Search

Fjords on the west coast of Spitsbergen experience variable Arctic and Atlantic climate signals that drive seasonal and inter‐annual variability of phytoplankton productivity and composition, by mechanisms that are not fully resolved. To this end, a time series (2013–2018) of Kongsfjorden (N 78°54.2, E 11°54.0) phytoplankton pigments, ocean physics, nutrient concentrations, and microbial abundances was investigated. Kongsfjorden phytoplankton dynamics were predominantly governed by solar radiation and cycles of warming and freshwater discharge that caused pronounced changes in light and nutrient availability. Phytoplankton growth after the polar night commenced in March in a mixed, nutrient loaded water column, and accelerated in April after weak thermal stratification. Spring (weeks 10–22) showed high diatom relative abundance that ceased when silicic acid and nitrate reached limiting concentrations. Summer (weeks 23–35) was characterized by sixfold stronger stratification due to increased freshwater discharge and continued ocean heating. This caused a warm, low salinity surface layer with low nutrient concentrations. Small and diverse flagellates, together with high bacterial and viral abundances, thrived inthis regenerative, N or P‐limited system. Elevated late summer chlorophyll a (Chl a), and ammonium suggested increased regeneration and nutrient pulses by glacial upwelling. Fall (weeks 36–48) caused rapidly declining Chl a and increasing diatom relative abundance, which persisted throughout the polar night, causing high diatom relative abundance during spring. Despite inter‐annual variability in ocean temperature and salinity we observed relatively stable seasonal phytoplankton taxonomic composition and Chl a.

Through cell lysis, viruses shape phytoplankton community composition and stimulate biogeochemical cycling in the oceans. Earlier studies indicate that reduced phosphorus (P) availability can affect phytoplankton virus proliferation. The effects of nitrogen (N) availability are claimed to be weaker than those for P, but this has not been thoroughly studied. Here, we investigated how N-limiting growth conditions, resulting in altered algal elemental stoichiometry and physiology, affected virus proliferation in the phytoplankters Micromonas pusilla and Phaeocystis globosa. Algal cultures were adapted to balanced nutrient-limited growth, i.e. N-, P- and NP-controlled growth, before infection with their respective viruses MpV-08T and PgV-07T. The viral infection experiments were then performed in batch cultures to allow optimal 1-step virus growth cycles. Compared to the nutrient-replete cultures, infection of nutrient-controlled hosts resulted in elongated latent periods (time until first virus release) and reduced viral burst sizes (viruses lysed host cell-1) for both MpV and PgV. For MpV, the viral burst size was reduced by 70%, independent of the type of nutrient. The burst size of PgV was most reduced under N-limitation, by as much as 92%, compared to 70% under P-limitation. Overall, our results demonstrate that algal virus production can be strongly impaired by N-limitation and that the effects are of a similar magnitude to or even larger than for P. Our study indicates that viral control of natural phytoplankton populations might be strongly driven by both P- and N-availability.

The intact polar lipid (IPL) composition of phytoplankton is plastic and dependent on environmental factors. Previous studies have shown that phytoplankton under low phosphorus (P) availability substitutes phosphatidylglycerols (PGs) with sulfoquinovosyldiacylglycerols (SQDGs) and digalactosyldiacylglycerols (DGDGs). However, these studies focused merely on P depletion, while phytoplankton in the natural environment often experience P limitation whereby the strength depends on the supply rate of the limiting nutrient. Here we report on the IPL composition of axenic cultures of the picophotoeukaryote Micromonas pusilla under different degrees of P limitation, i.e., P-controlled chemostats at 97 and 32 % of the maximum growth rate, and P starvation (obtained by stopping P supply to these chemostats). P-controlled cultures were also grown at elevated partial carbon dioxide pressure (pCO2) to mimic a future scenario of strengthened vertical stratification in combination with ocean acidification. Additionally, we tested the influence of viral infection for this readily infected phytoplankton host species. Results show that both SQDG : PG and DGDG : PG ratios increased with enhanced P limitation. Lipid composition was, however, not affected by enhanced (750 vs. 370 µatm) pCO2. In the P-starved virally infected cells the increase in SQDG : PG and DGDG : PG ratios was lower, whereby the extent depended on the growth rate of the host cultures before infection. The lipid membrane of the virus MpV-08T itself lacked some IPLs (e.g., monogalactosyldiacylglycerols; MGDGs) in comparison with its host. This study demonstrates that, besides P concentration, also the P supply rate, viral infection and even the history of the P supply rate can affect phytoplankton lipid composition (i.e., the non-phospholipid : phospholipid ratio), with possible consequences for the nutritional quality of phytoplankton.

Viruses are a major source of mortality for phytoplankton and bacteria and are therefore seen as drivers of food web dynamics and biogeochemical cycling in the marine pelagic environment. Previous studies have shown that aquatic viruses adsorb to suspended sediment, which theoretically decreases the mortality pressure on their microbial hosts. This process is of particular ecological importance in the Arctic, where coastal systems contain large amounts of suspended fine-sediment, supplied by melting and calving glaciers. The aim of this study was to investigate the effects of glacier-derived fine sediment on marine Arctic microbes during summer in Storfjorden, Svalbard (78 degrees N, 20 degrees E). We sampled for microbial abundances over transects with increasing sediment concentration towards three different glaciers, and examined the adsorption of the natural virus community to previously collected glacier-derived sediment. Our data show declined abundances of phytoplankton (

Previous studies have shown that viral infection can affect the lipid distribution of phytoplankton, specifically the fatty acid (FA) distribution, and has been hypothesized to affect the nutritional value of phytoplankton for higher trophic levels. Here, we report the bulk FA distribution as well as the FA distribution of individual intact polar lipid (IPL) classes of the alga Phaeocystis globosa infected with the lytic virus PgV-07T. Analysis of the virus PgV-07T itself showed that it contained shorter, more saturated bulk and IPL-bound FAs than the host. Viral infection did not affect the bulk or IPL-bound FA distribution after 24 h post-infection when cell lysis was initiated, but after 48 h the bulk FAs remaining in the particulate phase of the infected cultures contained 22% less polyunsaturated FAs (PUFAs) compared to the control cultures. This change in the bulk FAs was mainly due to the generation of PUFAs that occurred in the control cultures, suggesting that infection prevented P. globosa PUFA accumulation. Two of the 7 IPL classes, the monogalactosyldiacylglycerols and the sulfoquinovosyldiacylglycerols, showed about a 10% reduction in the percentage of PUFAs upon viral infection. In contrast, the digalactosyldiacylglycerols exhibited a 15% increase in PUFAs. This difference between the IPL-PUFAs and the bulk FAs is possibly due to a contribution to the bulk FA pool of e.g. triacylglycerols. Overall, these results suggest that grazing on infected cells and filter feeder uptake of post-lysis cell debris could lead to a transfer of relatively lower amounts of PUFAs to higher trophic levels.

Recent studies showed changes in phytoplankton lipid composition during viral infection and have indicated roles for specific lipids in the mechanisms of algal virus-host interaction. To investigate the generality of these findings and obtain a better understanding of the allocation of specific lipids to viruses, we studied the intact polar lipid (IPL) composition of virally infected and non-infected cultures of the prymnesiophyte Phaeocystis globosa G(A) and its lytic virus PgV-07T. The P. globosa IPL composition was relatively stable over a diel cycle and not strongly affected by viral infection. Glycolipids, phospholipids and betaine lipids were present in both the host and virus, although specific groups such as the diacylglyceryl-hydroxymethyltrimethyl-β-alanines and the sulfoquinovosyldiacylglycerols, were present in a lower proportion or were not detected in the virus. Viral glycosphingolipids (vGSLs), which have been shown to play a role in the infection strategy of the virus EhV-86, infecting the prymnesiophyte Emiliania huxleyi CCMP374, were not encountered. Our results show that the involvement of lipids in virus–algal host interactions can be very different amongst virus–algal host systems.

NetherlandsUnder natural conditions phytoplankton are often simultaneously subjected to phosphorus (P) limitation and suboptimal light levels. Potential interacting effects of P-limitation and light availability on phytoplankton virus-host interactions have thus far not been reported. We studied the influence of three environmentally relevant light levels (low; 25, medium; 100, and high; 250 μmol quanta m−2 s−1) in combination with P-limitation (vs. P-replete conditions) on virus proliferation in the key phytoplankton species Micromonas pusilla and Phaeocystis globosa. Cultures were acclimated to balanced P-limited growth at 3 light levels by semi-continuous culturing, before one-step infection experiments were carried out in batch mode. Under optimal conditions (medium light, P-replete), the latent period (time until first release of progeny viruses) was 6–9 and 9–12 h, and the burst size (number of viruses released per lysed host cell) was 241 ± 5 and 690 ± 28 for M. pusilla virus MpV and P. globosa virus PgV, respectively. Low light intensity under P-replete conditions prolonged the latent period of PgV (with maximally 3 h). The PgV burst size was 2.8-fold reduced under low light and 2.2-fold reduced under high light. The 10-fold range in light intensity did not affect viral latent period or burst size in P-replete M. pusilla. However, P-limitation (under optimal light) also led to elongated latent periods (with maximally 3 h compared to P-replete) and the viral burst sizes decreased by 2.7-fold for MpV and 3.5-fold for PgV. Finally, infectivity assays showed that PgV progeny from the P-limited high and low light cultures largely lost their infectivity, reducing their infective burst sizes to only 2–4 infective viruses per lysed host cell. Our study demonstrates that the effects of specific light and P-availability on virus-phytoplankton interaction are not only species specific, but can also strengthen each other's effects. Relatively small differences in environmental conditions with depth, geography or time have the potential to drastically affect viral infection of phytoplankton, with consequent effects on host species composition and biogeochemical fluxes.

In the Kongsfjorden-Krossfjorden system (Spitsbergen), increasing temperatures enhance glacier melting and concomitant intrusion of freshwater. These altered conditions affect the timing, intensity, and composition of the phytoplankton spring bloom in Kongsfjorden; yet, the effects on prokaryotes (bacteria and archaea) are not well understood. The aim of this study was to examine springtime prokaryote communities in both fjords as a function of hydrographic and phytoplankton variability. Prokaryote community composition was studied in two consecutive years by molecular fingerprinting of the 16S rRNA gene. In addition, we measured bacterial abundance, productivity (H-3-Leucine uptake), and single-cell activity using catalyzed reporter deposition fluorescence in situ hybridization combined with microautoradiography. Differences in bacterial and archaeal communities were found between Kongsfjorden and Krossfjorden. Furthermore, an increase in productivity, abundance, and proportion of active bacterial cells was observed during the course of spring. Bacteroidetes were the most abundant bacterial group among the assessed taxa in both Kongsfjorden and Krossfjorden. Multivariate analysis of the microbial community fingerprints revealed a strong temporal shaping of both the bacterial and archaeal communities in addition to a spatial separation between the two fjords. A significant part of the observed bacterial variation could be explained by cyanobacterial biomass, as deduced from pigment analysis, and by phosphate concentration. Archaea were mainly controlled by abiotic factors. We speculate that the bacterial response to hydrographic changes and glacier meltwater is mediated through shifts in phytoplankton abundance and composition, whereas archaea are directly influenced by abiotic environmental variables.

Growth and viral infection of the marine picoeukaryote Micromonas pusilla was studied under a future-ocean scenario of elevated partial CO 2 (pCO 2 ; 750 μatm versus the present-day 370 μatm) and simultaneous limitation of phosphorus (P). Independent of the pCO 2 level, the ratios of M. pusilla cellular carbon (C) to nitrogen (N), C:P and N:P, increased with increasing P stress. Furthermore, in the P-limited chemostats at growth rates of 0.32 and 0.97 of the maximum growth rate (μ max ), the supply of elevated pCO 2 led to an additional rise in cellular C:N and C:P ratios, as well as a 1.4-fold increase in M. pusilla abundance. Viral lysis was not affected by pCO 2 , but P limitation led to a 150% prolongation of the latent period (6 to 12 h) and an 80% reduction in viral burst sizes (63 viruses per cell) compared to P-replete conditions (4 to 8 h latent period and burst size of 320). Growth at 0.32 μ max further prolonged the latent period by another 150% (12 to 18 h). Thus, enhanced P stress due to climate change-induced strengthened vertical stratification can be expected to lead to reduced and delayed virus production in picoeukaryotes. This effect is tempered, but likely not counteracted, by the increase in cell abundance under elevated pCO 2 . Although the influence of potential P-limitation-relieving factors, such as the uptake of organic P and P utilization during infection, is unclear, our current results suggest that when P limitation prevails in future oceans, picoeukaryotes and grazing will be favored over larger-sized phytoplankton and viral lysis, with increased matter and nutrient flow to higher trophic levels.

Earlier studies show that the proliferation of phytoplankton viruses can be inhibited by depletion of soluble reactive phosphorus (SRP; orthophosphate). In natural marine waters, phytoplankton phosphorus (P) availability is, however, largely determined by the supply rate of SRP (e.g. through remineralization) and potentially by the source of P as well (i.e. the utilization of soluble non-reactive P; SNP). Here we show how a steady low supply of P (mimicking natural P recycling) to virally infected P-limited Micromonas pusilla stimulates virus proliferation. Independent of the degree of P limitation prior to infection (0.32 and 0.97μmax chemostat cultures), SRP supply resulted in 2-fold higher viral burst sizes (viruses lysed per host cell) as compared with no addition (P starvation). Delaying these spikes during the infection cycle showed that the added SRP was utilized for extra M. pusilla virus (MpV) production far into the lytic cycle (18 h post-infection). Moreover, P-limited M. pusilla utilized several SNP compounds with high efficiency and with the same extent of burst size stimulation as for SRP. Finally, addition of virus-free MpV lysate (representing a complex SNP mixture) to newly infected cells enhanced MpV production, implicating host-associated alkaline phosphatase activity, and highlighting its important role in oligotrophic environments.