Impact of ocean warming and acidification on coccolithophore ecology and calcification in the North Pacific

Abstract

Coccolithophores are cosmopolitan unicellular calcifying phytoplankton involved in important biogeochemical global cycles. This PhD thesis focuses on the impacts of ocean warming and acidification on the morphology and calcification of coccolithophores, with special attention to the cosmopolitan and dominant species Emiliania huxleyi. The study includes temperature experiments using three strains of E. huxleyi isolated in the NW Pacific Ocean, and the analysis of water samples collected along a transect covering the Gulf of California and NE Pacific margin waters. This type of work is timely since global warming and the rapid increase in anthropogenic atmospheric CO2 have remarkable consequences on the marine environment. To clarify discrepancies featuring in the literature about the calcification response to temperature changes, we compared three strains of E. huxleyi grown under non-limiting nutrient and light conditions, at 10, 15, 20 and 25 ºC of temperature. All three strains displayed similar growth rate versus temperature relationships, with an optimum at 20–25 ºC. Over the sub-optimum to optimum temperature range (10–25 ºC), elemental production (particulate inorganic carbon (PIC), particulate organic carbon (POC), total particulate nitrogen (TPN)), coccolith mass, coccolith size, width of the tube element cycle, number of attached coccoliths per coccosphere, coccosphere mass, individual sinking velocity, individual cell PIC : POC ratio, and coccolith production rate, were positively correlated with temperature. The correlation between PIC production and coccolith mass/size supports the notion that coccolith mass can be used as a proxy for PIC production in sediment samples. We found that incompleteness of coccoliths is not due to time shortage at high PIC production. Sub-optimal growth temperatures lead to an increase in the percentage of malformed coccoliths in a strain-specific fashion. The PIC : POC ratio showed a minimum at optimum growth temperature in all investigated strains. In the context of climate change, global warming might cause a decline in coccolithophore’s PIC contribution to the rain ratio, as well as improved fitness in some genotypes due to fewer coccolith malformations; our data also point to an important influence of global warming on sinking velocities. Given that warming, acidification, and lowered nutrient availability might occur simultaneously under climate change scenarios, there is the question about what the net effect of different influential factors will be. Therefore, we analysed 68 summertime samples along a transect at different stations and depths, giving a large range of conditions. The studied regions are expected to be particularly susceptible to both warming and acidification, and are characterized by high seasonal primary production through upwelling events. We focused the study on the coccosphere standing stock variations of the main morphotypes and morphological variations of the species E. huxleyi and on the different species of the genus Gephyrocapsa. We found that E. huxleyi type O is a colder morphotype enhanced by higher ammonium concentrations and is more tolerant to lower pH values than the morphotype A. The over-calcified E. huxleyi type A shared niche with the morphotype O. The observed morphological aberrations and the under-calcified morphology were associated to unfavorable conditions for the cell such as low (though not the lowest) nutrient concentrations. The PIC contribution per liter of G. oceanica was higher than that of E. huxleyi, in third place was that of G. muellerae. Gephyrocapsa oceanica presented the highest affinity to warmer waters with lower nutrient concentrations, and G. muellerae the highest tolerance to lower pH. In the context of climate change, G. oceanica and G. muellerae might increase their relative abundance with subsequent changes in the coccolithophore PIC production.

Date of Award	13 Jul 2018
Original language	English
Awarding Institution	Universitat Autònoma de Barcelona (UAB)
Supervisor	Patrizia Ziveri . (Director) & Gerald Langer . (Director)