Environment and Climate Change Canada, Canada Centre for Inland Waters, 867 Lakeshore Road, Burlington, ON L7S 1A1, Canada.
Joseph J. Zilber School of Public Health, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA.
Toxins (Basel). 2021 Jun 27;13(7):445. doi: 10.3390/toxins13070445.
The understanding of deep chlorophyll layers (DCLs) in the Great Lakes-largely reported as a mix of picoplankton and mixotrophic nanoflagellates-is predominantly based on studies of deep (>30 m), offshore locations. Here, we document and characterize nearshore DCLs from two meso-oligotrophic embayments, Twelve Mile Bay (TMB) and South Bay (SB), along eastern Georgian Bay, Lake Huron (Ontario, Canada) in 2014, 2015, and 2018. Both embayments showed the annual formation of DCLs, present as dense, thin, metalimnetic plates dominated by the large, potentially toxic, and bloom-forming cyanobacteria cf. . The contribution of cf. to the deep-living total biomass (TB) increased as thermal stratification progressed over the ice-free season, reaching 40% in TMB (0.6 mg/L at 9.5 m) and 65% in South Bay (3.5 mg/L at 7.5 m) in 2015. The euphotic zone in each embayment extended down past the mixed layer, into the nutrient-enriched hypoxic hypolimnia, consistent with other studies of similar systems with DCLs. The co-occurrence of the metal-oxidizing bacteria spp. and bactivorous flagellates within the metalimnetic DCLs suggests that the microbial loop plays an important role in recycling nutrients within these layers, particularly phosphate (PO) and iron (Fe). Samples taken through the water column in both embayments showed measurable concentrations of the cyanobacterial toxins microcystins (max. 0.4 µg/L) and the other bioactive metabolites anabaenopeptins (max. ~7 µg/L) and cyanopeptolins (max. 1 ng/L), along with the corresponding genes (max. in 2018). These oligopeptides are known to act as metabolic inhibitors (e.g., in chemical defence against grazers, parasites) and allow a competitive advantage. In TMB, the 2018 peaks in these oligopeptides and genes coincided with the cf. DCLs, suggesting this species as the main source. Our data indicate that intersecting physicochemical gradients of light and nutrient-enriched hypoxic hypolimnia are key factors in supporting DCLs in TMB and SB. Microbial activity and allelopathy may also influence DCL community structure and function, and require further investigation, particularly related to the dominance of potentially toxigenic species such as cf. .
大湖中的深叶绿素层（DCL）主要被报道为微微型浮游生物和混合营养型纤毛虫的混合体，其理解主要基于对深（> 30 m）、近海地区的研究。在这里，我们记录并描述了 2014 年、2015 年和 2018 年安大略省休伦湖东乔治亚湾两个中营养浅湾——十二英里湾（TMB）和南湾（SB）的近岸 DCL。这两个海湾都出现了 DCL 的年度形成，表现为密集、薄的、金属层状板，主要由大型、潜在有毒和形成水华的蓝细菌 cf. 组成。随着无冰季节热分层的发展，cf. 对深层总生物量（TB）的贡献增加，在 2015 年 TMB 中达到 40%（9.5 m 处为 0.6 mg/L），在南湾达到 65%（7.5 m 处为 3.5 mg/L）。每个海湾的透光层延伸到混合层以下，进入富营养缺氧的底层，与其他具有 DCL 的类似系统的研究一致。金属氧化细菌 spp. 和噬菌性鞭毛虫在金属层 DCL 中的共同存在表明，微生物环在这些层中循环利用营养物质方面发挥着重要作用，特别是磷（PO）和铁（Fe）。在两个海湾的水柱中取样显示，可测量的蓝细菌毒素微囊藻毒素（最大 0.4 µg/L）和其他生物活性代谢物拟鱼腥藻肽（最大约 7 µg/L）和蓝藻肽（最大 1 ng/L）以及相应的基因（最大 2018 年）的浓度。这些寡肽已知具有代谢抑制剂的作用（例如，在抵御食草动物、寄生虫的化学防御中），并具有竞争优势。在 TMB 中，这些寡肽和基因的 2018 年峰值与 cf. DCL 同时出现，表明该物种是主要来源。我们的数据表明，光照和富营养缺氧底层的理化梯度的交叉是支持 TMB 和 SB 中 DCL 的关键因素。微生物活性和化感作用也可能影响 DCL 群落结构和功能，需要进一步研究，特别是与潜在产毒物种如 cf. 的优势有关。
