open access

https://doi.org/10.14943/doctoral.k14192

https://hdl.handle.net/2115/96078

オホーツク海におけ海氷融解が海氷珪藻類の増殖と光合成生理能力に及ぼす影響

英語

2020-09-25

Hokkaido University

iii, 113p

Sea ice microalgal communities are dominated by diatoms and they play important roles in primary production at high latitudes. Growth of microalgae in ice-covered areas is primarily controlled by the seasonal light climate. In Arctic and subarctic seas, more light penetrates through sea ice in early spring than that during the winter time. This increase in light availability is a key driver of bloom in ice-covered seawaters. During ice melt in spring, ice algae are released from sea ice and could be exposed to changeable temperature, salinity and irradiance levels in surface water. Large variations of those environmental factors are likely to influence photophysiological performance of ice algae. Species with greater flexibility in photoacclimation are afforded a higher chance of survival and seed the following phytoplankton bloom. The Sea of Okhotsk is the southernmost sea ice zone in the northern hemisphere with a sizeable seasonal ice cover, thus ice algae of the Sea of Okhotsk have a large potential to seed the early spring diatom bloom in the water column. However, little is known about the Okhotsk ice algal communities and their seeding effects. We investigated the dynamics of the composition and the photophysiological performances of an ice algal community in a 6-day laboratory incubation experiment that simulated the natural ice melt conditions. Centric diatoms, especially Thalassiosira spp., overwhelmingly dominated the ice algal community throughout the incubation, whereas pennate diatoms, mostly Naviculaand Nitzschia, showed little growth with much higher mortality. The maximum photochemical efficiency of Photosystem II (Fv/Fm) was the lowest at the beginning of the ice melt, suggesting a suppressed photosynthetic functioning by changes in salinity. The cellular pigment contents decreased by 30% due to cellular damage, evidenced by deformed plastids under a microscope. The transcript level of the rbcL gene that encodes the large subunit of RubisCO was significantly higher during the ice melt and decreased in the no-ice period, suggesting an urgent need for osmoprotectants under the melt condition. Full recovery of the photosynthesis and growth was also made after complete ice melt. The results indicated high seeding potential of Thalassiosira to spring blooms owing to their photophysiological plasticity to dynamic salinity changes. Saroma Lagoon is an embayment with two inlets leading to the Sea of Okhotsk. There is a seasonal development of sea icein this lagoon. To investigate the living and photoprotective strategies of ice algae in such a coastal water system, we grew Nitzschia cf. neglecta, an ice diatom isolated from the sea ice of this lagoon, under irradiance levels of 30 and 100 μmol photons m⁻²s-¹, and temperatures of 2 and 10℃. Then the acclimated cells were exposed to high light in order to investigate the plasticity of their photosynthetic functioning. At 10℃, cells grew faster and showed a weaker susceptibility to high light. Highly efficient photoprotection was achieved through the diadinoxanthin-diatoxanthin cycle-dependent non-photochemical quenching (NPQ). The wide tolerance to both temperature and light changes suggests that the thinning of sea ice and higher temperatures in a warmer climate will lead to more intense blooms in Saroma Lagoon.

2020-09-25

甲第14192号

博士(環境科学)

(主査) 教授 鈴木 光次, 教授 力石 嘉人, 准教授 西岡 純, 助教 豊田 威信, 助教 野坂 裕一（東海大学生物学部）

環境科学院（地球圏科学専攻）

博士論文

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