Influence of nutrients and light on autotrophic, mixotrophic and heterotrophic freshwater chrysophytes

Chrysophyte algae show wide variation in their nutritional modes, which can be especially advantageous in oligotrophic conditions. However, the capacities and strategies of the algae to adapt to changing conditions may vary due to different nutritional requirements of individual species. In this context, comparative analyses evaluating the physiological range of mixotrophic chrysophytes are important to predict possible changes in phytoplankton composition. We compared 4 freshwater chrysophytes-Poteriospumella lacustris, Poterioochromonas malhamensis, Dinobryon divergens and Mallomonas annulata-under different growth conditions. The diatom Phaeodactylum tricornutum served as a photoautotrophic reference strain. We demonstrate active growth of P. lacustris and the mixotrophic P. malhamensis under chemoheterotrophic conditions. Neither of the chrysophytes were growing photoautotrophically; however, P. malhamensis showed some photosynthetic activity and survived longer when kept in the light. M. annulata, P. tricornutum and the mixotrophic D. divergens showed light-limited growth when kept in inorganic media. Even though D. divergens consumed living bacteria, it did not grow in the dark in the presence of the bacteria only. We detected large differences in the general nutritional preferences between the 2 mixotrophs in relation to the nutritional features of all investigated strains.