Dive Dry with Dr. Bill

#628: Red Algae Do "It" Deeper

No, this isn't a column about sex for you readers with the prurient minds! I guess I've trained you well with my titillating columns in the past. Yes, some algae do have sexual reproduction but if I were to devote an entire column to algal sex, you'd probably fall asleep by the second paragraph! I have something completely different in store for you this week. No, don't run away... read on!

Many have wondered why the ocean is often a deep blue in color (or sometimes turquoise or green in the shallows). We all know a glass of fresh or salt water appears transparent so it has no real intrinsic color of its own. Some of you are familiar with the nature of sunlight as it penetrates the ocean's surface. Water, along with the stuff in it, is a killer of sunlight. Obviously if you dive deep enough you are in a completely dark environment where sunlight never reaches. Here off Catalina, even at 200 ft there is enough light left to see. But water is a very selective serial killer of light... it picks its victims based on their wavelength or color.

Red light receives the first obituary in the sea, followed by orange. The low energy wavelengths just don't make it very far into the briny deep. That is one reason many deep water or nocturnal fish are reddish in color... you can't see them easily because they appear black. Sunlight is selectively reduced based on that old mnemonic that you learned in physics (or perhaps art class for the non-scientists)... ROY G BIV. The higher energy, shorter wavelength colors from green to blue to violet are filtered out last. Because of this, they are the colors scattered back to our eyes from the depths..

Since the colors left in deeper water are at the blue-violet end of the spectrum, you see the light reflected back to us from the depths and it appears blue. Shallow or murky water may appear more turquoise or green because more of the green light is present at those shallower depths. Most of you have seen what happens to the ocean's color near shore when storms resuspend sand and other sediments.

Now algae (aka seaweeds) are photosynthetic organisms, relying on sunlight to turn nutrients into food in the form of sugars and other carbohydrates that they can store for future use. The major groups of algae in our waters include reds, greens and browns. Their color is due to the light they reflect back to our eyes. Therefore a green alga bounces the green light back at us and absorbs the red and blue-violet light at either end of the spectrum. Likewise, a red alga reflects back the red-orange wavelengths and absorbs the green-violet wavelengths.

Based on that you'd expect to find red algae at deeper depths where the blue end of the spectrum is more predominant. To a large degree, this is true. On my deepest dives down to 200 ft, there is still enough light to see and the algae I find down there are mostly reds that appear vibrant red to purple under my video lights. That is because the video lights give off the full spectrum of light so red is present in their beams. However the red algae down there use the light that is available to them... the blue-violet end of the spectrum. Their pigments are ones that can trap that range and use it for photosynthesis.

Now green algae are somewhat similar to terrestrial plants. Their predominant pigment is chlorophyll which reflects light in the middle of the spectrum and utilizes both the red and the blue ends of the spectrum for photosynthesis. Green algae as well as the marine grasses (eelgrass and surfgrass) tend to be found in shallower water.

Since I'm supposed to be a high falutin' kelp forest ecologist, I can't forget my favorites... the brown algae (which include as a subgroup the kelps). Yep, our giant kelp (Macrocystis pyrifera) has a combination of pigments which make it appear brown in color. So do our other kelps such as the southern sea palm (Eisenia arborea) and feather boa (Egregia laevigata) not to mention that nasty non-native Sargassum horneri. The deeper water kelps like Pacific collander kelp (Agarum) and oarweed (Laminaria), generally found outside the influence of the giant kelp canopy, often have very large blades to gather more of the limited sunlight that reaches those depths.

Now brown algae have a real mix of pigments. Obviously they do not fall directly into one of the ROY G BIV categories, but if you are an artist or a scientist, you recognize that brown is a combination of different colors of the spectrum. One way to create it is to mix red and green pigments. In the brown algae, color may range from brown to an olive-green depending on the amount of the brown pigments like fucoxanthin that are present. This pigment absorbs light primarily in the blue-green and yellow-green regions of the spectrum. Therefore it can be found from the intertidal to fairly deep depths if light gets to it.

So seaweeds with red pigments are often found deeper where they can still photosynthesize by absorbing the blue end of the spectrum and reflecting the red (what little makes it to those depths). However, you will also find certain types of red algae in the shallows and even in the intertidal regions. These red algal species usually have calcium deposits in their cell walls which help them resist drying out (desiccation). Although often seen in tropical coral reef ecosystems, our waters include many species both encrusting and erect in habit. Now you know everything about seaweeds that you never wanted to know!

© 2015 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Brown and green algae; and erect and coralline red algae

This document maintained by Dr. Bill Bushing.
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