| Abstract |
Marine microbial eukaryotes have crucial roles in the water column ecosystem; however,
there are regional gaps in the investigation of natural microbial eukaryote communities and
uncertainties concerning their distribution persevere. This study combined 18S rRNA
metabarcoding, biomass measurements and statistical analyses of multiple environmental
variables to examine the ecology, the diversity and the composition of planktonic microbial
eukaryotes at the Eastern Mediterranean Sea, while trying to deepen the understanding of the
relationship between plankton and nutrient supply.
Initially, the composition and distribution of microbial plankton eukaryotes at the ultraoligotrophic
Eastern Mediterranean open sea, was studied. Our results showed that microbial
eukaryotic communities were structured according to seawater depth. In surface waters,
different sites shared high percentages of molecular operational taxonomic units (MOTUs), but
this was not the case for deep-sea communities. Plankton biomass, on the other hand, was
significantly different among sites, implying that communities of a similar composition may not
support the same activity or population size. Water temperature and dissolved organic matter
were found to significantly affect community distribution. Micro-eukaryotic distribution was
additionally affected by the nitrogen-to-phosphorus ratio (N/P) and viral abundance, while
smaller-in-size communities were affected by zooplankton. Therefore, it is underlined that, even
within restricted oceanic areas, marine plankton may follow distribution patterns that are largely
controlled by environmental variables.
External inputs into marine coastal ecosystems increase as a result of expanding human
activities. Following the changes in the water column physiochemistry after land inputs, the
planktonic community is affected. The second part of this work studied the structure and
composition of planktonic eukaryotic communities from coastal areas of the Eastern
Mediterranean. In particular, three coastal areas that receive high nutrient input from adjacent
rivers and nearby agricultural activity were studied, in comparison to three areas that receive
lower nutrient input and showed lower nutrient concentration in the water column. Dinoflagellata
prevailed regardless of the nutrient disturbance, however, phylla as Bacillariophyta and
Chlorophyta were more prominent in the most nutrient affected areas. Eukaryotic communities
of the most affected areas were associated with variables such as chlorophyll, dissolved nitrogen
and oxygen concentration and prey abundance; thus, the altered community composition of the
affected areas was associated to resources availability and higher trophic status. To further study
the ecological relationships created under the different enrichment conditions, coexistence
networks of eukaryotic microorganisms were constructed. The ecological network of the areas
receiving greater inputs from the land exhibited fewer species-to-species relationships, was less
coherence and specific microorganisms were found to play a greater role. Overall, differences
were found both in the ecological relationships and in the diversity and composition of the
eukaryotic plankton community between areas that receive varying anthropogenic nutrient
inputs.
Finally, the effects of the abrupt input of high quantities of dissolved inorganic nitrogen
and phosphorus were investigated in an attempt to simulate the nutrient disturbances caused by
eutrophication and climate change. Two nutrient levels were created through the addition of
different quantities of dissolved nutrients in a mesocosm experiment. During the developed
blooms, compositional differences were found within bacteria and microbial eukaryotes, and
communities progressed towards species of faster metabolisms. Regarding the different nutrient
concentrations, different microbial species were associated with each nutrient treatment and
community changes spanned from the phylum to the operational taxonomic unit (OTU) level.
Network analyses revealed important differences in the biotic connections developed: more
competitive relationships were established in the more intense nutrient disturbance and
networks of contrasting complexity were formed around species of different ecological strategies.
According to these results sudden disturbances in water column chemistry lead to the
development of entirely different microbial food webs with distinct ecological characteristics.
In conclusion, plankton biomass responds even in slight nutrient concentration changes,
however, it seems that the composition of eukaryotic community responds only to intense
nutrient level changes. The network analysis of plankton suggests that the overall changes of the
water column system are highly depended to the relationships among microorganisms. Overall,
this study gives fresh perspective to the ecological approach of plankton microbial eukaryotes in
connection to the nutrient supply in the water column.
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