Filomena Romano:
Temporal and vertical dynamics of mixoplanktonic pelagic ciliates in the oligotrophic Eastern Mediterranean Sea

Date: 31-07-2021    Supervisor: Per Juel Hansen & Paraskevi Pitta



Mixotrophy is the combination of phototrophic and heterotrophic nutrition in a single organism. At the beginning, mixotrophy was confined to phototrophic protists (constitutive mixotrophs), but in the last decades, mixotrophy includes also protists that lack built-in chloroplasts, and instead they host symbionts or retain functional chloroplasts from their prey (non-constitutive mixotrophs). This thesis has studied the vertical and temporal distribution of generalist non-constitutive mixotrophs (GNCMs); referring to the behavior where C-fixation can be acquired from different types of prey organisms. Ciliates are part of GNCMs, and they are found in both marine and freshwater systems, but information on the contribution of mixotrophic species to ciliate assemblages and their relationships with the microbial food web are still fragmented.

The present thesis assesses the temporal and vertical variability and dynamics of mixotrophic and heterotrophic ciliates and their interactions with the rest of biotic and/or abiotic variables in the ultra-oligotrophic Eastern Mediterranean Sea. A coastal and an offshore stations were sampled during 2019, on a monthly basis for the former, and seasonally for the latter, and vertical profiles from surface to 120 m are provided (down to 1500 m for the offshore station). Two parallel series of samples were examined for ciliate analysis. One series of samples was fixed with acid Lugol’s solution in order to ensure the minimum loss of specimens and therefore obtain a better estimation of the ciliate numerical abundance and biomass; the second series of samples was fixed with buffered formaldehyde, which allows the capture and identification of mixotrophic ciliate species, both micro- and nano-sized. A considerable microscopic effort was put into the assignment of ciliate specimens to different categories in terms of groups (oligotrichs, choreotrichs, tintinnids), trophic modes (mixotrophic, heterotrophic), size classes (from <18 μm to >50 μm), and taxonomic level (genus or species, when possible). Besides ciliates, all planktonic organisms, i.e. bacterio-, phyto- and protozooplankton, were also counted and identified to genus or species, when possible. With the assistance of an image analysis system, biomass was also estimated for all above mentioned organisms.

This approach, applied to a complete series of vertical (surface to 120 m – or 1500 m) and monthly, over a year, samples resulted in a unique data set. Based on this data set, we tried to find answers to the following questions: (1) What is the vertical distribution of different size classes of mixotrophic vs. heterotrophic ciliates in this oligotrophic environment? (2) Do the previous two features change with time? If so, how? (3) What is the contribution of both mixotrophic and heterotrophic nanociliates to the total ciliate community in terms of abundance and biomass? Does this change with time and depth, and how? (4) How are mixotrophic and heterotrophic ciliates related with the other components of the pelagic microbial food web? (5) Does this relationship change with time and depth, and how? Because the Eastern Mediterranean is among the most oligotrophic areas of the world, the findings of our study also apply for the open ocean.