Portrait of a Geothermal Spring, Hunter's Hot Springs, Oregon

Abstract

Although alkaline Hunter's Hot Springs in southeastern Oregon has been studied extensively for over 40 years, most of these studies and the subsequent publications were before the advent of molecular methods. However, there are many field observations and laboratory experiments that reveal the major aspects of the phototrophic species composition within various physical and chemical gradients of these springs. Relatively constant temperature boundaries demark the upper boundary of the unicellular cyanobacterium, Synechococcus at 73-74 °C (the world-wide upper limit for photosynthesis), and 68-70 °C the upper limit for Chloroflexus. The upper limit for the cover of the filamentous cyanobacterium, Geitlerinema (Oscillatoria) is at 54-55 °C, and the in situ lower limit at 47-48 °C for all three of these phototrophs due to the upper temperature limit for the grazing ostracod, Thermopsis. The in situ upper limit for the cyanobacteria Pleurocapsa and Calothrix is at ~47-48 °C, which are more grazer-resistant and grazer dependent. All of these demarcations are easily visible in the field. In addition, there is a biosulfide production in some sections of the springs that have a large impact on the microbiology. Most of the temperature and chemical limits have been explained by field and laboratory experiments.

Figure 1

Hunter’s Springs from over ~93 °C at source (gray) to the green biofilm of Synechococcus (73–74 °C) that ends at 54–55 °C with the dark brown cover of Geitlerinema (Oscillatoria) cf. terebriformis, some of which has contracted to expose a salmon-colored undermat of Chloroflexus that had been hidden by the top-mat of the cyanobacterium, G. terebriformis.

Figure 2

Hunter’s top mat of Synechococcus (green) with under-mat of Chloroflexus (yellow-orange) exposed in a thermal pool at 61–64 °C by blowing off the Synechococcus with a stream of water ejected from a syringe.

Figure 3

Photomicrograph of co-culture of Synechococcus (rod-like cells, 1.2 µm wide), some dividing, and Chloroflexus aurantiacus trichomes (~0.5 µm wide) in D medium at 45 °C.

Figure 4

Geitlerinema terebriformis in culture (trichomes ~5 µm wide) in D medium at 45 °C.

Figure 5

Geitlerinema terebriformis “mat” on right, with edge at ~55 °C, “riding over” Synechococcus topmat during low light period. The length of the view is ~6 cm (Figure 3 in [16]).

Figure 6

Geitlerinema terebriformis in contracted fascicles over Synechococcus topmat as a result of high, mid-day, summer light intensity. Temperature ~50 °C.

Figure 7

(a) Geitlerinema terebriformis “mat” in shade and 45 °C over sediment at 9:25 am, 27 July (Kahneeta Hot Springs) (Figure 4A in [15]); (b) Same “mat” at 10:00 (full sun in summer) with G. terebriformis largely absent due to downward vertical migration. Lower mat termination at meeting with cold water of river (Figure 4C in [15]).

Figure 8

Geitlerinema terebriformis mat contracted and retreating under high midday light intensity at 49–53 °C, revealing Chloroflexus undermat, originally covered by the motile G. terebriformis mat during period of low light (Figure 7 in [15]).

Figure 9

Synechococcus cover (>55 °C) with Geitlerinema terebriformis cover beginning at 55 °C and ending at about 48 °C due to grazing by the ostracod, Thermopsis thermophile (small white dots). (Figure 19.5 in [27]).

Figure 10

Geitlerinema terebriformis cover in Hunter’s stream that has been terminated at ~48 °C by ostracod grazing, revealing dark patches of Pleurocapsa/Calothrix in ostracod zone below 47–48 °C.

Figure 11

Pleurocapsa, leathery nodules with population of ostracods (white dots) over the area (horizontal length ~12 mm) in a Hunter’s stream at ~45 °C.

Figure 12

Newly established Synechococcus/Geitlerinema terebriformis population on soft gray substrate after 30+ days, but without the natural introduction of ostracods. In this case, the G. terebriformis cover gradually tapered out below about 35 °C much lower than 47–48 °C that would have been the temperature of truncation if ostracods were present.

Figure 13

Calothrix (tapered filaments) extended from base of aggregated cells of Pleurocapsa, (non-filamentous); from piece of nodule from ~45 °C, scraped off hard substrate. Bar = 5 µm.

Figure 14

Agar nutrient slab on which Pleurocapsa had been grown uniformly in lab (in petri dish) and then placed in “Fenceline” Spring at Hunter’s at about 45 °C and allowed to be grazed upon by a natural ostracod population shown grazing here (small dark specks), after about 12 h with about 800–1000 animals cm⁻², see [20].

Figure 15

Portion of a southern Hunter’s stream at about 50 °C with high sulfide production, at night, showing Thermochromatium cf. tepidum (pink) at surface of soft substrate mixed with some dark areas of G. terebriformis.

Figure 16

Swarm of swimming, ascending Thermochromatium tepidum (pink cloud at right) that had been covered by an opaque can for 2 h during 10 am on a cloudy day. In uncovered area, daytime cover of Geitlerinema terebriformis remained.

Figure 17

(a) Hunter’s pool at 9:25 am in April (~40 °C) with Geitlerinema terebriformis/Oscillatoria princeps cover (dark brown) but showing small patches of Beggiatoa leptomitiformis below the surface cover. Ostracods absent (Figure 2B in [18]); (b) Hunter’s pool at ~40 °C at 7:00 am (before sunrise, same day in April. Beggiatoa leptomitiformis (white) has migrated to the surface (complete darkness), with O₂-sulfide interface 0.2–0.3 mm below surface of mat, and covering the Geitlerinema terebriformis below (Figure 2C in [18]).

Figure 17

(a) Hunter’s pool at 9:25 am in April (~40 °C) with Geitlerinema terebriformis/Oscillatoria princeps cover (dark brown) but showing small patches of Beggiatoa leptomitiformis below the surface cover. Ostracods absent (Figure 2B in [18]); (b) Hunter’s pool at ~40 °C at 7:00 am (before sunrise, same day in April. Beggiatoa leptomitiformis (white) has migrated to the surface (complete darkness), with O₂-sulfide interface 0.2–0.3 mm below surface of mat, and covering the Geitlerinema terebriformis below (Figure 2C in [18]).

Figure 18

Cartoon summary of phototroph and ostracod distributions in Hunter’s streams. Upper portion with visible field distributions. Lower line below indicates Chloroflexus undermat distribution (47–68 °C) and ostracod, Pleurocapsa, Calothrix distributiions below 47 °C. The lower horizontal bars indicate ranges in culture of 4 Synechococcus thermotypes [3] and those of Chloroflexus, Oscillatoria (=Geitlerinema), Pleurocapsa, Calothrix, and Potamocypris (former name of the ostracod, Thermopsis thermophile). The range between arrows indicates the approximate optimal range for growth in culture.