Anderson Ruth, Jürgens Klaus, Hansen Per J
Marine Biological Section, Department of Biology, University of CopenhagenHelsingør, Denmark.
Leibniz Institute for Baltic Sea ResearchRostock, Germany.
Front Microbiol. 2017 Jul 26;8:1398. doi: 10.3389/fmicb.2017.01398. eCollection 2017.
Bacterivory among small (≤20 μm) phytoflagellates (SP) is increasingly recognized as a globally relevant phenomenon, impacting a wide range of aspects from primary production levels to marine fisheries. However, to correctly parametrize mixotrophic SP in biogeochemical and food web models, a better understanding of the magnitude and regulation of SP feeding is urgently needed. Current methods to determine SP bacterivory in the field may introduce biases by treating these organisms as equivalent to heterotrophic nanoflagellates (HNF). In the present case study we experimentally tested two generally employed assumptions of such studies: (A) bacterivory rates of the whole SP community and of distinct SP groups remain constant over 'short' time scales (hours to a day) and (B) SP community ingestion rates approximate the average ingestion rate of all feeding individuals. Food vacuole markers (acidotropic probes), were applied along the diel cycle at three stations in December 2015, and May and June 2016. In December and June, surrogate prey (fluorescently labeled bacteria) were used in parallel at one sampling station. Sampling at different times of day produced an up to fourfold difference in estimates of SP daily bacterivorous impact. In contrast, daily bacterivory estimates for HNF remained constant in almost all cases. The perceived principal SP bacterivorous groups also shifted strongly. As an example, picoeukaryotes dominated total SP bacterivory in daylight hours but completely ceased to feed at night. Finally, a large fraction of the SP community was not feeding at all time points tested. This lead to significant errors in estimated ingestion rates determined using the whole SP community, being up to 16 times lower than those determined solely for actively feeding mixotrophic SP. Overall, this case study indicates that applying the two commonly used premises outlined above can introduce significant biases and considerably alter our perception of mixotrophy in a given system.
小型（≤20 微米）植物性鞭毛虫（SP）中的噬菌作用日益被视为一种具有全球意义的现象，它影响着从初级生产水平到海洋渔业等广泛的方面。然而，为了在生物地球化学和食物网模型中正确地对混合营养型 SP 进行参数化，迫切需要更好地了解 SP 摄食的规模和调控机制。目前在野外测定 SP 噬菌作用的方法可能会因将这些生物等同于异养纳米鞭毛虫（HNF）而引入偏差。在本案例研究中，我们通过实验检验了此类研究中两个普遍采用的假设：（A）整个 SP 群落和不同 SP 群体的噬菌率在“短”时间尺度（数小时至一天）内保持恒定，以及（B）SP 群落的摄食率近似于所有摄食个体的平均摄食率。在 2015 年 12 月以及 2016 年 5 月和 6 月，沿着日周期在三个站点应用了食物液泡标记物（嗜酸性探针）。在 12 月和 6 月，在一个采样站点同时使用了替代猎物（荧光标记细菌）。在一天中的不同时间进行采样，得出的 SP 每日噬菌影响估计值相差高达四倍。相比之下，几乎在所有情况下，HNF 的每日噬菌估计值都保持恒定。被认为主要的 SP 噬菌群体也发生了强烈变化。例如，微微型真核生物在白天主导了总 SP 噬菌作用，但在夜间完全停止摄食。最后，很大一部分 SP 群落在所有测试时间点都不摄食。这导致使用整个 SP 群落确定的估计摄食率出现显著误差，比仅针对活跃摄食的混合营养型 SP 确定的摄食率低多达 16 倍。总体而言，本案例研究表明，应用上述两个常用前提可能会引入显著偏差，并极大地改变我们对给定系统中混合营养的认知。
