Gemma Kulk

The effect of iron limitation on the photophysiology of Phaeocystis antarctica (Prymnesiophyceae) and Fragilariopsis cylindrus (Bacillariophyceae) under dynamic irradiance

The effects of iron limitation on photoacclimation to dynamic irradiance were studied in Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) W. Krieg. in terms of growth rate, photosynthetic parameters, pigment composition, and fluorescence characteristics. Under dynamic light conditions mimicking vertical mixing below the euphotic zone, P. antarctica displayed higher growth rates than F. cylindrus both under iron (Fe)–replete and Fe‐limiting conditions. Both species showed xanthophyll de‐epoxidation that was accompanied by low levels of nonphotochemical quenching (NPQ) during the irradiance maximum of the light cycle. The potential for NPQ at light levels corresponding to full sunlight was substantial in both species and increased under Fe limitation in F. cylindrus. Although the decline in Fv/Fm under Fe limitation was similar in both species, the accompanying decrease in the maximum rate of photosynthesis and growth rate was much stronger in F. cylindrus. Analysis of the electron transport rates through PSII and on to carbon (C) fixation revealed a large potential for photoprotective cyclic electron transport (CET) in F. cylindrus, particularly under Fe limitation. Probably, CET aided the photoprotection in F. cylindrus, but it also reduced photosynthetic efficiency at higher light intensities. P. antarctica, on the other hand, was able to efficiently use electrons flowing through PSII for C fixation at all light levels, particularly under Fe limitation. Thus, Fe limitation enhanced the photophysiological differences between P. antarctica and diatoms, supporting field observations where P. antarctica is found to dominate deeply mixed water columns, whereas diatoms dominate shallower mixed layers.

Temperature-dependent photoregulation in oceanic picophytoplankton during excessive irradiance exposure

The phytoplankton community of open oligotrophic oceans is dominated by prokaryotic Prochlorococcus spp., Synechococcus spp., and eukaryotic picoand nanophytoplankton [1-3]. The competitive success of these phytoplankton species depends on different factors, including the response to the (dynamic) irradiance conditions encountered in the water column. With the occurrence of different ecotypes, picophytoplankton species such as Prochlorococcus spp., Synechococcus spp., and Ostreococcus spp. can grow over a broad range of irradiance conditions [4-7]. For example, the low light adapted ecotypes of Prochlorococcus are well adapted to the irradiance intensity and spectral composition of the deep chlorophyll maximum with high chlorophyll b/a ratios and low optimal growth irradiances [4,5,8]. In contrast, the high light adapted ecotypes of Prochlorococcus spp. can competitively grow in the (upper) mixed layer with low chlorophyll b/a ratios and higher optimal growth irradiances [4,5,8]. Similar differ‐ ences in pigmentation, absorption, and photosynthetic characteristics have been found in ecotypes of marine Synechococcus spp. [9-11] and Ostreococcus spp. [7,12,13]. In addition to the genetically defined (photo)physiology of the different ecotypes, the photoacclimation poten‐ tial of specific (pico)phytoplankton species may play an important role in the response to (dynamic) irradiance conditions [11].

Assessing drivers of coastal primary production in northern Marguerite Bay, Antarctica

The coastal ocean of the climatically-sensitive west Antarctic Peninsula is experiencing changes in the physical and (photo)chemical properties that strongly affect the phytoplankton. Consequently, a shift from diatoms, pivotal in the Antarctic food web, to more mobile and smaller flagellates has been observed. We seek to identify the main drivers behind primary production (PP) without any assumptions beforehand to obtain the best possible model of PP. We employed a combination of field measurements and modeling to discern and quantify the influences of variability in physical, (photo)chemical and biological parameters on PP in northern Marguerite Bay. Field data of high-temporal resolution (November 2013 – March 2014) collected at a long-term monitoring site here were combined with estimates of PP derived from photosynthesis-irradiance incubations and modeled using mechanistic and statistical models. Daily PP varied greatly and averaged 1764 mg C m-2 d-1 with a maximum of 6908 mg C m-2 d-1 after the melting of sea ice and the likely release of diatoms concentrated therein. A non-assumptive random forest model (RF) with all possibly relevant parameters (MRFmax) showed that variability in PP was best explained by light availability and chlorophyll a followed by physical (temperature, mixed layer depth and salinity) and chemical (phosphate, total nitrogen and silicate) water column properties. The predictive power from the relative abundances of diatoms, cryptophytes and haptophytes (as determined by pigment fingerprinting) to PP was minimal. However, the variability in PP due to changes in species composition was most likely underestimated due to the contrasting strategies of these phytoplankton groups as we observed significant negative relations between PP and the relative abundance of flagellates groups. Our reduced model (MRFmin) showed how light availability, chlorophyll a and total nitrogen concentrations can be used to obtain the best estimate of PP (R2 = 0.93). The resulting estimates from our models suggest summer PP to have been between 214.4 g and 176.1 g C m-2. Through the employment of a modeling technique without any assumptions apart from a representative sampling strategy, we showed and estimated how PP in this climatically sensitive and changing region can best be predicted and described.