Phytoplankton embraces a large diversity of life forms, from pioneer oxygenic photosynthetic cyanobacteria to a broad spectrum of phylogenetically distant eukaryotic organisms. In many of the evolutionary branches, colonial organisms have appeared. The evolutionary reasons for the transition to larger sizes are not yet fully understood, but multicellularity is thought to be one of its consequences. Phytoplankton ecological success or failure, under certain conditions is the result of a balance between gains and losses. Unicellular and colonial organisms have to adapt their respective functional traits related to photosynthesis, resource acquisition, and predation, to changes in the environment. The advantages of a certain life-form (unicellular or colonial), could, hence, rely in the relevance of gain processes (light, nutrient related traits), or of losses as main drivers of phytoplankton evolution. Despite coloniality could suppose an opportunity in finding new paths to succeed, previous knowledge pointed to larger phytoplankton cells as worse competitors for nutrients than smaller cells and hence, as the size of the colonies may start to become a constraint for nutrient uptake and utilization. However, other issues changing with size can become advantageous for colonial phytoplankton. For example, large flagellate colonies can move and hence cover a greater space of resources likely to be exploited than small unicellulars, and also can have a larger storage capacity. Large cells (and colonies in particular) can take more advantage of the production of external enzymes since colonial forms, specially mucilaginous ones, could maintain exoenzymes close to the cells in this external matrix. In summary, any strategy by which there is not a proportional increase in the need for nutrients as body size enlarges can be regarded as a competitive advantage for colonial organisms. Besides, a possible advantage for large unicellulars and colonies could be related to the top-down control of the systems by grazers, as smaller unicellulars are subject of grazing by both, small and large filter feeders, whereas large colonies can override the edible size spectrum of some. Also the aggregation of cells to form large colonies harder to gulp or filter by zooplankton is considered a relative widespread defense strategy. The ubiquitousness of colonial forms of phytoplankton and its endurance until today is the basis for believing that there has been a significant selection for it in the ancient unicellular world. Yet it is difficult to guess what the main drivers for coloniality have been, Can we find out the selective forces favoring multicellular colonial forms in phytoplankton? Understanding the ecological advantage that colonial forms could hold in phytoplankton was the main objective of this thesis.

Los orígenes de los organismos que componen la comunidad fitoplanctónica se remontan a distintos eventos endosimbióticos; por ello, una de las principales características del fitoplancton es una enorme diversidad que atañe tanto en sus atributos morfológicos y fisiológicos como en sus formas de vida. A pesar de que la multicelularidad en algunas especies de fitoplancton podría suponer una estrategia para asegurar la conservación de la línea germinal, o para generar un medio interno estable que proteja a las células que forman la colonia del cambiante ambiente que la rodea, actualmente se desconoce el motivo que pudo haber favorecido la transición desde la unicelularidad a la colonialidad. Sin embargo, la existencia de organismos coloniales evolutivamente más avanzados en los que se observa una diferenciación celular y división de tareas, podrían empezar a definir una estrategia de vida compleja y jerarquizada más allá de la simple agregación celular. La gran variedad de formas, tamaños y configuraciones presentadas por las células y colonias, es una muestra de que no hay una estructura óptima, sino un gran abanico de posibilidades exitosas para afrontar los obstáculos a los que estos organismos se enfrentan durante su ciclo de vida. Por otro lado, el paso de una forma de vida unicelular a una colonial en los organismos fitoplanctónicos, implica un número de condicionantes ecológicos que determinan cuándo y cómo las formas coloniales pueden ser más viables. En cualquier caso, el hecho de que las formas unicelulares y coloniales sean ubicuas en el fitoplancton y hayan perdurado durante millones de años, nos da a entender que deben existir ciertas ventajas ecológicas conferidas por la colonialidad. Este trabajo se centra en investigar en qué modo los organismos coloniales han llegado a compensado las desventajas derivadas de la agregación para haberse convertido en una alternativa evolutivamente exitosa a la unicelularidad.

Tesi realitzada al Centre d'Estudios Avançats de Blanes (CEAB-CSIC)