Instituto Universitario de Investigación del Agua, Universidad de Granada, C/ Ramón y Cajal, nº4, 18071, Granada, Spain.
Instituto Universitario de Investigación del Agua, Universidad de Granada, C/ Ramón y Cajal, nº4, 18071, Granada, Spain; Departamento de Ecología, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada, Spain.
Sci Total Environ. 2024 Jun 1;927:172220. doi: 10.1016/j.scitotenv.2024.172220. Epub 2024 Apr 6.
The microbial carbon (C) flux in the ocean is a key functional process governed by the excretion of organic carbon by phytoplankton (EOC) and heterotrophic bacterial carbon demand (BCD). Ultraviolet radiation (UVR) levels in upper mixed layers and increasing atmospheric dust deposition from arid regions may alter the degree of coupling in the phytoplankton-bacteria relationship (measured as BCD:EOC ratio) with consequences for the C-flux through these compartments in marine oligotrophic ecosystem. Firstly, we performed a field study across the south-western (SW) Mediterranean Sea to assess the degree of coupling (BCD:EOC) and how it may be related to metabolic balance (total primary production: community respiration; PP:CR). Secondly, we conducted a microcosm experiment in two contrasting areas (heterotrophic nearshore and autotrophic open sea) to test the impact of UVR and dust interaction on microbial C flux. In the field study, we found that BCD was not satisfied by EOC (i.e., BCD:EOC >1; uncoupled phytoplankton-bacteria relationship). BCD:EOC ratio was negatively related to PP:CR ratio across the SW Mediterranean Sea. A spatial pattern emerged, i.e. in autotrophic open sea stations uncoupling was less severe (BCD:EOC ranged 1-2), whereas heterotrophic nearshore stations uncoupling was more severe (BCD:EOC > 2). In the experimental study, in the seawater both enriched with dust and under UVR, BCD:EOC ratio decreased by stimulating autotrophic processes (particulate primary production (PP) and EOC) in the heterotrophic nearshore area, whereas BCD:EOC increased by stimulating heterotrophic processes [heterotrophic bacterial production (HBP), bacterial growth efficiency (BGE), bacterial respiration (BR)] in the autotrophic open sea. Our results show that this spatial pattern could be reversed under future UVR × Dust scenario. Overall, the impact of greater dust deposition and higher UVR levels will alter the phytoplankton-bacteria C-flux with consequences for the productivity of both communities, their standing stocks, and ultimately, the ecosystem's metabolic balance at the sea surface.
海洋中的微生物碳（C）通量是一个关键的功能过程，由浮游植物的有机碳排泄（EOC）和异养细菌的碳需求（BCD）控制。上层混合层中的紫外线辐射（UVR）水平和来自干旱地区的大气尘埃沉积的增加可能会改变浮游植物-细菌关系的耦合程度（以 BCD:EOC 比来衡量），从而对海洋贫营养生态系统中这些隔室的 C 通量产生影响。首先，我们在西南（SW）地中海进行了实地研究，以评估耦合程度（BCD:EOC）以及它如何与代谢平衡（总初级生产力：群落呼吸；PP:CR）相关。其次，我们在两个对比区域（异养近岸和自养开阔海域）进行了微宇宙实验，以测试 UVR 和尘埃相互作用对微生物 C 通量的影响。在实地研究中，我们发现 BCD 未被 EOC 满足（即，BCD:EOC>1；浮游植物-细菌关系解偶联）。BCD:EOC 比与整个 SW 地中海的 PP:CR 比呈负相关。出现了一个空间模式，即在自养开阔海域站，解偶联程度较轻（BCD:EOC 范围为 1-2），而在异养近岸站，解偶联程度较重（BCD:EOC>2）。在实验研究中，在富含有机物的海水中并在 UVR 下，在异养近岸地区，通过刺激自养过程（颗粒初级生产力（PP）和 EOC），BCD:EOC 比降低，而在自养开阔海域，通过刺激异养过程[异养细菌生产力（HBP）、细菌生长效率（BGE）、细菌呼吸（BR）]，BCD:EOC 比增加。我们的结果表明，在未来的 UVR×尘埃情景下，这种空间模式可能会逆转。总的来说，更高的尘埃沉积和更高的 UVR 水平的影响将改变浮游植物-细菌的 C 通量，从而对两个群落的生产力、它们的现存量以及最终对海洋表面生态系统的代谢平衡产生影响。
