Michael I. Latz

MARINE BIOLUMINESCENCE SPECTRA MEASURED WITH AN OPTICAL MULTICHANNEL DETECTION SYSTEM

The emission spectra of 70 bioluminescent marine species were measured with a computer controlled optical multichannel analyzer (OMA). A 350 nm spectral window is simultaneously measured using a linear array of 700 silicon photodiodes, coupled by fiber optics to a microchannel plate image intensifier on which a polychromator generated spectrum is focused. Collection optics include a quartz fiber optic bundle which allows spectra to be measured from single photophores. Since corrections are not required for temporal variations in emissions, it was possible to acquire spectra of transient luminescent events that would be difficult or impossible to record with conventional techniques. Use of this system at sea on freshly trawled material and in the laboratory has permitted acquisition of a large collection of bioluminescence spectra of precision rarely obtained previously with such material. Among unusual spectral features revealed were organisms capable of emitting more than one color, including: Umbellula magniflora and Stachyptilum superbum (Pennatulacea), Parazoanthus lucificum (Zoantharia), and Cleidopus gloria-maris (Pisces). Evidence is presented that the narrow bandwidth of the emission spectrum for Argyropelecus affinis (Pisces) is due to filters in the photophores.

Far Red Bioluminescence from Two Deep-Sea Fishes

Spectral measurements of red bioluminescence were obtained from the deep-sea stomiatoid fishes Aristostomias scintillans (Gilbert) and Malacosteus niger (Ayres). Red luminescence from suborbital light organs extends to the near infrared, with peak emission at approximately 705 nanometers in the far red. These fishes also have postorbital light organs that emit blue luminescence with maxima between 470 and 480 nanometers. The red bioluminescence may be due to an energy transfer system and wavelength-selective filtering.

The reversible effect of flow on the morphology of ceratocorys horrida (PERIDINIALES, DINOPHYTA)*

Most cells experience an active and variable fluid environment, in which hydrodynamic forces can affect aspects of cell physiology including gene regulation, growth, nutrient uptake, and viability. The present study describes a rapid yet reversible change in cell morphology of the marine dinoflagellate Ceratocorys horrida Stein, due to fluid motion. Cells cultured under still conditions possess six large spines, each almost one cell diameter in length. When gently agitated on an orbital shaker under conditions simulating fluid motion at the sea surface due to light wind or surface chop, as determined from digital particle imaging velocimetry, population growth was inhibited and a short‐spined cell type appeared that possessed a 49% mean decrease in spine length and a 53% mean decrease in cell volume. The reduction in cell size appeared to result primarily from a 39% mean decrease in vacuole size. Short‐spined cells were first observed after 1 h of agitation at 20°C; after 8 to 12 d of continuous agitation, long‐spined cells were no longer present. The morphological change was completely reversible; in previously agitated populations devoid of long‐spined cells, cells began to revert to the long‐spined morphology within 1 d after return to still conditions. During morphological reversal, spines on isolated cells grew up to 10 μm·d−1. In 30 d the population morphology had returned to original proportions, even though the overall population growth was zero during this time. The reversal did not occur as a result of cell division, because single‐cell studies confirmed that the change occurred in the absence of cell division and much faster than the 16‐d doubling time. The threshold level of agitation causing morphology change in C. horrida was too low to inhibit population growth in the shear‐sensitive dinoflagellate Lingulodinium polyedrum. At the highest level of agitation tested, there was negative population growth in C. horrida cultures, indicating that fluid motion caused cell mortality. Small, spineless cells constituted a small percentage of the population under all conditions. Although their abundance did not change, single‐cell studies and morphological characteristics suggest that the spineless cells can rapidly transform to and from other cell types. The sinking rate of individual long‐spined cells in still conditions was significantly less than that of short‐spined cells, even though the former are larger and have a higher cell density. These measurements demonstrate that the long spines of C. horrida reduce cell sinking. Shorter spines and reduced swimming would allow cells to sink away from turbulent surface conditions more rapidly. The ecological importance of the morphological change may be to avoid conditions that inhibit population growth and potentially cause cell damage.

THE EFFECT OF SALINITY UPON PHOTOTAXIS AND GEOTAXIS IN A LARVAL CRUSTACEAN

1. Experiments were conducted to determine the effect of salinity on phototaxis and geotaxis by Stages I and IV zoeae of the crab, Rhithropanopeus harrisii.2. Larvae were exposed to sudden salinity changes and stimulated with various intensities of 500-nm light in the horizontal plane. Although the pattern of phototaxis of larvae exposed to 40‰ was unchanged from that at 20‰ (acclimation salinity), the level of positive phototaxis to higher intensities was significantly greater and the level of negative phototaxis to low intensities significantly lower at 40‰. Exposure to low salinity sea water (5‰) generally reverses the sign of phototaxis, since a significantly higher level of negative phototaxis and lower level of positive phototaxis occurs at light intensities above 10-2 Wm-2.3. The minimum amount of salinity decrease from the acclimation salinity that induces a reversal in phototactic sign from positive to negative phototaxis at 0.19 Wm-2 ranges from 1 to 2‰, and appears to be independent of acclimat...

MECHANISMS OF FLUID SHEAR-INDUCED INHIBITION OF POPULATION GROWTH IN A RED-TIDE DINOFLAGELLATE1

Net population growth of some dinoflagellates is inhibited by fluid shear at shear stresses comparable with those generated during oceanic turbulence. Decreased net growth may occur through lowered cell division, increased mortality, or both. The dominant mechanism under various flow conditions was determined for the red‐tide dinoflagellate Lingulodinium polyedrum (Stein) Dodge. Cell division and mortality were determined by direct observation of isolated cells in 0.5‐mL cultures that were shaken to generate unquantified fluid shear. Larger volume cultures were exposed to quantified laminar shear in Couette‐flow chambers (0.004–0.019 N·m−2 shear stress) and to unquantified flow in shaken flasks. In these larger cultures, cell division frequency was calculated from flow cytometric measurements of DNA·cell−1. The mechanism by which shear inhibits net growth of L. polyedrum depends on shear stress level and growth conditions. Observations on the isolated cells showed that shaking inhibited growth by lowering cell division without increased mortality. Similar results were found for early exponential‐phase cultures exposed to the lowest experimental shear stress in Couette‐flow chambers. However, mortality occurred when a late exponential‐phase culture was exposed to the same low shear stress and was inferred to occur in cultures exposed to higher shear stresses. Elevated mortality in those treatments was confirmed using behavioral, morphological, and physiological assays. The results predict that cell division in L. polyedrum populations will be inhibited by levels of oceanic turbulence common for near‐surface waters. Shear‐induced mortality is not expected unless shear‐stress levels are unusually high or when cellular condition resembles late exponential/stationary phase cultures.

Spectral Composition of Bioluminescence of Epipelagic Organisms from the Sargasso Sea

The spectral characteristics of single identified epipelagic sources of bioluminescence from the western Sargasso Sea were measured with an optical multichannel analyzer (OMA) system during the April, 1985, Biowatt cruise. The emission spectra of specimens representing 45 species from 8 phyla were measured. Peak bioluminescence emissions typically occurred between 440 and 500 nm, in the blue region of the visible spectrum. Three exceptions involved emission in the green, yellow, and red spectral regions. Intraspectific variability in spectra, was noted in several species. One shrimp species exhibited two modes of light emission, each with different emission spectra. Other cases involved dynamic color shifts of 10 to 14 nm; the source of the spectral variability is unknown, but may involve optical filtering or differences in the color of luminescence from multiple sites of light emission. Measurements from independent samples of unsorted plankton revealed different spectral distributions. This suggests that the spectral emissions of bioluminescence in the upper water column will vary, based on species assemblage.

Green fluorescent protein regulation in the coral Acropora yongei during photoacclimation

SUMMARY Reef-building corals inhabit high light environments and are dependent on photosynthetic endosymbiotic dinoflagellates for nutrition. While photoacclimation responses of the dinoflagellates to changes in illumination are well understood, host photoacclimation strategies are poorly known. This study investigated fluorescent protein expression in the shallow-water coral Acropora yongei during a 30 day laboratory photoacclimation experiment in the context of its dinoflagellate symbionts. Green fluorescent protein (GFP) concentration measured by Western blotting changed reversibly with light intensity. The first 15 days of the photoacclimation experiment led to a ∼1.6 times increase in GFP concentration for high light corals (900 μmol quanta m–2 s–1) and a ∼4 times decrease in GFP concentration for low light corals (30 μmol quanta m–2 s–1) compared with medium light corals (300 μmol quanta m–2 s–1). Green fluorescence increased ∼1.9 times in high light corals and decreased ∼1.9 times in low light corals compared with medium light corals. GFP concentration and green fluorescence intensity were significantly correlated. Typical photoacclimation responses in the dinoflagellates were observed including changes in density, photosynthetic pigment concentration and photosynthetic efficiency. Although fluorescent proteins are ubiquitous and abundant in scleractinian corals, their functions remain ambiguous. These results suggest that scleractinian corals regulate GFP to modulate the internal light environment and support the hypothesis that GFP has a photoprotective function. The success of photoprotection and photoacclimation strategies, in addition to stress responses, will be critical to the fate of scleractinian corals exposed to climate change and other stressors.

Cryptic Bioluminescence in a Midwater Shrimp

The mesopelagic shrimp Sergestes similis emits ventrally directed bioluminescence that closely matches the intensity of downward-directed illumination and is able to rapidly modify its light output to match changes in background intensity. Masking experiments show that the photoreceptors involved are the compound eyes or adjacent tissues. Light emission originates from modified portions of the hepatopancreas and is similar to oceanic light in angular distribution and spectral characteristics. Normally oriented animals respond minimally to upward-directed light.

SPONTANEOUS AND STIMULATED BIOLUMINESCENCE OF THE DINOFLAGELLATE CERATOCORYS HORRZDA (PERIDINIALES)1

This is the first report of spontaneous bioluminescence in the autotrophic dinoflagellate Ceratocorys horrida von Stein. Bioluminescence was measured, using an automated data acquisition system, in a strain of cultured cells isolated from the Sargasso Sea. Ceratocorys horrida is only the second dinoflagellate species to exhibit rhythmicity in the rate of spontaneous flashing, flash quantum flux (intensity), and level of spontaneous glowing. The rate of spontaneous flashing was maximal during hours 2–4 of the dark phase [i.e. circadian time (CT)16–18 for a 14:10 h LD cycle (LD14:10)], with approximately 2% of the population flashing‐min−1, a rate approximately one order of magnitude greater than that of the dinoflagellate Gonyaulax polyedra. Flash quantum flux was also maximal during this period. Spontaneous flashes were 134 ms in duration with a maximum flux (intensity) of 3.1×109 quanta‐s−1. Light emission presumably originated from blue fluorescent microsources distributed in the cell periphery and not from the spines. Values of both spontaneous flash rate and maximum flux were independent of cell concentration. Isolated cells also produced spontaneous flashes. Spontaneous glowing was dim except for a peak of 6.4× 104quanta‐s−1 cell−1, which occurred at CT22.9 for LD14:10 and at CT22.8 for LD12:12. The total integrated emission of spontaneous flashing and glowing during the dark phase was 4×109 quantacell−1, equivalent to the total stimulable luminescence. The rhythms for C. horrida flash and glow behavior were similar to those of Gonyaulax polyedra, although flash rate and quantum flux were greater. Spontaneous bioluminescence in C. horrida may be a circadian rhythm because it persisted for at least three cycles in constant dark conditions. This is also the first detailed study of the stimulated bioluminescence of C. horrida, which also displayed a diurnal rhythm. Cultures exhibited >200 times more mechanically stimulated bioluminescence during the dark phase than during the light phase. Mechanical stimulation during the dark phase resulted in 6.7 flashes. cell−1; flashes were brighter and longer in duration than spontaneous flashes. Cruise‐collected cells exhibited variability in quantum flux with few differences in flash kinetics. The role of dinoflagellate spontaneous bioluminescence in the dynamics of near‐surface oceanic communities is unknown, but it may be an important source of natural in situ bioluminescence.

Shear-Stress Dependence of Dinoflagellate Bioluminescence

Fluid flow stimulates bioluminescence in dinoflagellates. However, many aspects of the cellular mechanotransduction are incompletely known. The objective of our study was to formally test the hypothesis that flow-stimulated dinoflagellate bioluminescence is dependent on shear stress, signifying that organisms are responding to the applied fluid force. The dinoflagellate Lingulodinium polyedrum was exposed to steady shear using simple Couette flow in which fluid viscosity was manipulated to alter shear stress. At a constant shear rate, a higher shear stress due to increased viscosity increased both bioluminescence intensity and decay rate, supporting our hypothesis that bioluminescence is shear-stress dependent. Although the flow response of non-marine attached cells is known to be mediated through shear stress, our results indicate that suspended cells such as dinoflagellates also sense and respond to shear stress. Shear-stress dependence of flow-stimulated bioluminescence in dinoflagellates is consistent with mechanical stimulation due to direct predator handling in the context of predator-prey interactions.