Data from: More, smaller bacteria in response to ocean's warming?

Heterotrophic bacteria play a major role in organic matter cycling in the ocean. Although the high abundances and relatively fast growth rates of coastal surface bacterioplankton make them suitable sentinels of global change, past analyses have largely overlooked this functional group. Here, time-series analysis of a decade of monthly observations in temperate Atlantic coastal waters revealed strong seasonal patterns in the abundance, size and biomass of the ubiquitous flow cytometric groups of low (LNA) and high nucleic acid content (HNA) bacteria. Over this relatively short period we also found that bacterioplankton cells were significantly smaller, a trend that is consistent with the hypothesized temperature-driven decrease in body size. Although decadal cell shrinking was observed for both groups, it was only LNA cells that were strongly coherent with ecological theories linking temperature, abundance and individual size on both the seasonal and interannual scale. We explain this finding because, relative to their HNA counterparts, marine LNA bacteria are less diverse, dominated by members of the SAR11 clade. Temperature manipulation experiments in 2012 confirmed a direct effect of warming on bacterial size. Concurrent with rising temperatures in spring, significant decadal trends of increasing standing stocks (3% year-1) accompanied by decreasing mean cell size (-1% year-1) suggest a major shift in community structure, with a larger contribution of LNA bacteria to total biomass. The increasing prevalence of these typically oligotrophic taxa may severely impact marine food webs and carbon fluxes by an overall decrease in the efficiency of the biological pump.

异养细菌（Heterotrophic bacteria）在海洋有机质循环中发挥着核心作用。尽管沿岸表层浮游细菌（coastal surface bacterioplankton）丰度高、生长速率相对较快，是全球变化研究的理想哨兵类群，但过往研究在很大程度上忽略了这一功能类群。本研究对大西洋温带近岸海域十年的月度观测数据开展时间序列分析，结果显示广泛存在的流式细胞术分群的低核酸含量（LNA）与高核酸含量（HNA）细菌，其丰度、细胞体积及生物量均呈现显著的季节变化模式。在这一相对较短的观测周期内，我们还发现浮游细菌细胞平均体积显著减小，这一趋势与温度驱动的体型减小假说高度吻合。尽管两类细菌均出现了十年尺度的细胞缩小现象，但仅低核酸含量细菌在季节和年际尺度上，均与联结温度、丰度及个体体型的生态学理论存在强相关性。我们对此现象作出合理解释：相较于高核酸含量细菌，海洋低核酸含量细菌的多样性更低，且类群组成以SAR11进化枝（SAR11 clade）为主导。2012年的温度操控实验证实了升温对细菌体型的直接影响。伴随春季水温持续升高，十年间观测到的现存生物量显著增长（年均增幅3%）与平均细胞体积减小（年均降幅1%）的同步变化，表明群落结构发生了重大转变，低核酸含量细菌对总生物量的贡献大幅提升。这类典型贫营养类群的占比不断升高，可能通过降低生物泵（biological pump）的整体运作效率，对海洋食物网和碳通量造成严重冲击。