Unexpected presence of the nitrogen‐fixing symbiotic cyanobacterium UCYN‐A in Monterey Bay, California

Abstract

In the last decade, the known biogeography of nitrogen fixation in the ocean has been expanded to colder and nitrogen‐rich coastal environments. The symbiotic nitrogen‐fixing cyanobacteria group A (UCYN‐A) has been revealed as one of the most abundant and widespread nitrogen‐fixers, and includes several sublineages that live associated with genetically distinct but closely related prymnesiophyte hosts. The UCYN‐A1 sublineage is associated with an open ocean picoplanktonic prymnesiophyte, whereas UCYN‐A2 is associated with the coastal nanoplanktonic coccolithophore Braarudosphaera bigelowii , suggesting that different sublineages may be adapted to different environments. Here, we study the diversity of nifH genes present at the Santa Cruz Municipal Wharf in the Monterey Bay (MB), California, and report for the first time the presence of multiple UCYN‐A sublineages, unexpectedly dominated by the UCYN‐A2 sublineage. Sequence and quantitative PCR data over an 8‐year time‐series (2011–2018) showed a shift toward increasing UCYN‐A2 abundances after 2013, and a marked seasonality for this sublineage which was present during summer‐fall months, coinciding with the upwelling‐relaxation period in the MB. Increased abundances corresponded to positive temperature anomalies in MB, and we discuss the possibility of a benthic life stage of the associated coccolithophore host to explain the seasonal pattern. The dominance of UCYN‐A2 in coastal waters of the MB underscores the need to further explore the habitat preference of the different sublineages in order to provide additional support for the hypothesis that UCYN‐A1 and UCYN‐A2 sublineages are different ecotypes.

Nitrogen Fixation and Diazotroph Community in the Subarctic Sea of Japan and Sea of Okhotsk

“…However, it is still possible that diazotroph groups, especially Clusters 1A and III, play a certain role in the regional nitrogen cycle where subtropical diazotrophs are unable to flourish. These nifH clusters have been widely reported in both pelagic and sediment samples from temperate, cold, and/or N-rich regions, including the English Channel (Rees et al, 2009), Monterey Bay (Cabello et al, 2020), Danish Strait (Bentzon-Tilia et al, 2015), northwestern upwelling coast of Iberia (Moreira-Coello et al, 2019), and Chukchi Sea (Shiozaki et al, 2018). This is consistent with the detection of these groups in the cold waters (−0.1 to 10.0°C) of our study region.…”

Nitrogen Fixation in a Changing Arctic Ocean: An Overlooked Source of Nitrogen?

“…How will increasing temperatures in the AO affect growth rates and BNF potential of diazotrophs? Ultimately, warming and altered circulation patterns (e.g., Bluhm et al, 2015;Woodgate, 2018) may lead to range contraction and/or expansion for diazotrophs (Sherwood et al, 2014;Cabello et al, 2020), depending on respective autecology. How will warming and circulation changes affect the biogeography of diazotrophs, e.g., to what extent may northward spreading of warmer-water diazotrophs and/or habitat contraction for potentially coldadapted diazotrophs alter community composition and BNF activity?…”

“…D: Ocean acidification alters the carbonate system (Terhaar et al, 2020), stimulating some diazotrophs while suppressing others (Eichner et al, 2014;Luo et al, 2019). What group-specific responses of diazotrophs can be expected in the AO, e.g., will increased undersaturation of calcite selectively affect the diazotroph UCYN-A2 due to the coccolithophore host having a calcifying life-stage (Thompson et al, 2014;Cabello et al, 2020)? Ocean acidification can lower the bioavailability of iron (Shi et al, 2010), a key regulating nutrient for diazotrophs (Sohm et al, 2011).…”

Response of Posidonia oceanica (L.) Delile and its associated N2 fixers to different combinations of temperature and light levels

“…The results obtained for UCYN-C might be due to the different light and temperature requirements its representatives possess, consequently, the species have optimum N 2 fixation rates under distinct conditions. On the other hand, although it has been denoted that UCYN-A exhibits a broad temperature range, more associated with cooler waters and a lower temperature optimum than the remaining cyanobacterial groups (Moisander et al, 2010; Cabello et al, 2020), the results obtained from the experiment do not reflect a clear pattern. Taking into consideration how the different diazotrophic species are adapted to grow and function under differing conditions, further investigation is required to achieve better understanding of their potential response to the interaction between climate change factors and other stressors.…”