Sand-Jensen Kaj
Freshwater Biological Laboratory, Biological Institute, University of Copenhagen, Universitetsparken 4, 2100 Copenhagen, Denmark
Ann Bot. 2014 Jul;114(1):17-33. doi: 10.1093/aob/mcu085.
The cyanobacterial genus Nostoc includes several species forming centimetre-large gelatinous colonies in nutrient-poor freshwaters and harsh semi-terrestrial environments with extended drought or freezing. These Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates.
The objective of this review is to evaluate the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources.
Simple models predict the decline in uptake of dissolved inorganic carbon (DIC) and a decline in the growth rate of spherical freshwater colonies of N. pruniforme and N. zetterstedtii and sheet-like colonies of N. commune in response to a thicker diffusion boundary layer, lower external DIC concentration and higher organic carbon mass per surface area (CMA) of the colony. Measured growth rates of N. commune and N. pruniforme at high DIC availability comply with general empirical predictions of maximum growth rate (i.e. doubling time 10-14 d) as functions of CMA for marine macroalgae and as functions of tissue thickness for aquatic and terrestrial plants, while extremely low growth rates of N. zetterstedtii (i.e. doubling time 2-3 years) are 10-fold lower than model predictions, either because of very low ambient DIC and/or an extremely costly colony matrix. DIC uptake is limited by diffusion at low concentrations for all species, although they exhibit efficient HCO3(-) uptake, accumulation of respiratory DIC within the colonies and very low CO2 compensation points. Long light paths and light attenuation by structural substances in large Nostoc colonies cause lower quantum efficiency and assimilation number and higher light compensation points than in unicells and other aquatic macrophytes. Extremely low growth and mortality rates of N. zetterstedtii reflect stress-selected adaptation to nutrient- and DIC-poor temperate lakes, while N. pruniforme exhibits a mixed ruderal- and stress-selected strategy with slow growth and year-long survival prevailing in sub-Arctic lakes and faster growth and shorter longevity in temperate lakes. Nostoc commune and its close relative N. flagelliforme have a mixed stress-disturbance strategy not found among higher plants, with stress selection to limiting water and nutrients and disturbance selection in quiescent dry or frozen stages. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range (0-35 °C; maximum at 25 °C). Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.
蓝藻念珠藻属包含多个物种，它们在营养匮乏的淡水以及干旱或冰冻期延长的恶劣半陆地环境中形成厘米级大小的凝胶状菌落。这些念珠藻物种具有丝状结构，其中包含正常的光合细胞和固氮异形胞，它们嵌入在由多糖和许多其他有机物质构成的广泛凝胶状基质中，从而提供生物和环境保护。较大的菌落尺寸对外部资源的利用造成了限制，而凝胶状基质意味着额外的成本并降低了生长速率。
本综述的目的是评估凝胶状念珠藻菌落生长和死亡率低、抵御环境危害、持久性和长寿的背后机制，以及它们在资源高度有限的情况下节约资源的能力。
简单模型预测，随着扩散边界层变厚、外部溶解无机碳（DIC）浓度降低以及菌落每表面积有机碳质量（CMA）增加，球形淡水菌落的普鲁士念珠藻和泽特斯特念珠藻以及片状菌落的普通念珠藻对DIC的摄取量会下降，生长速率也会下降。在高DIC可用性条件下测得的普通念珠藻和普鲁士念珠藻的生长速率符合一般经验预测，即最大生长速率（即倍增时间为10 - 14天）是海洋大型藻类CMA的函数，也是水生和陆生植物组织厚度的函数，而泽特斯特念珠藻极低的生长速率（即倍增时间为2 - 3年）比模型预测低10倍，这可能是由于极低的环境DIC和/或极其昂贵的菌落基质。对于所有物种而言，低浓度时DIC摄取受扩散限制，尽管它们表现出高效的HCO₃⁻摄取、菌落在呼吸过程中对DIC的积累以及非常低的CO₂补偿点。大型念珠藻菌落中较长的光程和结构物质对光的衰减导致其量子效率和同化数低于单细胞和其他水生大型植物，光补偿点则更高。泽特斯特念珠藻极低的生长和死亡率反映了其对营养和DIC匮乏的温带湖泊的应激选择适应，而普鲁士念珠藻表现出一种杂草型和应激选择混合策略，在亚北极湖泊中生长缓慢且能存活一整年，在温带湖泊中生长较快但寿命较短。普通念珠藻及其近亲发状念珠藻具有一种高等植物中未发现的应激 - 干扰混合策略，在限制水和养分方面进行应激选择，在静止的干燥或冰冻阶段进行干扰选择。尽管不同物种之间存在深刻的生态差异，但温带样本的活跃生长大多局限于相同的温度范围（0 - 35°C；25°C时达到最大值）。未来的研究应旨在揭示念珠藻物种在沉积物和土壤表面环境资源供应条件下极端持久性和低代谢背后的过程。
