Low nutrient availability reduces high‐irradiance‐induced viability loss in oceanic phytoplankton

Abstract

In situ viability of oceanic phytoplankton may be relatively low in open oceans. This is assumed to be related to the high-irradiance and low-nutrient conditions typical for oligotrophic regions. However, experimental evidence for this phenomenon was not yet available. In the present study, the importance of nutrient availability in highirradiance-induced viability loss was therefore studied for three key oceanic phytoplankton species. Prochlorococcus marinus, Ostreococcus sp., and Thalassiosira oceanica were acclimated to two different N : P ratios. Growth, viability, and photophysiology were assessed under nutrient-replete and N-and P-starved conditions. Simultaneously, high-irradiance-induced photoinhibition and viability loss were measured and three inhibitors were used to investigate the underlying physiological mechanisms contributing to viability loss. Highirradiance exposure caused viability loss in P. marinus and Ostreococcus sp., but not in T. oceanica. Low-nutrient availability enhanced survival during high-irradiance exposure, although species-specific differences were observed. The lower sensitivity to high-irradiance intensities at low-nutrient availability was related to conformational changes in photosystem II in P. marinus, to enhanced photoprotection by the xanthophyll pigment cycle and alternative electron transport in Ostreococcus sp., and to enhanced photoprotection by the xanthophyll pigment cycle in T. oceanica. Climate change may lead to enhanced stratification in the open ocean. The resulting increase in the average irradiance intensity phytoplankton experience may promote viability loss in the smallest phytoplankton size fraction. However, this effect may partially be counteracted by the simultaneously expected decrease in nutrient availability.

Growth and biochemical composition of a microphytobenthic diatom (Entomoneis paludosa) exposed to shorebird (Calidris alpina) droppings

“…The pigment's qualitative composition did not vary but total pigment content per cell showed significant differences between growing conditions according to initial nitrogen availability (Table 4), and cells growing in the M882 condition had the highest pigment concentration. This is consistent with previous studies where pigment content and pigment ratio were used as indicators of the diatom physiological status (Geider et al, 1993;Kulk et al, 2013;Van Leeuwe et al, 2008) and where nitrogen availability affected total chlorophyll a cellular content. However, in our study, the light-harvesting pigment ratio Chl c + fucoxanthin/Chl a was unaffected by nitrogen limitation (Table 4).…”

Photophysiology of nitrate limited phytoplankton communities in Kongsfjorden, Spitsbergen

“…Samples used for the incubations were unfiltered and therefore included all grazers present in the natural seawater samples ( see Van de Poll et al in press). In the experimental setup, 20 μ mol photons m −2 s −1 was provided continuously (no dark period) by a 250 W MHNTD lamp (Philips), which closely matched the diurnal cycle in Kongsfjorden in June (Kirk ) and a natural PAR spectrum (Kulk et al ). Photoacclimation to the relatively lower irradiance intensity in the experimental setup (Table ) was thereby established within 24 h (as measured by fast repetition rate fluorometry [FRRf]).…”

“…Detailed laboratory studies with single phytoplankton species have revealed that both nutrient starvation and limitation may have considerable effects on phytoplankton photophysiology. Generally, the light harvesting capacity is reduced during nutrient starvation by a reduction in the cellular chlorophyll a (Chl a ) concentration and quantum yield of photosystem II (PSII) ( F v / F m ), as well as an increase in the relative amount of photoprotective carotenoids per Chl a (XC/Chl a ) and subsequent nonphotochemical quenching processes (NPQ NSV ) (Geider et al , ; Berges et al ; Kulk et al ). However, Chl a specific absorption and the absorption cross section of the remaining PSII ( σ PSII ) may increase during nutrient starvation (Kolber et al ; Geider et al ; Berges et al ), partially counteracting the reduced light harvesting capacity.…”

Combined Phosphorus Limitation and Light Stress Prevent Viral Proliferation in the Phytoplankton Species Phaeocystis globosa, but Not in Micromonas pusilla

“…These light conditions are ecologically relevant for the surface waters of the North Sea and Wadden Sea, whereby ML represents a daily average for the spring period (Dring et al, 2001;Ly et al, 2014). We used a 10-fold difference between LL and HL, which is a natural range experienced by phytoplankton in temperate waters (Kulk et al, 2013), and whereby LL represents a level at which the maximum growth rate of natural phytoplankton populations is halved (Cloern, 1999) and HL shows significant inhibiting effects on the photosynthetic efficiency (Fv/Fm) of our model species under P replete conditions. Triplicate 50 ml cultures were grown semi-continuously under P-replete and P-limiting conditions with daily dilution 3 h into the light period.…”

“…Irradiance can either limit phytoplankton growth and production by a reduced availability (turbidity, season, water depth or mixing to deep waters; see Huisman et al, 2002) or by an excess of it, causing photoinhibition (mixing to shallow waters, stratification; Long et al, 1994). The effects of these growth-controling factors can even be cumulative (Cloern, 1999;Colijn and Cadée, 2003;van de Poll et al, 2005;Kulk et al, 2013;Arteaga et al, 2014).…”