Physiological and metabolic characteristics of polar diatoms: insights into cold adaptation and potential biotechnology
The polar regions represent two of the most extreme environments on Earth, with sub-zero temperatures, sustained light in the summer and complete darkness in the winter. Marine diatoms are prevalent in the polar oceans, significantly contributing to primary productivity and ecosystem functioning. They are characterized by having optimal growth at low temperatures (<10°C) and have developed various genetic adaptations to cope with these extreme environments. The polar regions, however, are experiencing unprecedented environmental changes because of regional climate change. We currently lack detailed knowledge on polar diatom ecophysiology and metabolism, which means we do not know how these species may be impacted by future environmental changes. One barrier to our understanding of polar diatoms is the availability of living strains for experimental study. The number of polar diatom strains in public collections is low and of those that are available, many have been kept in culture for many decades. Hence, there is an urgent requirement to isolate new environmental strains that have not been exposed to artificial conditions for many decades and also to increase the number and diversity of species available. In this project I conducted fieldwork in Antarctica to isolate several new Antarctic strains and used these alongside Arctic and Antarctic strains from culture collections to study whether physiological and metabolic characteristics are conserved between diatoms from the two regions. My studies focused primarily on the response to temperature with the aim of predicting the effects of future climate change on polar diatom physiology and metabolism. The project also aimed to identify potential candidate strains for biotechnological exploitation. Conducting fieldwork on the Western Antarctic Peninsula, I isolated 36 strains encompassing 12 different species of polar diatoms. These have been genetically and morphologically identified and representatives deposited in the Culture Collection for Algae and Protozoa. I investigated the ecophysiology of two diatom species found in both polar regions, Fragilariopsis cylindrus and Porosira glacialis. My results showed how the thermal tolerance differed between Arctic and Antarctic strains with optimal growth temperatures of the Arctic F. cylindrus being 3°C higher than the Antarctic strain. On the contrary, P. glacialis from the Arctic had an optimal growth temperature 5.5°C lower than the Antarctic strain. In addition, the thermal tolerance of newly isolated strains of F. cylindrus was investigated and found to be similar to the laboratory strain except for one, which was able to grow at 12°C. The difference in temperature tolerance between the Arctic and Antarctic F. cylindrus strains led me to investigate the vitamin B12 requirements of these strains. Previously it had been suggested that there are links between B12 dependency and temperature tolerance, and the Antarctic F. cylindrus was previously found to be B12 independent. My results demonstrated that all three Arctic F. cylindrus strains had an obligate requirement for exogenous vitamin B12, whereas all three Antarctic strains did not. Further investigation failed to isolate and identify the B12-independent methionine synthase gene (METE) in Arctic strains, but this gene was identified in Antarctic strains. In contrast, the B12-dependent methionine synthase gene (METH) was identified and partially sequenced in all strains. Several single nucleotide polymorphisms were identified in the Arctic strains which are hypothesised to be non-deleterious. These results provide a potential explanation for the B12 dependency in Arctic isolates, and reasons for how this might have arisen are considered. Lastly, polar diatoms are thought to offer a unique resource for biotechnology offering potentially novel metabolites or increased biomass production during winter months when temperatures decrease. Pilot experiments indicate that there is no advantage in growth over a temperate strain during the winter months in the UK, but further work focusing on optimisation is needed.