and isooctanoic acids, which are present in the amide linkage with the α-amino group of the terminal α, γ-diaminobutyric acid residue of colistin A and B, respectively was investigated. From the isotopic experiments using isoleucine-U-¹⁴C, valine-U-¹⁴C and acetic acid-2-¹⁴C, it was concluded that 6-methyloctanoic and isooctanoic acids were derived from isoleucine and valine, respectively.

Amino acids pooled in colistin-producing cells grown in the synthetic medium were abundant in isoleucine, valine and leucine, which were probable precursors of the abovedescribed fatty acid components of colistin and cellular fatty acids. On the other hand, 6-methyloctanoic and isooctanoic acids were not found in the cellular fatty acids, while C-15 and C-16 branched chain fatty acids usually found in Bacillus sp. were abundantly contained in the cells, indifferently of an improved capacity of colistin formation.     

Establishment of two continuous T-cell strains from a single plaque of a patient with mycosis fungoides

Summary From a plaque biopsy of a patient with mycosis fungoides, two different continuous cell lines were established by including both IL-2 and IL-4 in culture medium. Both continuous cell lines appeared with characteristic chromosome markers after approximately 40 cell population doublings. The initial karyotype recognized in T cells from the skin biopsy was 46,XY and the karyotypes of the continuous cell strains were 46,XY, -18, + i(18q) and another with multiple chromosome aberrations as described in Sezary T-cell leukemia. Phenotyping with monoclonal antibodies and T-cell receptor analysis indicates that the latter cell strain represents a minority of T-cells in the plaque. Due to its many chromosomal aberrations it probably represents the malignant cell, which may be a central cell in the immune stimulation taking place in the skin.     

Gamma irradiating chromatography columns enables bioburden-free integrated continuous biomanufacturing

An important consideration for integrated continuous biomanufacturing is that the downstream chromatography steps integrated with the bioreactor should maintain a low bioburden state throughout the entire duration of the operation. One potential strategy to achieve this is to start bioburden-free and functionally close the chromatography system. While chromatography skids themselves can be rendered bioburden-free, limitations exist in applying these methods to chromatography columns. The small column sizes used in continuous multicolumn chromatography enable gamma irradiation of disposable columns to render them bioburden-free. However, this approach has not been widely implemented, likely because gamma irradiation can negatively impact resin performance. Here, several protective mobile-phase modifiers were screened and shown to help chromatography resins retain naïve-like performance. Gamma irradiated columns were then integrated into perfusion bioreactors for continuous capture. Successful integrated continuous capture downstream of perfusion bioreactors for greater than 40 days using protein A, custom affinity, and non-affinity capture resins for multiple biologic modalities is demonstrated in development and commercial settings. No indications of time-based performance decline or bioburden growth have been observed. This strategy enables bioburden-free integrated continuous biomanufacturing operations and could allow full process closure and decreased environmental control requirements for facilities; thus, permitting simultaneous multi-product operations in a ballroom arrangement.     

苜蓿和小麦长期连作对土壤酶活性及养分的影响

在长武长期轮作与施肥定位试验的基础上,对黄土高原沟壑区苜蓿和小麦连作27年后的黄盖黏黑垆土土壤酶活性及土壤养分进行了测定,探讨苜蓿和小麦长期连作对土壤酶活性及肥力的影响.结果表明： 苜蓿和小麦连作施肥都能提高土壤酶活性;同一作物不同施肥措施的土壤蔗糖酶、脲酶、磷酸酶活性差异不显著,但相同施肥条件下,苜蓿连作比小麦连作的土壤蔗糖酶、脲酶、磷酸酶活性高;不同作物种类及施肥措施的土壤过氧化氢酶活性差异不明显;氮磷肥和有机肥配施(NPM)的土壤脲酶、磷酸酶和过氧化氢酶活性较高,而蔗糖酶活性较低;苜蓿连作比小麦连作更有利于土壤有机质、全氮及速效氮的积累,氮磷肥和有机肥配施有利于培肥土壤.
     

Process optimization of continuous gluconic acid fermentation by isolated yeast-like strains of Aureobasidium pullulans

This study was focused on the optimization of a new fermentation process for continuous gluconic acid production by the isolated yeast-like strain Aureobasidium pullulans DSM 7085 (isolate 70). Operational fermentation parameters were optimized in chemostat cultures, using a defined glucose medium. Different optima were found for growth and gluconic acid production for each set of operation parameters. Highest productivity was recorded at pH values between 6.5 and 7.0 and temperatures between 29 and 31 degrees C. A gluconic acid concentration higher than 230 g/L was continuously produced at residence times of 12 h. A steady state extracellular gluconic acid concentration of 234 g/L was measured at pH 6.5. 122% air saturation yielded the highest volumetric productivity and product concentration. The biomass-specific productivity increased steadily upon raising air saturation. An intracellular gluconic acid concentration of about 159 g/L (0.83 mol) was determined at 31 degrees C. This is to be compared with an extracellular concentration of 223 g/L (1.16 mol), which indicates the possible existence of an active transport system for gluconic acid secretion, or the presence of extracellular glucose oxidizing enzymes. The new process provides significant advantages over the traditional discontinuous fungi operations. The process control becomes easier, thus offering stable product quality and quantity.     

Performance comparison of ethanol and butanol production in a continuous and closed-circulating fermentation system with membrane bioreactor

Since both ethanol and butanol fermentations are urgently developed processes with the biofuel-demand increasing, performance comparison of aerobic ethanol fermentation and anerobic butanol fermentation in a continuous and closed-circulating fermentation (CCCF) system was necessary to achieve their fermentation characteristics and further optimize the fermentation process. Fermentation and pervaporation parameters including the average cell concentration, glucose consumption rate, cumulated production concentration, product flux, and separation factor of ethanol fermentation were 11.45?g/L, 3.70?g/L/h, 655.83?g/L, 378.5?g/m²/h, and 4.83, respectively, the corresponding parameters of butanol fermentation were 2.19?g/L, 0.61?g/L/h, 28.03?g/L, 58.56?g/m²/h, and 10.62, respectively. Profiles of fermentation and pervaporation parameters indicated that the intensity and efficiency of ethanol fermentation was higher than butanol fermentation, but the stability of butanol fermentation was superior to ethanol fermentation. Although the two fermentation processes had different features, the performance indicated the application prospect of both ethanol and butanol production by the CCCF system.     

Studies on Halimeda IV. An endogenous rhythm of chloroplast migration in the siphonous green alga,H. distorta 1

Abstract

Halimeda has been found particularly suitable for studies of long‐distance chloroplast migration by virtue of its coenocytic structure and calcium carbonate skeleton. A circadian rhythm of chloroplast migration in Halimeda distorta was monitored by videography of segment surface pigmentation. In normal 12 h light/12 h dark treatments synchronised with dawn and dusk, the segments were green all day, began to become pale immediately the light was turned off, and then remained almost white for most of the night until beginning to re‐green a few hours before dawn. As a result of that, they were already quite green by the time the light was turned on. In continuous darkness a similar cycle, albeit with reducing amplitude and a period of about 23 hours, was maintained for at least 7 days. However, this cycle differed significantly from the normal one in that the segments did not remain green after the light was not switched on at dawn, but rather began to pale immediately thereafter. Conversely, in continuous light the segments did not become pale at any time. Thus, the rhythmical re‐emergence of the chloroplasts before dawn and their subsequent withdrawal appears to be controlled by an endogenous rhythm which is independent of light. However, light does completely, but reversibly, inhibit the chloroplast withdrawal component of the cycle. This behaviour of the chloroplasts in Halimeda is very similar to that in the related alga, Caulerpa, but it is quite different from that in another extensively Studied but unrelated siphonous green alga, Acetabularia, in which the circadian rhythm of chloroplast migration is maintained in continuous light.