Highly variable inland lakes of the Bahamas create unique opportunities for research on the evolution of marine organisms in extreme environments. For example, some lakes vary from 0.5x to 2.0x ocean salinity depending on precipitation. Despite extreme fluctuations, these sites often support rich communities that include mangroves, marine macro-algae, and a variety of endemic vertebrates and invertebrates. The persistence of these communities suggests a large degree of adaptive evolution which has yet to be demonstrated for most species. We ask whether or not inland populations of the model, single-celled macro-alga Acetabularia crenulata from hypersaline Bahamian lakes evolved to tolerate extreme changes in salinity. If so, might evolution explain observations of unusual population densities relative to nearby ocean sites? We measured oceanic A. crenulata growth at salinities ranging from 0.5x to 2.0x global average sea water (~35ppt). Intact specimens were placed into 1X Erd-Schreiber media and allowed to acclimate. Subsets were distributed across lower or higher salinities in a step-wise manner and allowed to acclimate for one-day, minimum. After acclimation, caps were excised, leaving approximately 2 cm of stalk and rhizoid intact. Length of regenerative tissue was recorded over time and differences among treatments analyzed by one-way ANOVA and post-hoc tests.
Results/Conclusions
ANOVA revealed significant differences in A. crenulata growth across salinities (p ≤ 1.26e-9). Post-hoc tests suggested that individuals grown at the four most “extreme” concentrations (0.5x, 0.75x,1.75x, 2.0x) did not differ significantly (p > 0.05) with average growth of 3.3cm (SD = 1.87). Likewise, there were no significant differences at the “middle” ranges (1.0x, 1.25, 1.5x) with average growth of 8.9cm (SD = 1.97). All pairwise comparisons of “extreme” vs. “middle” treatments were, in contrast, significant (p ≤ 0.05). Our results show that oceanic A. crenulata regeneration is reduced by nearly 40% outside 1.0x to 1.5x sea water salinity. We are extending our study to inland lakes on four Bahamian islands in Spring of 2018. Some of these lakes can exceed the range tested here, but it remains to be seen whether A. crenulata merely persists during hyper- or hypo-saline conditions, or is able to grow and reproduce in ways that nearby ocean counterparts cannot. Given the historic role of A. crenulata as a model cell, its ecological significance in marine and lake ecosystems, and broad concerns over the evolutionary consequences of global change, our results are likely to have important implications for understanding adaptive potential in extreme, novel environments.