Co-occurrence patterns between bacterial communities and
phytoplankton taxa
In
the natural environment, the existence of microorganisms is not isolated
from the world. Each microorganism needs to be in contact with the
natural environment and other microorganisms during its growth process,
thereby form the entire microbial communities. Aquatic ecosystems are
heterogeneous environment, which contains enormously diverse
prokaryotic and eukaryotic
communities. The interactions between prokaryotic and eukaryotic
communities also can decide the community structure (Prosser et al.,
2007), therefore, we perform the
non-random co-occurrence networks and important inter-taxa relationships
using network analysis approach. The interactions between phytoplankton
and bacteria play a significant role in shaping the microenvironment
around these organisms. This
microenvironment, known as phycosphere, is thought to shape the
diversity of bacteria around phytoplankton, thereby stimulating various
interactions between the two groups and affecting the global
biogeochemical cycle (Gregory et al., 2018). Interrelations between
bacteria and microalgae are multifaceted and complicated, for example,
bacteria naturally can rely on photosynthetic phytoplankton to obtain
the organic carbon needed to maintain their growth (Falkowski, Fenchel,
& Delong, 2008); in turn, phytoplankton can depend on bacteria to
remineralize organic matter into inorganic substitutes, ultimately
supporting the growth of algae (Worden et al., 2015). Therefore, the
research of phycosphere and bacterial communities is important because
they control the metabolic interactions of algal-bacteria in the
microenvironment.
TheMychonasteswas distributed worldwide and has a wide range of habitats, including
streams and large still waters, for example, Baringo and Victoria Lakes
in Kenya, Erken Lake in Sweden, and Stechlin Lake in Germany, etc
(Krienitz, Bock, Dadheech, & Proschold, 2011). However, as the most
abundant genus in spring, summer, and autumn in our study,Mychonastes was first reported by Li et al (2013) as a new
recorded genus of freshwater Chlorophyceae isolated from Lake Dianchi.Synechococcus is a common prokaryotic picocyanobacterial, which
had a relatively high abundance in summer. Liu, Shi, Fan, Wu, and Lei
(2019) studied the growth and interspecies competition ofMychonastes and Synechococcus under different N:P ratios.
Although we did not find a competitive relationship betweenMychonastes and Synechococcus in our results,Mychonastes has a significantly negative correlation with other
cyanobacteria (Fig. 8). In
Proteobacteria taxa, Maliki a in spring, Brevundimonas andAeromonas in autumn could promote the growth ofDesmodesmus , whileFlavobacterium andMassilia were mutually exclusive with Desmodesmus .Flavobacteriumis considered as a plant growth promoting bacterium, which is reported
to exist during the algal blooms and responsible to improve the
biological phosphorus removal ability (Park et al., 2007; Pinhassi et
al., 2004). In our study,Flavobacterium contributed to
the growth of most green algae, Navicula (Bacillariophyta), andEustigmatos (Ochrophyta), while it had the significant negative
correlation with Desmodesmus , Pseudomuriella ,Parietochloris , and Goniomonas . Node green algaeLobosphaera had the highest connectivity, with 12 negatively
correlated nodes and 4 positively nodes. The stability of ecosystem
function lies in the interaction between species (Zhou et al., 2010).
Lower connectivity in the community will lead to higher functional
stability of the system (for example, a non-scale network), because the
entire network module is less affected by node loss (Yang et al., 2017).
As an abundant bacterium in autumn,Sphingobacterium was only
helpful to the growth of green algae Schroederia , Lee, Oh, Oh,
Kim, and Ahn (2016) also reported that this bacterium responsible for
the growth of algae. Comamonas belonging to Comamonadaceae family
had the relatively higher abundance in spring, summer, and autumn, which
was considered as denitrifying polyphosphate accumulating microorganism
(Calderer et al., 2014).
The
negative correlations in co-occurrence patterns may imply predation or
competition among taxa. Cyanobacteria are the only prokaryotes with
oxygen-producing photosynthesis. They exist in different niches and are
important participants in the global carbon and nitrogen cycle. Spring
cyanobacteria only had a significant negative correlation withAtlantibacter , and promoted the growth of the remaining bacteria
(mainly Firmicutes) and green algae (Golenkinia ). The planktonic
cyanobacterium Synechococcus is ubiquitous in the ocean and fresh
waters, and plays an important role in total carbon sequestration on a
global scale in oligotrophic aquatic environments (Callieri, Cronberg,
& Stockner, 2012). Summer cyanobacteria were more mutually exclusive
with Proteobacteria, specifically,Synechococcuswas significantly negative related to Lysobacter . WinterPseudoxanthomonas , Shigella , and Methylobacteriumwere the prevalent (negative) group among theSynechocystis -associated bacteria. Therefore, these results
demonstrated that inoculation of functional bacteria can rebuild the
microbial communities, which is conducive to the growth of microalgae
and inhibits the growth of cyanobacteria.
Numerous evidences have been
demonstrated that a link occurs between diatoms and microorganisms
(Gregory et al., 2018; Stanish et al., 2013), however, whether they
cooperate with each other or exclude each other still lacks relevant
knowledge. In our results, we found significant correlations between
diatom communities and bacteria communities. And
these
relationships were greatly subjected to seasonal variations.Discostella had no significant correlation with any bacterial
taxa in spring, summer, and winter, whereas could synergistically grow
and reproduce with Proteobacteria (especially Comamonas ,Stenotrophomonas , and Bosea ) in autumn. The blooms of the
small centric diatom Stephanodiscus were regularly found every
year in lakes and reservoirs under low temperature conditions (Ha, Jang,
& Joo, 2003; Kang et al., 2007). Stephanodiscus was only found
in spring in our study area, and was significantly positive correlated
with bacterial Pseudoxanthomonas .
The
relationships between Navicula and bacterial communities did not
appear to be one sided,Navicula constituted
symbiotic relationships with Flavobacterium , Massilia ,Gemmata , and Aquabacterium in spring, verifying that cell
yield and growth rate of Navicula and the bacterialFlavobacterium increased when grown in mixed culture, because the
organic material secreted by diatoms was absorbed by and would support
the growth of bacterium. The diatom Cymatopleura had high
connectivity, in addition to co-growing with Sphingobium , it also
repelled each other with bacteria Pedomicrobium and green algae.
The genus Amphora is one of the larger genera belonging to the
family Naviculaceae and widespread including freshwater, brackish water,
and marine habitats (Nagumo, 2003).Amphorawas significantly positive related with cyanobacteria and most
Proteobacteria (Legionella , Hyphomicrobium , andRhizobium ) except Rhodoferax , and negative related toChlorella and Choricystis .
From these results, network
analysis is potentially important for clarifying the internal mechanism
of interspecies interaction and grasping the functions of microbial
communities in ecological processes such as carbon, nitrogen, and
phosphorus dynamics.