47
heterocyst and are either epibionts (e.g. Calothrix on the diatom Chaetoceros), live
within the periplasmic space between the plasmalemma and frustule (e.g. Richelia
within Rhizosolenia) while some associations are believed to be truly endosymbiotic and live within the diatom cell membrane (Richelia with the diatom Hemiaulus)
(Caputo et al. 2019; Foster and Zehr 2019; Villareal 1990). Since they have heterocysts that protect them from photosynthetic O 2 (produced by both the cyanobacterium and the diatom) they fix N 2 during the light. There are dramatic differences in
the genomes of the epibionts compared to the endosymbionts with genome reduction, including in N metabolism pathways, in the endosymbionts (Hilton et al. 2013).
The unicellular cyanobacterium Crocosphaera (also called UCYN-B from nifH
phylogeny studies, Fig. 4.2) was first isolated from the South Atlantic Ocean in the
1980s (Waterbury 2006), but was not generally recognized as being widely distributed for some time although, in retrospect, flow cytometry studies sometimes
reported abundant populations of the size and fluorescence characteristics of
Crocosphaera (Campbell and Vaulot 1993; Neveux et al. 1999). Crocosphaera was
then observed in the North Pacific along with detection of its nifH gene in DNA and
from mRNA, providing the hint that Crocosphaera was a more common member of
the diazotrophic assemblages than previously thought (Moisander et al. 2010; Zehr
et al. 2001). There are a number of isolated strains, that differ in size and genome
composition, temperature optima, exopolysaccharide production and geographical
distribution (Webb et al. 2009; Bench et al. 2016). Crocosphaera is unusual in that
the strains and natural populations are generally greater than 95% identical at the
DNA level, although they have some distinct genome regions (Bench et al. 2011,
2013). Crocosphaera is often found as single cells, but also in aggregates. The
aggregates are comprised of exopolysaccharide-forming strains that are genetically
distinct (Bench et al. 2013, 2016). There is cell-to-cell variability in cellular N 2 fixation (Foster et al. 2013; Masuda et al. 2020). It is interesting to note that Crocosphaera
or a close relative is symbiotic in centric diatoms (Carpenter and Janson 2000).
Other unicellular genera found in the marine environment include Cyanothece,
although the genera Cyanothece and Crocosphaera are similar and taxonomic
nomenclature has recently been revised (Mareš et al. 2019). Cyanothece sp. ATCC
51142 (recently renamed Crocosphaera subtropica, Mareš et al. 2019) was isolated
from an intertidal environment, and is a model marine organism that has been extensively studied in the laboratory (e.g. Toepel et al. 2008), but significance in the
surface ocean plankton is less clear. nifH gene sequences and the isolate Cyanothece
sp. TW3 (Taniuchi et al. 2012) have appeared in samples from various locations.
Although N 2 -fixing unicellular cyanobacterial genera can be diverse in benthic habitats and microbial mats, in the open ocean there are only a few taxa known other
than Crocosphaera/ Cyanothece, and the uncultivated symbiotic unicellular cyanobacterium known as UCYN-A (name derived from PCR primer sets used early on
to detect it (Mazard et al. 2004; Zehr et al. 1998b)) now called Candidatus
Atelocyanobacterium thalassa (Thompson et al. 2012).
The UCYN-A unicellular cyanobacteria (Figs. 4.2e and 4.4) were discovered by
the nifH PCR approach in the 1990s although it was years before its significance
was known. The UCYN-A cyanobacteria (previously called Group A (Zehr et al.
4.1 The Surface Ocean (Pelagic)
heterocyst and are either epibionts (e.g. Calothrix on the diatom Chaetoceros), live
within the periplasmic space between the plasmalemma and frustule (e.g. Richelia
within Rhizosolenia) while some associations are believed to be truly endosymbiotic and live within the diatom cell membrane (Richelia with the diatom Hemiaulus)
(Caputo et al. 2019; Foster and Zehr 2019; Villareal 1990). Since they have heterocysts that protect them from photosynthetic O 2 (produced by both the cyanobacterium and the diatom) they fix N 2 during the light. There are dramatic differences in
the genomes of the epibionts compared to the endosymbionts with genome reduction, including in N metabolism pathways, in the endosymbionts (Hilton et al. 2013).
The unicellular cyanobacterium Crocosphaera (also called UCYN-B from nifH
phylogeny studies, Fig. 4.2) was first isolated from the South Atlantic Ocean in the
1980s (Waterbury 2006), but was not generally recognized as being widely distributed for some time although, in retrospect, flow cytometry studies sometimes
reported abundant populations of the size and fluorescence characteristics of
Crocosphaera (Campbell and Vaulot 1993; Neveux et al. 1999). Crocosphaera was
then observed in the North Pacific along with detection of its nifH gene in DNA and
from mRNA, providing the hint that Crocosphaera was a more common member of
the diazotrophic assemblages than previously thought (Moisander et al. 2010; Zehr
et al. 2001). There are a number of isolated strains, that differ in size and genome
composition, temperature optima, exopolysaccharide production and geographical
distribution (Webb et al. 2009; Bench et al. 2016). Crocosphaera is unusual in that
the strains and natural populations are generally greater than 95% identical at the
DNA level, although they have some distinct genome regions (Bench et al. 2011,
2013). Crocosphaera is often found as single cells, but also in aggregates. The
aggregates are comprised of exopolysaccharide-forming strains that are genetically
distinct (Bench et al. 2013, 2016). There is cell-to-cell variability in cellular N 2 fixation (Foster et al. 2013; Masuda et al. 2020). It is interesting to note that Crocosphaera
or a close relative is symbiotic in centric diatoms (Carpenter and Janson 2000).
Other unicellular genera found in the marine environment include Cyanothece,
although the genera Cyanothece and Crocosphaera are similar and taxonomic
nomenclature has recently been revised (Mareš et al. 2019). Cyanothece sp. ATCC
51142 (recently renamed Crocosphaera subtropica, Mareš et al. 2019) was isolated
from an intertidal environment, and is a model marine organism that has been extensively studied in the laboratory (e.g. Toepel et al. 2008), but significance in the
surface ocean plankton is less clear. nifH gene sequences and the isolate Cyanothece
sp. TW3 (Taniuchi et al. 2012) have appeared in samples from various locations.
Although N 2 -fixing unicellular cyanobacterial genera can be diverse in benthic habitats and microbial mats, in the open ocean there are only a few taxa known other
than Crocosphaera/ Cyanothece, and the uncultivated symbiotic unicellular cyanobacterium known as UCYN-A (name derived from PCR primer sets used early on
to detect it (Mazard et al. 2004; Zehr et al. 1998b)) now called Candidatus
Atelocyanobacterium thalassa (Thompson et al. 2012).
The UCYN-A unicellular cyanobacteria (Figs. 4.2e and 4.4) were discovered by
the nifH PCR approach in the 1990s although it was years before its significance
was known. The UCYN-A cyanobacteria (previously called Group A (Zehr et al.
4.1 The Surface Ocean (Pelagic)