High biomass turnover rates of endosymbiotic nitrogen‐fixing cyanobacteria in the western Bering Sea

Abstract

Recent studies have described active nitrogen fixation in high‐latitude waters, but the ecological controls on the occurrence or activity of nitrogen‐fixing organisms (diazotrophs) in such systems remain unknown. Turnover rates and top‐down controls are also general knowledge gaps for marine diazotrophs. We detected abundant UCYN‐A (endosymbiotic nitrogen‐fixing cyanobacteria) in the Gulf of Anadyr, western Bering Sea, which correlated with high dissolved iron to dissolved inorganic nitrogen ratios (Fe : DIN) due to riverine input. Growth and grazing mortality of UCYN‐A sublineages were almost balanced with higher biomass‐turnover rates compared to the whole phytoplankton community, indicating selective grazing of UCYN‐A in nitrogen‐depleted waters. Grazing rates on UCYN‐A1 (small cells) were higher than for UCYN‐A2/3/4 (large cells), consistent with the general size dependence of phytoplankton growth and grazing mortality. We found that Fe : DIN is a major determinant of UCYN‐A abundances in high‐latitude waters, where UCYN‐A could make substantial contributions to plankton food‐web cycling.

Microzooplankton Grazing Exerts a Strong Top‐Down Control on Unicellular Cyanobacterial Diazotrophs

“…Primers and probes designed in previous studies were used: UCYN‐A1 (Church et al., 2005), UCYN‐A2 (Thompson et al., 2014), UCYN‐B (Moisander et al., 2010), and UCYN‐C (Foster et al., 2007). It should be noted that the qPCR assay to estimate the abundances of UCYN‐A2 also amplifies the nifH genes of UCYN‐A3 and UCYN‐A4 due to cross‐reactivity between assays, so we defined the signal detected with the UCYN‐A2 qPCR assay as the abundance of UCYN‐A2/3/4 (Cheung et al., 2022; Farnelid et al., 2016). The standard curves were generated by 5‐fold serial dilution from linearized recombinant plasmids and the qPCR efficiency varied between 97% and 103%.…”

“…More recently, Cheung et al. (2022) reported that UCYN‐A sublineages displayed high biomass turnover rates compared to the total phytoplankton community in the western Bering Sea. Nevertheless, the grazing rates of all major UCD groups (UCYN‐A, UCYN‐B, and UCYN‐C) have not been compared in the same water column.…”

“…Turk-Kubo et al (2018) first measured in situ microzooplankton grazing rates on UCYN-A1 and UCYN-C at 15 and 125 m in the North Pacific Subtropical Gyre, showing that microzooplankton preferentially grazed on UCYN-A1 and grazing rates on these two UCDs was higher in the deeper layer of water column (125 m, near the deep chlorophyll maximum). More recently, Cheung et al (2022) reported that UCYN-A sublineages displayed high biomass turnover rates compared to the total phytoplankton community in the western Bering Sea. Nevertheless, the grazing rates of all major UCD groups (UCYN-A, UCYN-B, and UCYN-C) have not been compared in the same water column.…”

Comparison of bacterial carbon production estimates from dilution and 3H‐leucine methods across a strong gradient in ocean productivity

“…The dilution technique does, however, offer a distinct advantage for integrating bacteria into a broader food web context by providing estimates of their growth and grazing mortality rates in the same incubations that measure the growth and grazing of phototrophic microbes ( Prochlorococcus , Synechococcus , picoeukaryotes) as well as other phytoplankton (e.g., Landry et al 2022 a ). The expanding usage of sequence analyses with dilution experiments to resolve species‐ and clade‐specific differences in growth and grazing dynamics (Yokokawa and Hagata 2005; Cheung et al 2022) can also be expected to amplify this advantage in the future. It is therefore useful to know that BCP Dil results can be compatible with BCP Leu , given appropriate attention to bacteria cell carbon content.…”

“…Dilution experiments also differ from 3 H‐leucine assays in their potential to compare the vital rates of different microbial populations to one another in the same incubations. Therefore, if dilution results are compatible to 3 H‐leucine for bacteria in aggregate, the population‐distinguishing advantage of dilutions could be useful as a tool for integrating bacterial production into food web studies relative to phototrophic microbes as well as for using molecular sequence techniques to quantify potential differences and growth‐mortality tradeoffs among co‐occurring microbes (Yokokawa and Hagata 2005; Cheung et al 2022).…”