It is morning in Guaymas Basin, the seas are calm, and we will get going! We only have a total of ten dives under the best of circumstances, but will make an effort to accommodate especially Alvin novices. For this, we start with today’s dive team. Danik Forsman is piloting Alvin. Ultra-experienced deep-sea researcher Mandy Joye from the University of Georgia, co-proponent on the NSF grant that has ultimately brought us to Guaymas Basin, is accompanied by her graduate student, Andrew Montgomery. Andrew was supposed to join the previous cruise in 2016, but had to cancel in the last minute because of a soccer injury. This fall, he has not played any soccer at all.
The dive target, Cathedral Hill, is well known for its abundant microbial mats, that are visually dominated by the large sulfur-oxidizing filamentous Beggiatoaceae, some of the largest bacteria in existence. These organisms exist at interfaces where they have access to sulfide, coming up with hydrothermal fluid through the sediment, and oxygen or nitrate from the water column. The filaments oxidize sulfide to elemental sulfur and ultimately to sulfate, and use oxygen and nitrate as electron acceptors for this energy-yielding reaction. The energy is saved as Adenosin triphosphate, or ATP, and used for inorganic carbon fixation via the Calvin cycle (the same pathway as photosynthetic plants), or the reverse TCA cycle. Whhereas most Begiatoaceae are colorless, Guaymas Basin has many pigmented forms that are genetically and physiologically distinct. Investigating the physiology and genomics of these organisms is a major goal of this cruise.
Since sulfur-oxidizing bacterial mats grow on top of sediments with hydrothermal fluids, these sediments are implicitly hot; temperatures can reach over 100 Celsius within 30 centimeters. We use this link as a shortcut looking for hydrothermal sediments to sample. But first, Alvin’s in-situ heatflow probe is inserted 45 cm into the sediment to measure the temperature gradient, synchronous at 9 measurement points that are spaced 5 cm apart. For every sampling site, an entire temperature field is measured so that we can be sure all sediment cores have the desired temperature range.
The pilot places the core tubes artfully into the sediment, one next to the other; the top flaps (made from a flexible grey membrane) form a tight seal when the cores are pulled out and placed back into the sampling basket. When the sediments are very hot, the pilot has to race against the clock, as the plastic material of the core tubes begins to soften and to melt, in particular at the bottom end that is exposed to the highest temperatures; some cores could only be recovered as abstract art.
Unusual animal life can be found everywhere in Guaymas Basin. Here, a small forest of Spaghetti worms is populated by deep-sea shrimp. For reasons that are not very clear, these shrimp and other crustaceans are often bright pink or red-pigmented, although there is no natural light in Guaymas Basin.This dive turns out to be very successful; over 30 sediment cores are recovered from the hot hydrothermal areas of Cathedral Hill. The shipboard labs spring into action after Alvin and the dive team returns, fully equipped for everything. Core processing is the first order of the evening; sediments and their microbial inhabitants have to be as fresh as possible for microbial process rate measurements and assays of cellular activity. Cores are processed in the cold room at 4C, where Guangchao Zhang, postdoc in the Joye lab, and Andrew smile even after midnight – these are very good cores with that special smell of success, a heady mix of sulfide and hydrocarbons, and they are excellent candidates for high rates of anaerobic processes, such as sulfate reduction, methanogenesis and methane oxidation.
In a more temperate shipboard lab, Roland and Viola are starting to prep their sediment samples for BONCAT analysis; microbes are offered a fake fluorescent amino acid that they take up, and become fluorescent themselves. This staining method can be applied to many experimental manipulations, for example temperature variations or different chemical stimuli or stress factors, and it can be combined with staining methods that identify specific evolutionary groups of microorganisms. By testing how particular microbes retain their appetite under different conditions, it is possible to do physiological experiments with the natural diversity of environmentally abundant bacteria and archaea.