A key marine diazotroph in a changing ocean: the interacting effects of temperature, CO2 and light on the growth of Trichodesmium erythraeum IMS101

Trichodesmium is a globally important marine diazotroph that accounts for approximately 60-80% of marine biological N2 fixation and as such plays a key role in marine N and C cycles. We undertook a comprehensive assessment of how the growth rate of Trichodesmium erythraeum IMS101 was directly affected by the combined interactions of temperature, pCO2 and light intensity. Our key findings were: low pCO2 affected the lower temperature tolerance limit (Tmin) but had no effect on the optimum temperature (Topt) at which growth was maximal or the maximum temperature tolerance limit (Tmax); low pCO2 had a greater effect on the thermal niche width than low-light; the effect of pCO2 on growth rate was more pronounced at suboptimal temperatures than at supraoptimal temperatures; temperature and light had a stronger effect on the photosynthetic efficiency (Fv/Fm) than did CO2; and at Topt, the maximum growth rate increased with increasing CO2, but the initial slope of the growth-irradiance curve was not affected by CO2. In the context of environmental change, our results suggest that the (i) nutrient replete growth rate of Trichodesmium IMS101 would have been severely limited by low pCO2 at the last glacial maximum (LGM), (ii) future increases in pCO2 will increase growth rates in areas where temperature ranges between Tmin to Topt, but will have negligible effect at temperatures between Topt and Tmax, (iii) areal increase of warm surface waters (> 18°C) has allowed the geographic range to increase significantly from the LGM to present and that the range will continue to expand to higher latitudes with continued warming, but (iv) continued global warming may exclude Trichodesmium spp. from some tropical regions by 2100 where temperature exceeds Topt.

束毛藻（Trichodesmium）是全球范围内具有重要生态意义的海洋固氮生物，约贡献了海洋生物固氮总量的60%~80%，因此在海洋氮循环与碳循环中发挥关键作用。本研究针对红海束毛藻IMS101（Trichodesmium erythraeum IMS101）的生长速率如何受温度、二氧化碳分压（pCO₂）与光照强度的联合交互作用直接调控开展了全面评估。本研究的核心结论如下：低二氧化碳分压会影响低温耐受阈值（最低生长温度，T_min），但对生长速率达到峰值的最适温度（T_opt）以及最高生长温度耐受阈值（T_max）无显著影响；低二氧化碳分压对其热生态位宽度的影响强于弱光照；相较于高于最适温度的条件，二氧化碳分压对生长速率的调控效应在低于最适温度的环境中更为显著；温度与光照对光合效率（Fv/Fm）的影响强于二氧化碳；在最适生长温度条件下，最大生长速率随二氧化碳分压升高而提升，但生长-光照曲线的初始斜率不受二氧化碳分压影响。结合全球环境变化的背景，本研究结果表明：（i）末次冰盛期（LGM）的低二氧化碳分压会严重限制红海束毛藻IMS101在营养充足条件下的生长速率；（ii）未来二氧化碳分压升高将提升温度处于最低生长温度至最适温度区间海域的束毛藻生长速率，但在温度介于最适温度与最高耐受温度的海域中则几乎无影响；（iii）全球表层暖水（水温>18℃）的面积扩张已使束毛藻的地理分布范围自末次冰盛期至今显著扩大，且随着全球持续变暖，其分布范围将进一步向高纬度地区拓展；（iv）到2100年，持续的全球变暖可能会使束毛藻属（Trichodesmium）物种被排除出部分温度超过最适生长温度的热带海域。