高摄食压力下球形棕囊藻凝聚体的形成

棕囊藻包含囊体和游离单细胞两种生活史阶段。囊体是棕囊藻藻华爆发时的优势形态,藻华衰退时囊体能够形成凝聚体,但是棕囊藻游离单细胞的凝聚体极少被发现。本次研究将球形棕囊藻单细胞和高密度的海洋弯曲甲藻共同培养,使球形棕囊藻的生长承受高摄食压力,通过观察摄食者和棕囊藻的生长、凝聚体的数量和形态,阐明单细胞凝聚体的形成以及与摄食压力的关系。当球形棕囊藻进入衰退期时,高摄食压力引发游离单细胞聚合形成凝聚体,无摄食压力情况下,单细胞不形成凝聚体。凝聚体由无鞭毛细胞组成,细胞排列紧密,近似球体。凝聚体形成伊始,凝聚体内部可见凝胶状物质将细胞互相粘结,并且粘附了纤维等物质。凝聚体的体积和粘附的细胞数量逐渐提高,细胞排列愈加紧密,凝聚体内部形态和结构不易分辨。凝聚体的形成有效保护了部分单细胞免受摄食压力的影响,减少了摄食死亡率。凝聚体的形成是球形棕囊藻面临高摄食压力时采取的主动的防御策略。球形棕囊藻能够频繁引发大规模藻华的原因可能在于其在生长的各个阶段中均具有优越的竞争策略。     

球形棕囊藻的生长、囊体形态以及囊体细胞的分布

球形棕囊藻(Phaeocystis globosa)是中国近海海区常见有害藻华原因种, 其异型生活史中包含单细胞和球形囊体两种形态。游离单细胞直径一般为几微米, 囊体最大直径可达3 cm, 巨大的体积可能导致囊体具有特殊的结构和细胞分布。以球形棕囊藻汕头株为研究对象, 测定了囊体直径、囊体细胞丰度和游离单细胞丰度, 并探讨球形棕囊藻囊体形态与细胞分布的关系。研究结果表明, 囊体形态在其异型生活史中占优势, 囊体对生物量的贡献介于38%–95%之间, 在对数生长期的后期和稳定期, 囊体细胞与单细胞相比占绝对优势。囊体细胞数量与囊体直径的对数呈线性相关, 回归线斜率为1.34, 该值显著低于世界海区其它球形棕囊藻株系的研究结果, 表明汕头株单位囊体表面上分布的细胞数更少。中国海区的球形棕囊藻囊体结构和细胞分布与其它株系不同, 在爆发球形棕囊藻的海区, 巨大的囊体能够有效地抵御摄食, 可能对区域海洋食物链结构和功能有重要影响。     

球形棕囊藻囊体形成中光照、营养盐和共存硅藻的影响

在不同光照和营养盐结构条件下半连续培养球形棕囊藻和3种硅藻,研究光照、营养盐限制和硅藻竞争对球形棕囊藻囊体形成的影响。结果表明:高光照显著促进了藻类的生长,球形棕囊藻在低光环境下几乎不形成囊体。球形棕囊藻和3种硅藻对光限制和P限制更加敏感,而在N限制环境中均具有相对较高的生物量。粒径较小的球形棕囊藻游离单细胞和中肋骨条藻在营养盐和光限制条件下比粒径较大的细胞具有更强的竞争能力。硝酸盐是球形棕囊藻囊体形成的营养基础,但是营养盐结构并未改变棕囊藻囊体形态。具有两种生活史状态有利于球形棕囊藻度过资源限制的环境,从而有利于球形棕囊藻在硅藻藻华之后再次形成藻华。     

藻华发生过程中胶州湾水体颗粒有机物脂肪酸的组成与动态

胶州湾在2005年夏季发生了中肋骨条藻(Skeletonema costatum ( Grev.) Cleve)藻华,该优势种占细胞总量的比例在两个研究站位(C3站及A5站)分别达到59%和86%.对藻华发生过程中(7月,8月及9月份)的颗粒有机物进行了采样分析,测定了脂肪酸、叶绿素(Chl-a)和颗粒有机碳(POC)的浓度,分析了藻华发生过程中浮游植物和脂肪酸的组成特征,并通过标志脂肪酸对藻华发生过程中颗粒有机物组成的动态变化进行了分析.总脂肪酸浓度同叶绿素和颗粒有机碳浓度一样,在藻华发生期达到最高,两个采样站位(C3站及A5站)分别为29.0μg·L-1及185.5μg·L-1,比藻华发生前和消退后高2～3倍及20倍.胶州湾悬浮颗粒脂肪酸主要包括16:0,14:0,18:0等直链饱和脂肪酸(SSFA),16:1ω7、16:1ω5+ω9、18:1ω9,18:1ω7等单不饱和脂肪酸(MUFA)及以 20:5ω3(DHA)和22:6ω3(EPA)为主的多不饱和脂肪酸(PUFA)和以i-15:0及ai-15:0为主的支链饱和脂肪酸(BSFA).藻华期与发生前相比,脂肪酸中的不饱和组分(MUFA,PUFA)所占比例在两个取样站位均有提高.随着藻华的消退,藻华区域中心的A5 站的SSFA及BSFA比例有大幅提高,而在非藻华中心区域的C3站,不饱和脂肪酸尤其是ω3系列多不饱和脂肪酸(DHA, EPA等)比例的上升非常显著.藻华发生时浮游植物在颗粒有机物中的比重提高.藻华消退后,A5站颗粒有机物中碎屑有机物及细菌所占比重提高,而C3站颗粒有机物中浮游植物的比重仍然很高.浮游植物大量死亡是A5站藻华消退的主要形式,而C3站藻华的消退则可能与浮游动物的摄食有关.     

有害赤潮生物球形棕囊藻对卤虫的毒性研究

利用1997年10月和1998年6月分别在广东汕头饶平海域和香港海域分离到的球形棕囊藻,即球形棕囊藻香港株(Phaeocystis globosastrain HK)和球形棕囊藻汕头株(Phaeocystis globosastrain ST),在实验室条件下研究其对四日龄卤虫(Artemia sinica)的急性毒性。结果表明,对数生长期的HK株藻液24h对卤虫LC₅₀约为2.9×10⁵cell·mL^-1。对数期的ST株对卤虫的毒性难以达到半致死浓度,衰亡期的ST株24h对卤虫LC₅₀约为9.89×10⁶cell·mL^-1。在上述浓度下两者半致死时间分别为20.91h和26.62h,而对数生长期的HK株和衰亡期的ST株的培养物过滤液24h对卤虫LC₅₀则分别为7.1×10⁵cell·mL^-1和1.457×10⁷cell·mL^-1,且在该浓度下的半致死时间分别为26.56h和28.02h。实验表明,HK株藻液和滤液的毒性均大于ST株,且藻液的毒性均大于滤液。     

摄食信息对球形棕囊藻和布氏双尾藻竞争结果的影响

浮游动物的摄食信息能增大棕囊藻囊体体积,囊体形成被认为是棕囊藻的诱导性防御机制。利用桡足类火腿伪镖水蚤和异养甲藻海洋尖尾藻释放的摄食信息,研究了诱导性防御对球形棕囊藻和布氏双尾藻的竞争的影响。结果表明,球形棕囊藻接收了火腿伪镖水蚤和海洋尖尾藻释放的摄食信息之后形成更大的囊体。防御启动后的球形棕囊藻比未接收摄食信息的球形棕囊藻更快地形成囊体,且囊体维持的时间更长。对照组和火腿伪镖水蚤摄食信息诱导的球形棕囊藻的生物体积比布氏双尾藻更高,且球形棕囊藻在竞争中占优势;而海洋尖尾藻摄食信息诱导的球形棕囊藻生物体积低于布氏双尾藻,且球形棕囊藻相对布氏双尾藻的竞争力下降。微型浮游动物海洋尖尾藻摄食信息导致球形棕囊藻相对硅藻布氏双尾藻的竞争力的下降,有利于解释硅藻先于棕囊藻发生藻华。     

Effects of small-scale turbulence on Phaeocystis globosa (Prymnesiophyceae) growth and life cycle

The response of Phaeocystis globosa to small-scale turbulence was studied in 5 l microcosms. Turbulence was generated by oscillating grids. The effect of small-scale turbulence was examined under 3 turbulence levels representative of the P. globosa natural environment, and in non-turbulent control cultures. Single cell numbers, nitrogen concentrations and colony formation (number and diameter) were followed over 13 days in each experimental culture. Small-scale turbulence decreased single cell growth and also influenced colony formation. More colonies were formed when turbulence increased to a given threshold, but above this turbulence level, fewer and smaller colonies were observed in P. globosa cultures. The ecological significance of these results, particularly, the potential influence of small-scale turbulence on competition mechanisms between P. globosa and diatoms are finally discussed and suggested as a key factor to understand phytoplankton successions in the Eastern English Channel.     

Red tide blooms of Cochlodinium polykrikoides in a coastal cove

Successive blooms of the dinoflagellate Cochlodinium polykrikoides occurred in Pettaquamscutt Cove, RI, persisting from September through December 1980 and again from April through October 1981. Cell densities varied from <100 cells L⁻¹ at the onset of the bloom and reached a maximum density exceeding 3.4 × 10⁶ cells L⁻¹ during the summer of 1981. The bloom was mainly restricted to the mid to inner region of this shallow cove with greatest concentrations localized in surface waters of the southwestern region during summer/fall periods of both years. Highly motile cells consisting of single, double and multiple cell zooids were found as chains of 4 and 8 cells restricted to the late August/September periods. The highest cell densities occurred during periods when annual temperatures were between 19 and 28 °C and salinities between 25 and 30. A major nutrient source for the cove was Crying Brook, located at the innermost region at the head of the cove. Inorganic nitrogen (NH₃ and NO₂ + NO₃) from the brook was continually detectable throughout the study with maximum values of 57.5 and 82.5 μmol L⁻¹, respectively. Phosphate (PO₄-P) was always present in the source waters and rarely <0.5 μmol L⁻¹; silicate always exceeded 30 μmol L⁻¹ with maximum concentrations reaching 226 μmol L⁻¹. Chlorophyll a and ATP concentrations during the blooms varied directly with cell densities. Maximum Chl a levels were 218 mg m⁻³ and ATP-carbon was >20 g C m⁻³. Primary production by the dinoflagellate-dominated community during the bloom varied between 4.3 and 0.07 g C m⁻³ d⁻¹. Percent carbon turnover calculated from primary production values and ATP-carbon varied from 6 to 129% d⁻¹. The dinoflagellates dominated the entire summer period; other flagellates and diatoms were present in lesser amounts. A combination of low washout rate due to the cove dynamics, active growth, and life cycles involving cysts allowed C. polykrikoides to maintain recurrent bloom populations in this area.     

Differentiation betweenPhaeocystis pouchetii (Har.) Lagerheim andPhaeocystis globosa Scherffel

An Arctic clone ofPhaeocystis pouchetii LAGERHEIM was compared toPhaeocystis globosa SCHERFFEL isolated from the southern North Sea with regard to temperature tolerance and colony shapes. Already youngP.pouchetii colonies (<100 m) show the typical distribution of the cells in groups, separated from each other by wide zones of cell-free mucilage; the maximum colony size is ca 2 mm in diameter.P.pouchetii colonies form clouds with bubble-like vesicles, spherical colony-shapes are seldom found.P.globosa colonies are spherical up to a size of 2 mm; the cells are distributed homogeneously over the periphery of the colonies. A pouchetii-like distribution of cells never occurs either in the spherical young colonies or in the pear-shaped old colonies (size up to 8 mm). A development from the colony shape of the globosa-type to the pouchetii-type or vice versa was never found. Therefore the colony shape has to be considered a constant distinctive character. Single cells ofP.pouchetii andP.globosa cannot be separated from each other by using the light microscope; this also holds for the flagellates and the non-motile cells.P.pouchetii grows well between 0°C and 14°C,P.globosa between 4°C and 22°C, respectively. Because of the distinctive differences in the morphology of the colonies and the differences in temperature tolerances we propose thatPhaeocystis globosa should no longer be considered conspecific withPhaeocystis pouchetii.     

Inhibition of copepod feeding by exudates and transparent exopolymer particles (TEP) derived from a Phaeocystis globosa dominated phytoplankton community

We investigated if (1) dissolved compounds excreted by Phaeocystis globosa and (2) transparent exopolymer particles (TEP) formed from carbohydrates excreted into the water affect the feeding of nauplii and females of the calanoid copepod Temora longicornis during a P. globosa bloom. Copepod grazing on the diatom Thalassiosira weissflogii in the presence of these possible grazing deterrents was measured during three successive weeks of a mesocosm study, simulating the development of a P. globosa bloom. Our results demonstrate no indication for the presence of feeding deterrents in the dissolved phase, but a strong inhibitory effect of transparent exopolymer particles (TEP) on the consumption of algae by both nauplii and adult copepods. The inhibitory effect of TEP was connected to the accumulation of DOM during the progress of the bloom. We suggest that a reduction in the grazing pressure of zooplankton may increase the survival of the liberated single cells during disruption of colonies and allow seeding populations to persist. Furthermore, P. globosa reduces the trophic efficiency of the food web not only by withdrawal of its colonies from grazing but also by a relaxation of the grazing pressure on co-occurring phytoplankton and by alteration of the food web structure via TEP production.     

泰国狐尾藻(小二仙草科)，中国水生植物一新记录种

报道了近期在我国海南岛发现的小二仙草科狐尾藻属新记录种：泰国狐尾藻[Myriophyllum siamense (Craib) Tardieu],并对该种进行了描述。该植物半水生,无羽状叶,雄花高度退化,在形态特征上同马达加斯加、澳洲和大洋洲的一些种类存在相关性,为狐尾藻属跨洲际散布提供了佐证。其发现也说明了海南岛至今还有不少植物采集的薄弱地区和盲区。     

Intra-population clonal variability in allelochemical potency of the toxigenic dinoflagellate Alexandrium tamarense

Clonal variability in exponential growth rate and production of secondary metabolites was determined from clonal isolates of Alexandrium tamarense originating from a single geographical population from the east coast of Scotland. To assess variability in the selected phenotypic characteristics over a wide spectrum, 10 clones were chosen for experimentation from 67 clonal isolates pre-screened for their lytic capacity in a standardized bioassay with the cryptophyte Rhodomonas salina. Specific growth rates (μ) of the 10 clonal isolates ranged from 0.28 to 0.46 d⁻¹ and were significantly different among clones. Cell content (fmol cell⁻¹) and composition (mol%) of paralytic shellfish toxins (PSTs), analyzed by liquid chromatography with fluorescence detection (LC–FD), varied widely among these isolates, with total PST quotas ranging from 20 to 89 fmol cell⁻¹. Except for strain 3, the toxins C1/C2, neosaxitoxin (NEO), saxitoxin (STX), and gonyautoxins-1 and -4 (GTX1/GTX4), were consistently the most relatively abundant, with lesser amounts of GTX2/GTX3 evident among all isolates. Only clone 3 contained >20 mol% of toxin B1, with C1/C2, GTX2/GTX3 and NEO in almost equimolar ratios.Eight of the 10 clones caused cell lysis of both R. salina and the heterotrophic dinoflagellate Oxyrrhis marina, as quantified from the dose–response curves from short-term (24 h) co-incubation bioassays. For two clones, no significant mortality even at high Alexandrium cell concentrations (ca. 10⁴ mL⁻¹) was observed. Allelochemical activity expressed as EC₅₀ values, defined as the Alexandrium cell concentration causing lysis of 50% of target cells, varied by about an order of magnitude and was significantly different among clones. No correlation was observed between growth rate und allelochemical potency (as EC₅₀) indicating that at least under non-limiting growth conditions no obvious growth reducing costs are associated with the production of allelochemically active secondary metabolites.     

Use of dissolved inorganic and organic phosphorus by axenic and nonaxenic clones of Karenia brevis and Karenia mikimotoi

Nearly annual blooms of the marine dinoflagellate Karenia brevis, which initiate offshore on the West Florida Shelf in oligotrophic waters, cause widespread environmental and economic damage. The success of K. brevis as a bloom-former is partially attributed to its ability to use a diverse suite of nutrients from natural and anthropogenic sources, although relatively little is known about the ability of K. brevis and the closely related Karenia mikimotoi to use a variety of organic sources of phosphorus, including phosphomonoesters, phosphodiesters, and phosphonates. Through a series of bioassays, this study characterized the ability of axenic and nonaxenic K. brevis and K. mikimotoi clones isolated from Florida waters to use a variety of organic phosphorus compounds as the sole source of phosphorus for growth, comparing this utilization to that of inorganic sources of phosphate. Differing abilities of axenic and nonaxenic K. brevis and K. mikimotoi cultures to use phosphorus from the compounds evaluated were documented. Specifically, growth of axenic cultures was greatest on inorganic phosphorus and was not supported on the phosphomonoester phytate, or generally on phosphodiesters or phosphonates. The nonaxenic cultures were able to use organic compounds that the axenic cultures were not able to use, often after lags in growth, highlighting a potential role of co-associated bacterial communities to transform nutrients to bioavailable forms. Given the ability of K. brevis and K. mikimotoi to use a diverse suite of inorganic and organic phosphorus, bloom mitigation strategies should consider all nutrient forms.