Always coexisting: The story of two dominant Prochlorococcus ecotypes

Prochlorococcus is the most abundant photosynthetic organism on earth, although they were only discovered in 1988 the ever increasing ability to study genomes has revealed that the species is incredibly diverse forming many different ecotypes which vary significantly with depth and latitudes due to the wide availability of niches.

Although oligotrophic oceans contain a diverse community of phytoplankton they are usually numerically dominated by one of two ecotypes of Prochlorococcus, eMIT9312 or eMED4 both of which are high light adapted. A recent study (Chandler et al, 2016), Surveyed transects in the Pacific Ocean and compare it to data collected from the Atlantic Ocean (Johnson et al 2006). In order to determine the distribution and composition of the two numerically dominant ecotypes, and to determine if eMIT9312 and eMED4 were found to occur separately or if the populations coexisted together along a key environmental gradient. The two environmental gradients investigated were temperature and water stratification.

Temperature was shown to be the main driving factor with ∼21°C being the transition zone below this point eMED4 dominates while above this eMIT9312 dominates. These findings support previous work which looked at relative growth of strains in culture (Johnson et al 2006). Temperature was important in influencing the ratio of eMIT9312: eMED4 which was shown to follow a log- linearly relationship with temperature.

The influence of stratification while not directly impacting cell growth was shown to affect the community structure by its influence on nutrient availability. By looking at strains within the two ecotypes strains of eMED4 are able to take advantage of a wider range of organics as P sources (Moore et al, 2005). Some stains of eMIT9312 are able to use nitrate as an N source (Berube et al, 2015).While some stains are able to take advantage of extra organics other do not process the required genes to facilitated these processes. It is suggested that those who can take advantage of different nutrients are dominating and therefore contribution more to the relative ecotype abundance. The effect stratification has on light exposure is likely to be of lesser importance as both ecotypes are HL adapted.

Anomalies were found that did not appear to fit the ratio for example samples collected from the Gulf Stream revealed an anomalously high eMIT9312:eMED4 ratio, this higher than expected ratio was due to a lower than expected eMED4 abundances as opposed to a higher eMIT9312 abundance. It is believed that this anomaly is the result of a lag between temperature decline and population restructuring. It’s predicted that given enough time at the lower temperature the ratio of eMIT9312:eMED4 ratio would shift to what would be expected.

It was found that in every sample both ecotypes coexisted neither was out-competed even in areas where the driving factors such as temperature favoured one ecotype over the other of all the samples analysed neither ecotype was ever below detectable limits (0.65 cells ml−1). This observation of coexistence even when one ecotype may appear a superior competitor over the other may be explained by the frequency of disturbance events, including mixing and temperature shifts. The term paradox of the plankton is used to describe the unexpected high diversity of plankton where it would be expected that competitive exclusion would take place.

Something that requires further study is the relative importance of temperature and stratification (which themselves are correlated) as independent driving factors on ecotype abundance. Temperature was shown to be the biggest influence with stratification still being significant but having a less prominent role. One limit to this study was that although direct effects of temperature on the growth rate of the ecotypes was observed, it was not able to assess the indirect effects of temperature on growth which includes nutrient availability and mortality rate.

Although this paper provided a good starting point highlighting the distribution, coexistence and effects of temperature and stratification on eMIT9312 or eMED4 ecotypes in the Pacific and Atlantic Ocean, more research is needed particularly the ability to be able to culture all genotypes in the eMIT9312 and eMED4 clade to ensure that the full story is being revealed.  

References

Berube, P.M., Biller, S.J., Kent, A.G., Berta-Thompson, J.W., Roggensack, S.E., Roache-Johnson, K.H., et al. (2015) Physiology and evolution of nitrate acquisition in Prochlorococcus. ISME J 9: 1195–1207.

Chandler, J., Lin, Y., Gainer, P., Post, A., Johnson, Z. and Zinser, E. (2016). Variable but persistent coexistence of Prochlorococcus ecotypes along temperature gradients in the ocean's surface mixed layer. Environmental Microbiology Reports, 8(2), pp.272-284.

Johnson, Z.I., Zinser, E.R., Coe, A., McNulty, N.P., Malcolm, E., Woodward, S., and Chisholm, S.W. (2006) Niche partitioning among Prochlorococcus ecotypes along oceanscale environmental gradients. Science 311: 1737–1740.

Moore, L.R., Ostrowski, M., Scanlan, D.J., Feren, K., and Sweetsir, T. (2005) Ecotypic variation in phosphorusacquisition mechanisms within marine picocyanobacteria. Aquat Microb Ecol 39: 257–269.