Dynamics of Prochlorococcus Diversity and Photoacclimation During Short-Term Shifts in Water Column Stratification at Station ALOHA

Abstract

The cyanobacterium Prochlorococcus is the dominant phototroph in surface waters of the vast oligotrophic oceans, the foundation of marine food webs, and an important component of global biogeochemical cycles. The prominence of Prochlorococcus across the environmental gradients of the open ocean is attributed to its extensive genetic diversity and flexible chlorophyll physiology, enabling light capture over a wide range of intensities. What remains unknown is the balance between temporal dynamics of genetic diversity and chlorophyll physiology in the ability of Prochlorococcus to respond to a variety of short (approximately 1 day) and longer (months to year) changes in the environment. Previous field research established depth-dependent Prochlorococcus single cell chlorophyll distributions in the North Pacific Subtropical Gyre. Here, we examined whether the shifts in chlorophyll distributions correspond to changes in Prochlorococcus genetic diversity (i.e., ecotype-level community structure) or photoacclimation of stable communities over short time intervals. We report that community structure was relatively stable despite abrupt shifts in Prochlorococcus chlorophyll physiology, due to unexpected physiological plasticity of high-light adapted Prochlorococcus ecotypes. Through comparison with seasonalscale changes, our data suggest that variability on daily scales triggers shifts in Prochlorococcus photoacclimation, while seasonal-scale dynamics trigger shifts in community structure. Together, these data highlight the importance of incorporating the process of photoacclimation and chlorophyll dynamics into interpretations of phytoplankton population dynamics from chlorophyll data as well as the importance of daily-scale variability to Prochlorococcus ecology.

Prochlorococcusrely on microbial interactions rather than on chlorotic resting stages to survive long-term nutrient starvation

“…254Amplicon sequencing of the Internal Transcribed Spacer between the 16S and 23S genes (ITS, 255(Thompson et al 2018)) revealed that between ~100-137m LL cells (belonging primarily to the LL-I and 256 LL-IV clades) gradually replace the HL cells, consistent with previous studies (e.g. (Malmstrom et al 257 2010)) but contrasting with the sharp delineation between high-chlorophyll and low-chlorophyll 258Prochlorococcus populations (Fig.…”

“…However, when co-cultured with a heterotrophic 188 bacterium, Alteromonas HOT1A3, all Prochlorococcus strains were able to re-grow after 60 days of N 189 stress. Interestingly, strain MIT9313, which was initially inhibited by this Alteromonas strain ( Figure 4A consists of both HL and LL cells, implying that phenotypic heterogeneity (acclimation) can also 245 contribute to this phenomenon (Thompson et al 2018). We therefore asked whether the double 246 population could also be due to the presence of chlorotic cells, e.g.…”

“…All DNA samples were eluted in 100 μl DNA free distilled-water. 492 493 16S and ITS PCR amplification 494 PCR amplification of the ITS was carried out with specific primers for Prochlorococcus CS1_16S_1247F 495Thompson et al 2018). The first PCR was 497 performed in triplicate in a total volume of 25 μl containing 0.5 ng of template, 12.5 μl of MyTaq Red 498 Mix (Bioline) and 0.5 μl of 10 μM of each primer.…”

Size dependence of metabolism within marine picoplankton populations

“…Using this method, we observed bimodal size distributions in Prochlorococcus , which were not apparent in flow cytograms; however, it is likely that traditional flow cytometers do not sufficiently discriminate anisotropic scatterers in this size range to resolve peaks which are separated by only 0.15 μ m. For Prochlorococcus , this distribution may either be due to the cell cycle stage distribution (this interval corresponds to a 1.8‐ to 2.6‐fold fold change in cell volume) or due to physiological differences between strains. The latter has been observed at Station ALOHA, especially near the deep chlorophyll maximum depth (Campbell and Vaulot ) or following a deepening of the mixed layer, which transiently mixes the low‐light adapted and high‐light adapted Prochlorococcus populations (Thompson et al ); however, these bimodal distributions are more prominent in flow cytograms rather than the subtle differences we observed.…”

Monitoring Microbial Communities in the Marine Environment

“…In the oligotrophic ocean north of the Hawaiian Islands, Prochlorococcus is the numerically dominant photosynthetic organism (98% of fluorescent particles). Within the upper 100 m, the distribution of Prochlorococcus DNA ecotypes is relatively constant . Yet, chlorophyll fluorescence and low‐angle scatter measurements show a steady increase as one descends down the water column.…”