Congrats to Benjamin Klempay for his first, first-authored publication in the lab! (wow, didn’t I just write that??) Benjamin is part of the Oceans Across Space and Time (OAST) project and his paper, Microbial diversity and activity in Southern California salter and bitterns: analogues for ancient ocean worlds, appears in a special issue of the journal Environmental Microbiology. In the paper Benjamin does a deep dive into the microbial diversity of the network of lakes that make up the South Bay Salt Works, a little known industrial site/wildlife refuge on San Diego Bay that also happens to be the oldest continually operating solar salt harvesting facility in the US.
Our interest in hypersaline lakes – aside from that fact that they just really weird and fun environments to explore – is their value as analogues for evaporative environments on Mars and other ancient ocean worlds. Once upon a time Mars was wet, and may not have been so dissimilar to many environments on Earth today. As that water was lost the oceans, lakes, and wetlands were reduced by evaporation to saline lakes and ultimately salt pans. These end-state evaporative environments are key targets for Martian exploration today. Extremely salty lakes like those found at the Salt Works are a reasonable representation of the last potentially inhabited environments on the surface of Mars before it became too desiccated to support life. Thus the signatures of ancient Martian life might bear some similarities to contemporary life in these lakes.
The microbial diversity of hypersaline lakes has been studied in depth – as I mentioned before they’re weird and fun places to study – but Benjamin’s work looks at a couple of unexplored elements. First, he didn’t restrict his analysis to sodium chloride lakes at the Salt Works (salterns) but also included magnesium chloride lakes (bitterns) that are thought to be too toxic for life (see a nice discussion of this in a recent OAST paper here). He found an interesting pattern of microbial diversity across these lakes, with diversity decreasing as salinity decreases, then suddenly increasing in the magnesium chloride lakes. The reason for this is the absence of microbial growth in those lakes. Rather than hosting a specialized microbial community they collect microbes from dust, seaspray, and other sources (infall), and preserve this DNA but inactivating the enzymes that would normally degrade it.
Co-authors Anne Dekas and Nestor Arandia-Gorostidi at Stanford also applied nano-SIMS to evaluate single-cell activity levels across the salinity (water activity) gradient. Biomass can be very high in these lakes – 100 fold or more higher than seawater – so we assumed that activity would be high too. The nice thing about nano-SIMS is that it evaluates activity on a per-cell basis. Looked at in this way, most bacteria and archaea had surprisingly low levels of activity. We’re still trying to understand exactly what this means and Anne and Nestor undertook an impressive array of experiments as part of our 2020 field effort to try to get to the bottom of it. We think that the extraordinarily low levels of predation are partially responsible; the eukaryotic protists that typically prey on bacteria and archaea can’t grow at the salinity of the saltiest lakes at South Bay Salt Works. Viruses, the other major source of mortality for bacteria and archaea, don’t generally propagate through low-activity populations. So the haloarchaea that dominate in these lakes may have hit upon a winning evolutionary strategy of slow growth under the protection of a particularly extreme environment.
